Minerva Surgical, Inc. v. Hologic, Inc.

P R E L I M I N A R Y P R I N T
Volume 594 U. S. Part 2
Pages 559–594
OFFICIAL REPORTS
OF
T H E S U P R E M E C O U R T
June 29, 2021
REBECCA A. WOMELDORF
reporter of decisions
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OCTOBER
TERM, 2020
559
Syllabus
MINERVA SURGICAL, INC. v. HOLOGIC, INC., et al.
certiorari
to the united states court of appeals for
the federal circuit
No. 20–440. Argued April 21, 2021—Decided June 29, 2021
In the late 1990s, Csaba Truckai invented a device to treat abnormal
uterine bleeding. The device, known as the NovaSure System, uses a
moisture-permeable applicator head to destroy targeted cells in the
uterine lining. Truckai fled a patent application and later assigned the
application, along with any future continuation applications, to his com-
pany, Novacept, Inc. The PTO issued a patent for the device. Nova-
cept, along with its portfolio of patents and patent applications, was even-
tually acquired by respondent Hologic, Inc. In 2008, Truckai founded
petitioner Minerva Surgical, Inc. There, he developed a supposedly
improved device to treat abnormal uterine bleeding. Called the Min-
erva Endometrial Ablation System, the new device uses a moisture-
impermeable applicator head to remove cells in the uterine lining. The
PTO issued a patent, and the FDA approved the device for commercial
sale. Meanwhile, Hologic fled a continuation application with the
PTO, seeking to add claims to its patent for the NovaSure System.
Hologic drafted one of its claims to encompass applicator heads gener-
ally, without regard to whether they are moisture permeable. The PTO
issued the altered patent in 2015.
Hologic then sued Minerva for patent infringement. As relevant
here, Minerva rejoined that Hologic's patent was invalid because the
newly added claim did not match the invention's written description,
which addresses applicator heads that are water permeable. In re-
sponse, Hologic invoked the doctrine of assignor estoppel. Because
Truckai had assigned the original patent application, Hologic argued, he
and Minerva could not impeach the patent's validity. The District
Court agreed that assignor estoppel barred Minerva's invalidity defense.
The Court of Appeals for the Federal Circuit affrmed in relevant part.
Minerva now asks this Court to abandon or narrow assignor estoppel.
Held: Assignor estoppel is well grounded in centuries-old fairness princi-
ples, and the Federal Circuit was right to uphold it. But assignor
estoppel applies only when the assignor's claim of invalidity contradicts
explicit or implicit representations he made in assigning the patent.
Pp. 566–579.
(a) Courts have long applied the doctrine of assignor estoppel to deal
with inconsistent representations about a patent's validity. The doc-
trine got its start in late 18th-century England and crossed the Atlantic

560 MINER
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Syllabus
about a hundred years later. This Court frst considered and approved
the
doctrine in Westinghouse Elec. & Mfg. Co. v. Formica Insulation
Co., 266 U. S. 342. The Court grounded the doctrine in a principle of
fairness: “If one lawfully conveys to another a patented right,” Westing-
house reasoned, “fair dealing should prevent him from derogating from
the title he has assigned.” Id., at 350. The Court made clear, however,
that the doctrine has limits. Although the assignor cannot assert inva-
lidity in an infringement suit, he can argue about how to construe the
patent's claims. Id., at 350–351. The Court left for another day other
questions about the doctrine's scope, including how it would apply to
the assignment of patent applications. Id., at 352–353. Pp. 566–570.
(b) The Court rejects Minerva's contention that assignor estoppel
should be abandoned. Minerva's frst argument on that score—that
Congress abrogated the doctrine in the Patent Act of 1952—is unpersua-
sive. Minerva relies on statutory language providing that “[i]nvalidity”
of the patent “shall be [a] defense[ ] in any action involving ” infringe-
ment. 35 U. S. C. § 282(b). According to Minerva, that language “in-
structs that invalidity must be available as a defense in every action,”
thus leaving no room for assignor estoppel. Brief for Petitioner 17–18.
But similar language appeared in the patent statute when the Court
decided Westinghouse. Anyway, Minerva's view is untenable because
it would foreclose applying in patent cases a whole host of common-law
preclusion doctrines—a broad result that would confict with this
Court's precedents. See, e. g., SCA Hygiene Products Aktiebolag v.
First Quality Baby Products, LLC, 580 U. S. 328, 345–346. And it
would subvert congressional design, for Congress in 1952 “legislate[d]
against a background of common-law adjudicatory principles,” including
assignor estoppel. Astoria Fed. Sav. & Loan Assn. v. Solimino, 501
U. S. 104, 108.
The Court also rejects Minerva's view that two post-Westinghouse
decisions have already interred assignor estoppel. In Scott Paper Co.
v. Marcalus Mfg. Co., 326 U. S. 249, the Court did nothing more than
decline to apply assignor estoppel in a novel and extreme circumstance.
The Court did not question—indeed, it restated—the “basic principle”
of fairness on which the doctrine rests. Id., at 251. In Lear, Inc. v.
Adkins, 395 U. S. 653, the Court considered and toppled a different pat-
ent estoppel doctrine—licensee estoppel—but did not purport to decide
the fate of assignor estoppel. To the contrary, the Court stated that
the patent holder's “equities” in the assignment context “were far more
compelling than those presented in the typical licensing arrangement.”
Id., at 664. Together, Scott Paper and Lear maintained assignor estop-
pel, but suggested that the doctrine needed to stay attached to its equi-
table moorings.
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Syllabus
Finally, the Court rejects Minerva's claim that contemporary patent
pol
icy—specifcally, the need to weed out bad patents—supports over-
throwing assignor estoppel. Assignor estoppel refects a demand for
consistency in dealing with others. When a person sells his patent
rights, he makes an (at least) implicit representation to the buyer that
the patent at issue is valid. In later raising an invalidity defense, the
assignor disavows that implied warranty. By saying one thing and then
saying another, the assignor wants to proft doubly—by gaining both
the price of assigning the patent and the continued right to use the
invention it covers. That course of conduct by the assignor is unfair
dealing. And the need to prevent such unfairness outweighs any loss
to the public from leaving an invalidity defense to someone other than
the assignor. Pp. 570–575.
(c) Assignor estoppel comes with limits: it applies only when its un-
derlying principle of fair dealing comes into play. That principle de-
mands consistency in representations about a patent's validity. When
an assignor warrants that a patent claim is valid, his later denial of
validity breaches norms of equitable dealing. But when the assignor
has made neither explicit nor implicit representations in confict with an
invalidity defense, then there is no unfairness in its assertion—and so
there is no ground for applying assignor estoppel. One example of non-
contradiction is when an assignment occurs before an inventor can possi-
bly make a warranty of validity as to specifc patent claims. That situa-
tion arises in certain employment arrangements, when an employee
assigns to his employer patent rights in any future inventions he may
develop during his employment. A second example is when a later
legal development renders irrelevant the warranty given at the time of
assignment. Third, and most relevant here, a post-assignment change
in patent claims can remove the rationale for applying assignor estoppel.
The last situation arises most often when an inventor assigns a patent
application, rather than an issued patent. There, the assignee may re-
turn to the PTO to enlarge the patent's claims. Assuming that the new
claims are materially broader than the old ones, the assignor did not
warrant to the new claims' validity. And if he made no such represen-
tation, then he can challenge the new claims in litigation: Because there
is no inconsistency in his positions, there is no estoppel.
The Federal Circuit failed to recognize these boundaries. Minerva
argued that estoppel should not apply because it was challenging a claim
that was materially broader than the ones Truckai had assigned. The
Federal Circuit declined to consider the alleged disparity, deeming
“irrelevant” the question whether Hologic had expanded the assigned
claims. But if Hologic's new claim is materially broader than the ones
Truckai assigned, then Truckai could not have warranted its validity in

562 MINER
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Syllabus
making the assignment. And without such a prior inconsistent repre-
sent
ation, there is no basis for estoppel. The judgment of the Federal
Circuit is therefore vacated, and the case is remanded for the Court of
Appeals to address whether Hologic's new claim is materially broader
than the ones Truckai assigned. Pp. 576–579.
957 F. 3d 1256, vacated and remanded.
Kagan, J., delivered the opinion of the Court, in which Roberts, C. J.,
and Breyer, Sotomayor, and Kavanaugh, JJ., joined. Alito, J., fled a
dissenting opinion, post, p. 579. Barrett, J., fled a dissenting opinion,
in which Thomas and Gorsuch, JJ., joined, post, p. 583.
Robert N. Hochman argued the cause for petitioner.
With him on the briefs were Vera M. Elson and Edward
G. Poplawski.
Morgan L. Ratner argued the cause for the United States
as amicus curiae urging vacatur. With her on the brief
were Acting Solicitor General Prelogar, Acting Assistant
Attorney General Boynton, Deputy Solicitor General Stew-
ar t, Mel issa N. Pa t te rso n, Ka th e r ine Two mey All en,
Thomas W. Krause, Farheena Y. Rasheed, William La-
Marca, Meredith H. Schoenfeld, and Megan Heller.
Matthew M. Wolf argued the cause for respondents.
With him on the brief were Jennifer A. Sklenar, R. Stanton
Jones, William M. Jay, and David J. Zimmer.*
*Phillip R. Malone, Mark A. Lemley, pro se, and William H. Neukom
fled a brief for Intellectual Property Law Professors as amici curiae urg-
ing reversal.
Briefs of amici curiae urging affrmance were fled for Leading Tech-
nology Composites, Inc., et al. by Daniel R. Ortiz and Jean E. Lewis;
and for United Therapeutics Corp. by Charles L. McCloud, Amy Mason
Saharia, and Shaun R. Snader.
Briefs of amici curiae were fled for the American Intellectual Property
Law Association by Richard T. Matthews and Joseph R. Re; for Engine
Advocacy by Christopher T. Bavitz; for the Intellectual Property Owners
Association by Robert M. Isackson, Melvin Garner, Matthew Kaufman,
Lauren Sabol, and Kevin H. Rhodes; for the New York City Bar Asso-
ciation by Aaron L. J. Pereira, Timothy P. Heaton, and John Gladstone
Mills III; for the New York Intellectual Property Law Association by
Irena Royzman, Colman B. Ragan, Robert J. Rando, Charles R. Macedo,
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563
Opinion of the Court
Justice Kagan delivered the opinion of the Court.
In
Westinghouse Elec. & Mfg. Co. v. Formica Insulation
Co., 266 U. S. 342, 349 (1924), this Court approved the “well
settled” patent-law doctrine of “assignor estoppel.” That
doctrine, rooted in an idea of fair dealing, limits an inventor's
ability to assign a patent to another for value and later con-
tend in litigation that the patent is invalid. The question
presented here is whether to discard this century-old form
of estoppel. Continuing to see value in the doctrine, we
decline to do so. But in upholding assignor estoppel, we
clarify that it reaches only so far as the equitable principle
long understood to lie at its core. The doctrine applies
when, but only when, the assignor's claim of invalidity con-
tradicts explicit or implicit representations he made in as-
signing the patent.
I
Inventors look to the patent system to obtain valuable
rights. A typical patent application, fled with the U. S. Pat-
ent and Trademark Offce (PTO), includes a written descrip-
tion and drawing of the invention and one or more claims
par ticu lar ly setti ng out the i nventi on's scope. See 35
U. S. C. §§ 111–113. The application also usually contains an
inventor's oath—a statement attesting that the applicant is
“the original inventor” of the “claimed invention,” so that he
is entitled to the patent sought. § 115. If the PTO decides
that the invention meets the “conditions for patentability”—
mainly, that the invention is useful, novel, and non-obvious—
it will issue a patent to the inventor. See §§ 101–103. That
award gives the inventor the right to exclude others from
making, using, or selling the invention until the patent ex-
pires (currently, 20 years after the application date). See
§ 154.
David P. Goldberg, and Mark A. Chapman; and for Pharmaceutical Re-
search and Manufacturers of America by Thomas G. Saunders, James C.
Stansel, and David E. Korn.
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564 MINER
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Opinion of the Court
The invention sparking this lawsuit is a device to treat
abnor
mal uterine bleeding, a medical condition affecting
many millions of women. Csaba Truckai, a founder of the
company Novacept, Inc., invented the device—called the
NovaSure System—in the late 1990s. He soon afterward
fled a patent application, and assigned his interest in the
appl icati on—as wel l as i n any future “conti nuati on
applications”—to Novacept.
1
The NovaSure System, as de-
scribed in Truckai's patent application, uses an applicator
head to destroy targeted cells in the uterine lining. To
avoid unintended burning or ablation (tissue removal), the
head is “moisture permeable,” meaning that it conducts fuid
out of the uterine cavity during treatment. The PTO issued
a patent, and in 2001 the Food and Drug Administration
(FDA) approved the device for commercial distribution.
But neither Truckai nor Novacept currently benefts from
the NovaSure System patent. In 2004, Novacept sold its
assets, including its portfolio of patents and patent applica-
tions, to another company (netting Truckai individually
about $8 million). And in another sale, in 2007, respondent
Hologic, Inc. acquired all patent rights in the NovaSure Sys-
tem. Today, Hologic sells that device throughout the
United States.
Not through with inventing, Truckai founded in 2008 peti-
tioner Minerva Surgical, Inc. There, he developed a suppos-
edly improved device to treat abnormal uterine bleeding.
Called the Minerva Endometrial Ablation System, the device
(like the NovaSure System) uses an applicator head to re-
move cells in the uterine lining. But the new device, relying
1
A continuation application enables an inventor to add to or modify the
claims set out in his original application. See 35 U. S. C. § 120; Manual
of Patent Examining Procedure § 201.07 (9th ed., June 2020). But the
continuation application may not materially change the written description
of the invention. See Manual of Patent Examining Procedure § 211.05.
So the new or altered claims must align with the original description.
Ibid.; see 35 U. S. C. § 112.
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Opinion of the Court
on a different way to avoid unwanted ablation, is “moisture
i
mpermeable”: It does not remove any fuid during treat-
ment. The PTO issued a patent for the device, and in 2015
the FDA approved it for commercial sale.
Meanwhile, in 2013, Hologic fled a continuation application
requesting to add claims to its patent for the NovaSure Sys-
tem. Aware of Truckai's activities, Hologic drafted one of
those claims to encompass applicator heads generally, with-
out regard to whether they are moisture permeable. The
PTO in 2015 issued the altered patent as requested.
A few months later, Hologic sued Minerva for patent
infringement. Minerva rejoined that its device does not in-
fringe. But more relevant here, it also asserted that Holog-
ic's amended patent is invalid. The essential problem,
according to Minerva, is that the new, broad claim about ap-
plicator heads does not match the invention's description,
which addresses their water-permeability. See Defendant
Minerva's Opening Brief in Support of Its Motion for Partial
Summary Judgment in No. 15–cv–1031 (Del.), Doc. 300,
pp. 8–9, 13–15; see also Festo Corp. v. Shoketsu Kinzoku
Kogyo Kabushiki Co., 535 U. S. 722, 736 (2002) (“What is
claimed by the patent application must be the same” as what
is described). In response, Hologic invoked the doctrine of
assignor estoppel. Because Truckai assigned the original
patent application, Hologic argued, he and Minerva (essen-
tially, his alter-ego) could not impeach the patent's validity.
The District Court agreed that assignor estoppel barred
Minerva's invalidity defense, and also ruled that Minerva had
infringed Hologic's patent. See 325 F. Supp. 3d 507, 524–
525, 532 (Del. 2018). At a trial on damages, a jury awarded
Hologic about $5 million.
The Court of Appeals for the Federal Circuit mainly up-
held the judgment, focusing on assignor estoppel. The court
frst “decline[d] Minerva's invitation to `abandon [that] doc-
trine.' ” 957 F. 3d 1256, 1267 (2020). Citing both this
Court's precedents and equitable principles, the court af-
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frmed the doctrine's “continued vitality.” Id., at 1268. An
assig
nor, the court stated, “should not be permitted to sell
something and later to assert that what was sold is worth-
less, all to the detriment of the assignee.” Id., at 1265.
The assignor makes an “implicit representation” that the
rights “he is assigning (presumably for value) are not worth-
less.” Ibid. It would “work an injustice,” the court rea-
soned, to “allow the assignor to make that representation at
the time of assignment (to his advantage) and later to repudi-
ate it (again to his advantage).” Ibid. (quoting Diamond
Scientifc Co. v. Ambico, Inc., 848 F. 2d 1220, 1224 (CA Fed.
1988)). The court then applied assignor estoppel to bar
Truckai and Minerva from raising an invalidity defense.
Here, the court rejected Minerva's argument that because
“Hologic broadened the claims” after “Truckai's assign-
ment,” it would “be unfair to block Truckai (or Minerva) from
challenging the breadth of those claims.” 957 F. 3d, at 1268.
Relying on circuit precedent, the court deemed it “irrelevant
that, at the time of the assignment, the inventor's patent
application[ ] w[as] still pending ” and that the assignee “may
have later amended the claims” without the inventor's input.
Ibid. (quoting Diamond Scientifc, 848 F. 2d, at 1226).
We granted certiorari, 592 U. S. ––– (2021), to consider the
important issues raised in the Federal Circuit's judgment.
Assig nor estoppel, we now hold, is wel l grounded i n
centuries-old fairness principles, and the Federal Circuit was
right to uphold it. But the court failed to recognize the
doctrine's proper limits. The equitable basis of assignor
estoppel defnes its scope: The doctrine applies only when an
inventor says one thing (explicitly or implicitly) in assigning
a patent and the opposite in litigating against the patent's
owner.
II
Courts have long applied the doctrine of assignor estoppel
to deal with inconsistent representations about a patent's
validity. The classic case (different in certain respects from
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the one here) begins with an inventor who both applies for
and
obtains a patent, then assigns it to a company for value.
Later, the inventor/assignor joins a competitor business,
where he develops a similar—and possibly infringing—
product. When the assignee company sues for infringe-
ment, the assignor tries to argue—contrary to the (explicit
or implicit) assurance given in assigning the patent—that
the invention was never patentable, so the patent was never
valid. That kind of about-face is what assignor estoppel
operates to prevent—or, in legalese, estop. As one of the
early American courts to use the doctrine held: The assignor
is not “at liberty to urge [invalidity] in a suit upon his own
patent against a party who derives title to that patent
through him.” Woodward v. Boston Lasting Mach. Co., 60
F. 283, 284 (CA1 1894). Or as the Federal Circuit held in
modern times: The assignor's explicit or “implicit represen-
tation” that the patent he is assigning is “not worthless . . .
deprive[s] him of the ability to challenge later the [patent's]
validity.” Diamond Scientifc, 848 F. 2d, at 1224.
Assignor estoppel got its start in late 18th-century Eng-
land and crossed the Atlantic about a hundred years later.
In the frst recorded case, Lord Kenyon found that a patent
assignor “was by his own oath and deed estopped” in an in-
fringement suit from “attempt[ing] to deny his having had
any title to convey.” Oldham v. Langmead (1789), as de-
scribed in J. Davies, Collection of the Most Important Cases
Respecting Patents of Invention and the Rights of Patentees
442 (1816); see Hayne v. Maltby, 3 T. R. 439, 441, 100 Eng.
Rep. 665, 666 (K. B. 1789) (recognizing the Oldham holding).
That rule took inspiration from an earlier doctrine—estoppel
by deed—applied in real property law to prevent a conveyor
of land from later asserting that he had lacked good title at
the time of sale. See 2 E. Coke, The First Part of the Insti-
tutes of Laws of England 352a (Hargrave & Butler eds., 19th
ed. 1832) (1628). Lord Kenyon's new patent formulation of
the doctrine grew in favor throughout the 1800s as an aspect
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568 MINER
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Opinion of the Court
of fair dealing: When “the Defendant sold and assigned th[e]
patent
to the Plaintiffs as a valid one,” it “does not lie in
his mouth to say that the patent is not good.” Chambers v.
Crichley, 33 Beav. 374, 376, 55 Eng. Rep. 412 (1864); see Wal-
ton v. Lavater, 8 C. B. N. S. 162, 187, 141 Eng. Rep. 1127,
1137 (C. P. 1860) (“The defendant, who has received a large
sum for the sale of this patent, ought not to be allowed to
raise any question as to its validity”). The earliest Ameri-
can decision applying the doctrine dates from 1880. See
Faulks v. Kamp, 3 F. 898 (CC SDNY). Within a decade
or two, the doctrine was “so well established and generally
accepted that citation of authority is useless.” Griffth v.
Shaw, 89 F. 313, 315 (CC SD Iowa 1893); see 2 W. Robinson,
Law of Patents for Useful Inventions § 787 (1890) (collect-
ing cases).
This Court frst considered—and unanimously approved—
assignor estoppel in 1924, in Westinghouse v. Formica.
Speaking through Chief Justice Taft, the Court initially in-
voked the doctrine's uniform acceptance in the lower courts.
The frst decision applying assignor estoppel, the Court re-
counted, was soon “followed by a myriad.” 266 U. S., at 349.
“[L]ater cases in nearly all the Circuit Courts of Appeal”
were “to the same point” as the frst, adding up to a full
“forty-fve years of judicial consideration and conclusion.”
Ibid. Such a “well settled” rule, in the Court's view, should
“not [be] lightly disturb[ed].” Ibid. And so it was not dis-
turbed, lightly or otherwise. Rather, the Court added its
own voice to that pre-existing “myriad.” We announced
that an assignor “is estopped to attack” the “validity of a
patented invention which he has assigned.” Ibid. “As to
the rest of the world,” the Court explained, “the patent may
have no effcacy”; but “the assignor can not be heard to ques-
tion” the assignee's rights in what was conveyed. Ibid.
West ingho use, like its precursor decisi ons, grounded
assignor estoppel in a principle of fairness. “If one lawfully

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Opinion of the Court
conveys to another a patented right,” the Court reasoned,
“fair
dealing should prevent him from derogating from the
title he has assigned.” Id., at 350. After all, the “grantor
purports to convey the right to exclude others”; how can he
later say, given that representation, that the grantee in fact
possesses no such right? Ibid. The Court supported that
view of equity by referring to estoppel by deed. See supra,
at 567. Under that doctrine, the Court explained, “a
grantor of a deed of land” cannot “impeach[ ] the effect of his
solemn act” by later claiming that the grantee's title is no
good. Westinghouse, 266 U. S., at 350. “The analogy” was
“clear”: There was “no reason why the principles of estoppel
by deed should not apply to [the] assignment of a patent
right.” Id., at 348, 350. In the latter context too, the Court
held, the assignor could not fairly “attack” the validity of a
right he had formerly sold. Id., at 349.
After thus endorsing assignor estoppel, the Court made
clear that the doctrine has limits. Although the assignor
cannot assert in an infringement suit that the patent is
invalid, the Court held that he can argue about how to
construe the patent's claims. Here, the Court addressed the
role in patent suits of prior art—the set of earlier inven-
tions (and other information) used to decide whether the
specifed invention is novel and non-obvious enough to merit
a patent. Id., at 350. “Of course,” the Court said, the as-
signor cannot use prior art in an infringement suit “to de-
stroy the patent,” because he “is estopped to do this.” Id.,
at 351. But he can use prior art to support a narrow claim
construction—to “construe and narrow the claims of the
patent, conceding their validity.” Ibid. “Otherwise,” the
Court explained, a judge “would be denied” the “most satis-
factory means” of “reaching a just conclusion” about the pat-
ent's scope—a conclusion needed to resolve the infringement
charge. Id., at 350–351. “The distinction” thus estab-
lished, the Court thought, “may be a nice one, but seems to
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be workable.” Id., at 351. And, indeed, the Court applied
it
to decide the case at hand for the assignor, fnding that he
had not infringed the properly narrowed claim.
2
Finally, the Court left for another day several other ques-
tions about the contours of assignor estoppel. One con-
cerned privity: When was an assignor so closely affliated
with another party that the latter would also be estopped?
See id., at 355. Another related to consideration: What if
an assignor had received only a nominal amount of money
for transferring the patent? See ibid. But the question
that most interested the Court was whether estoppel should
operate differently if the assignment was not of a granted
patent but of a patent application—as in fact was true in that
case. The Court saw a possible distinction between the two.
In a patent application, the Court began, the inventor
“swor[e] to” a particular “specifcation.” Id., at 352. But
the exact rights at issue were at that point “inchoate”—not
“certainly defned.” Ibid. And afterward, the Court (pre-
sciently) observed, the claims might be “enlarge[d]” at “the
instance of the assignee” beyond what the inventor had put
forward. Id., at 353. That might weaken the case for es-
toppel. But the Court decided not to decide the issue, given
its holding that the assignor had not infringed the (narrowed)
patent claim anyway.
III
Minerva's main argument here, as in the Federal Circuit,
is that “assignor estoppel should be eliminated”—and indeed
2
The limit set out in Westinghouse is not often invoked today, because
modern courts construe patent claims (as they construe statutes) mainly
by reference to their text. See Lemley, Rethinking Assignor Estoppel,
54 Houston L. Rev. 513, 523 (2016). Only when the claims are “still ambig-
uous,” after consideration of text and canons, will a court think about
narrowing a claim by reference to prior art. Phillips v. AWH Corp., 415
F. 3d 1303, 1327 (CA Fed. 2005) (internal quotation marks omitted). The
critical point for our purpose is that even while affrming the assignor
estoppel doctrine, the Court made clear that it did not always bar assign-
ors from effectively defending against infringement suits.
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has been already. Brief for Petitioner 17. We reject that
v
iew. The doctrine has lasted for many years, and we con-
tinue to accept the fairness principle at its core. Minerva's
back-up contention is that assignor estoppel “should be con-
strained.” Id., at 41. On that score, we fnd that the Fed-
eral Circuit has applied the doctrine too expansively. Today,
we clarify the scope of assignor estoppel, including in the
way Westinghouse suggested.
A
In its quest to abolish assignor estoppel, Minerva lodges
three main arguments. The frst two offer different reasons
for why the doctrine is already defunct: because Congress
repudiated it in the Patent Act of 1952 and because, even if
not, this Court's post-Westinghouse cases “leave no room for
the doctrine to continue.” Brief for Petitioner 20. The
third, by contrast, is a present-day policy claim: that assignor
estoppel “imposes” too high a “barrier to invalidity chal-
lenges” and so keeps bad patents alive. Id., at 38. (The
principal dissent essentially endorses the frst two argu-
ments, but not the third. See post, at 583–584, 586–588, 593
(opinion of Barrett, J.).)
On the frst point, we do not agree that the Patent Act of
1952 abrogated assignor estoppel. The statutory language
Minerva relies on provides that “[i]nvalidity” of the patent
“shall be [a] defense[ ] in any action involving ” infringement.
35 U. S. C. § 282(b). According to Minerva, that language
“instructs that invalidity must be available as a defense in
every [infringement] action,” thus “leav[ing] no room for as-
signor estoppel.” Brief for Petitioner 17–18 (emphasis in
original). But to begin with, similar language, entitling a
defendant to plead invalidity in any infringement action, was
in the patent statute when Westinghouse was decided. See
Patent Act of 1897, ch. 391, § 2, 29 Stat. 692 (“In any action
for infringement the defendant may plead” invalidity). And
anyway, Minerva's view is untenable because it would fore-
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VA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court
close applying in patent cases a whole host of common-law
preclusi
on doctrines—not just assignor estoppel, but equita-
ble estoppel, collateral estoppel, res judicata, and law of the
case. That broad result would confict with this Court's
precedents. See, e. g., SCA Hygiene Products Aktiebolag v.
First Quality Baby Products, LLC, 580 U. S. 328, 345–346
(2017) (recognizing equitable estoppel in a patent suit). And
it would subvert congressional design. For Congress “leg-
islate[s] against a background of common-law adjudicatory
principles,” and it “expect[s]” those principles to “apply ex-
cept when a statutory purpose to the contrary is evident.”
Astoria Fed. Sav. & Loan Assn. v. Solimino, 501 U. S. 104,
108 (1991) (internal quotation marks omitted). Assignor es-
toppel was by 1952 just such a background principle of patent
adjudication, and Congress gave no indication of wanting to
terminate it or disturb its development. Nor has Congress
done so since that time.
We likewise do not accept Minerva's view that two of our
post-Westinghouse decisions have already interred assignor
estoppel. According to Minerva (quoting the case's dissent),
Scott Paper Co. v. Marcalus Mfg. Co. “eliminated any justif-
cation for assignor estoppel and `repudiated' the doctrine.”
Reply Brief 13 (quoting 326 U. S. 249, 264 (1945) (Frank-
furter, J., dissenting)). And if that were not enough, Min-
erva continues, our decision in Lear, Inc. v. Adkins, 395 U. S.
653 (1969), also “eviscerated any basis for assignor estoppel.”
Reply Brief 13. But we think the words “eliminated,”
“repudiated,” and “eviscerated” are far off. Scott Paper and
Lear in fact retained assignor estoppel; all they did was po-
lice the doctrine's boundaries ( just as Westinghouse did and
we do today).
Whatever a worked-up dissent charged, Scott Paper did
nothing more than decline to apply assignor estoppel in a
novel and extreme circumstance. The petitioner in Scott
Paper made the same ask Minerva does here: to abolish the
Westinghouse rule. The Court expressly declined that re-
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573
Opinion of the Court
quest. See 326 U. S., at 254. And it restated the “basic
pr
inciple” animating assignor estoppel, describing it as “one
of good faith, that one who has sold his invention may not,
to the detriment of the purchaser, deny the existence of that
which he has sold.” Id., at 251. The Court, to be sure, de-
clined to apply the doctrine in the case before it. There,
estoppel would have prevented the assignor from making a
device on which the patent had expired—a device, in other
words, that had already entered the public domain. The
Court could not fnd any precedent for applying estoppel in
that situation. See id., at 254. And the Court thought that
doing so would carry the doctrine too far, reasoning that the
public's interest in using an already-public invention out-
weighs the “interest in private good faith.” Id., at 256–257.
But the Court did not question—again, it reaffrmed—the
principle of fairness on which assignor estoppel rests in more
common cases, where the assignee is not claiming to control
a device unequivocally part of the public domain. See id.,
at 251. In those cases, the doctrine remained intact.
Lear gives Minerva still less to work with. In that case,
the Court considered and toppled a different patent estoppel
doctrine. Called licensee estoppel, it barred (as its name
suggests) a patent licensee from contesting the validity of
the patent on a device he was paying to use. Minerva's
basic claim is that as goes one patent estoppel rule, so goes
another. Brief for Petitioner 23–25. But Lear did not pur-
port to decide the fate of the separate assignor estoppel doc-
trine. To the contrary, the Court stated that the patent
holder's “equities” in the assignment context “were far more
compelling than those presented in the typical licensing ar-
rangement.” 395 U. S., at 664. And so they are. As ex-
plained earlier, assignor estoppel rests on the idea that the
assignor has made an explicit or implicit representation
about the patent's validity, and received some kind of pay-
ment in return. See supra, at 566–568. No rationale of that
kind supports licensee estoppel. The licensee is a buyer of
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VA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court
patent rights, not a seller. He has given no assurances of
the
patent's worth. All he has done is purchase the right to
use a patented device—which, if the patent is invalid, he
need not have done. Lear's refusal to bar a licensee's claim
of invalidity showed that the Court was alert to “the impor-
tant public interest in permitting full and free competition
in the use of ideas”—and so would not always apply patent
estoppel doctrines. 395 U. S., at 670. But that does not
mean, at the other extreme, always rejecting those doc-
trines. Lear counseled careful attention to the equities at
stake in discrete patent contexts—and expressly distin-
guished assignor from licensee estoppel.
In sum, Scott Paper and Lear left Westinghouse right
about where they found it—as a bounded doctrine designed
to prevent an inventor from frst selling a patent and then
contending that the thing sold is worthless. Westinghouse
saw that about-face as unfair; Scott Paper and Lear never
questioned that view. At the same time, Westinghouse real-
ized that assignor estoppel has limits: Even in approving the
doctrine, the Court made clear that not every assignor de-
fense in every case would fall within its scope. See supra,
at 569–570. Scott Paper and Lear adopted a similar stance.
They maintained assignor estoppel, but suggested (if in dif-
ferent ways) that the doctrine needed to stay attached to its
equitable moorings. The three decisions together thus show
not the doctrinal “eviscerat[ion]” Minerva claims, Reply
Brief 13, but only the kind of doctrinal evolution typical of
common-law rules.
Finally, we do not think, as Minerva claims, that contempo-
rary patent policy—specifcally, the need to weed out bad
patents—supports overthrowing assignor estoppel. In re-
jecting that argument, we need not rely on stare decisis:
“[C]orrect judgments have no need for that principle to prop
them up.” Kimble v. Marvel Entertainment, LLC, 576 U. S.
446, 455 (2015). And we continue to think the core of as-
signor estoppel justifed on the fairness grounds that courts

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575
Opinion of the Court
applying the doctrine have always given. Assignor estop-
pel,
like many estoppel rules, refects a demand for consist-
ency in dealing with others. See H. Herman, The Law of
Estoppel § 3 (1871) (“An estoppel is an obstruction or bar to
one's alleging or denying a fact contrary to his own previous
action, allegation or denial”). When a person sells his pat-
ent rights, he makes an (at least) implicit representation to
the buyer that the patent at issue is valid—that it will actu-
ally give the buyer his sought-for monopoly.
3
In later rais-
ing an invalidity defense, the assignor disavows that implied
warranty. And he does so in service of regaining access to
the invention he has just sold. As the Federal Circuit put
the point, the assignor wants to make a “representation at
the time of assignment (to his advantage) and later to repudi-
ate it (again to his advantage).” Diamond Scientifc, 848
F. 2d, at 1224; see supra, at 565–566. By saying one thing
and then saying another, the assignor wants to proft dou-
bly—by gaining both the price of assigning the patent and
the continued right to use the invention it covers. That
course of conduct by the assignor strikes us, as it has struck
courts for many a year, as unfair dealing—enough to out-
weigh any loss to the public from leaving an invalidity de-
fense to someone other than the assignor.
4
3
Recognizing this implicit representation is particularly appropriate
given the patent law's demand for honesty from patent applicants. In
applying for a patent, the inventor must ordinarily submit an oath—a
statement attesting that he is “the original inventor” of the “claimed in-
vention.” § 115(b)(2); see supra, at 563. And the inventor must comply
with “a duty of candor and good faith” in the patent process, including “a
duty to disclose” to the PTO all information he knows “to be material to
patentability.” 37 CFR § 1.56(a) (2020); see § 1.63(c). An inventor pre-
senting an application to the PTO thus states his good-faith belief that his
claims are patentable—that they will result in a valid patent. When the
inventor then assigns those claims to another, he effectively incorporates
that assurance.
4
Even beyond promoting fairness, assignor estoppel furthers some pat-
ent policy goals. Assignors are especially likely infringers because of
their knowledge of the relevant technology. By preventing them from

576 MINER
VA SURGICAL, INC. v. HOLOGIC, INC.
Sti
ll, our
Opinion of the Court
B
endorsement of assignor estoppel comes with
limits—true to the doctrine's reason for being. Just as we
guarded the doctrine's boundaries in the past, see supra, at
569–570, 572–574, so too we do so today. Assignor estoppel
should apply only when its underlying principle of fair deal-
ing comes into play. That principle, as explained above, de-
mands consistency in representations about a patent's valid-
ity: What creates the unfairness is contradiction. When an
assignor warrants that a patent is valid, his later denial of
validity breaches norms of equitable dealing. And the origi-
nal warranty need not be express; as we have explained, the
assignment of specifc patent claims carries with it an im-
plied assurance. See supra, at 575. But when the assignor
has made neither explicit nor implicit representations in con-
fict with an invalidity defense, then there is no unfairness
in its assertion. And so there is no ground for applying as-
signor estoppel.
One example of non-contradiction is when the assignment
occurs before an inventor can possibly make a warranty of
validity as to specifc patent claims. Consider a common
employment arrangement. An employee assigns to his em-
ployer patent rights in any future inventions he develops
during his employment; the employer then decides which,
if any, of those inventions to patent. In that scenario, the
assignment contains no representation that a patent is valid.
How could it? The invention itself has not come into being.
See Lemley, Rethinking Assignor Estoppel, 54 Houston
L. Rev. 513, 525–527 (2016). And so the employee's transfer
of rights cannot estop him from alleging a patent's invalidity
in later litigation.
raising an invalidity defense in an infringement suit, the doctrine gives
assignees confdence in the value of what they have purchased. That
raises the price of patent assignments, and in turn may encourage
invention.
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577
Opinion of the Court
A second example is when a later legal development ren-
ders
irrelevant the warranty given at the time of assignment.
Suppose an inventor conveys a patent for value, with the
warranty of validity that act implies. But the governing
law then changes, so that previously valid patents become
invalid. The inventor may claim that the patent is invalid in
light of that change in law without contradicting his earlier
representation. What was valid before is invalid today,
and no principle of consistency prevents the assignor from
saying so.
Most relevant here, another post-assignment develop-
ment—a change in patent claims—can remove the rationale
for applying assignor estoppel. Westinghouse itself antici-
pated this point, which arises most often when an inventor
assigns a patent application, rather than an issued patent.
As Westinghouse noted, “the scope of the right conveyed in
such an assignment” is “inchoate”—“less certainly defned
than that of a granted patent.” 266 U. S., at 352–353; see
supra, at 570. That is because the assignee, once he is the
owner of the application, may return to the PTO to “en-
large[ ]” the patent's claims. 266 U. S., at 353; see 35 U. S. C.
§ 120; 37 CFR § 1.53(b). And the new claims resulting from
that process may go beyond what “the assignor intended” to
claim as patentable. 266 U. S., at 353. Westinghouse did
not need to resolve the effects of such a change, but its liber-
ally dropped hints—and the equitable basis for assignor es-
toppel—point all in one direction. Assuming that the new
claims are materially broader than the old claims, the as-
signor did not warrant to the new claims' validity. And if
he made no such representation, then he can challenge the
new claims in litigation: Because there is no inconsistency in
his positions, there is no estoppel. The limits of the assign-
or's estoppel go only so far as, and not beyond, what he rep-
resented in assigning the patent application.
The Federal Circuit, in both its opinion below and prior
decisions, has failed to recognize those boundaries. Minerva
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578 MINER
VA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court
(recall, Truckai's alter-ego) argued to the court that estoppel
shou
ld not apply because it was challenging a claim that was
materially broader than the ones Truckai had assigned. But
the court declined to consider that alleged disparity. Citing
circuit precedent, the court held it “irrelevant” whether Ho-
logic had expanded the assigned claims: Even if so, Minerva
could not contest the new claim's validity. 957 F. 3d, at 1268
(quoting Diamond Scientifc, 848 F. 2d, at 1226); see supra,
at 566. For the reasons given above, that conclusion is
wrong. If Hologic's new claim is materially broader than
the ones Truckai assigned, then Truckai could not have war-
ranted its validity in making the assignment. And without
such a prior inconsistent representation, there is no basis
for estoppel.
We remand this case to the Federal Circuit to now address
what it thought irrelevant: whether Hologic's new claim is
materially broader than the ones Truckai assigned. The
parties vigorously disagree about that issue. In Truckai's
view, the new claim expanded on the old by covering non-
moisture-permeable applicator heads. In Hologic's view,
the claim matched a prior one that Truckai had assigned.
Resolution of that issue in light of all relevant evidence will
determine whether Truckai's representations in making the
assignment confict with his later invalidity defense—and so
will determine whether assignor estoppel applies.
IV
This Court recognized assignor estoppel a century ago,
and we reaffrm that judgment today. But as the Court rec-
ognized from the beginning, the doctrine is not limitless.
Its boundaries refect its equitable basis: to prevent an as-
signor from warranting one thing and later alleging another.
Assignor estoppel applies when an invalidity defense in an
infringement suit conficts with an explicit or implicit repre-
sentation made in assigning patent rights. But absent that
kind of inconsistency, an invalidity defense raises no concern
of fair dealing—so assignor estoppel has no place.
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579
Alito, J., dissenting
For these reasons, we vacate the judgment of the Federal
Circuit
and remand the case for further proceedings consist-
ent with this opinion.
It is so ordered.
Justice Alito, dissenting.
We granted review in this case to decide whether the doc-
trine of assignor estoppel bars petitioner from challenging
the validity of a patent indirectly assigned to respondents,
and I do not see how we can answer that question without
deciding whether Westinghouse Elec. & Mfg. Co. v. Formica
Insulation Co., 266 U. S. 342 (1924), which recognized as-
signor estoppel, should be overruled. Both the majority and
the principal dissent go to great lengths to avoid that ques-
tion, but in my judgment, their efforts are unsuccessful.
The majority says it has no need to invoke precedent, see
ante, at 574–575, but without that support, the majority's
holding cannot stand. Not one word in the patent statutes
supports assignor estoppel, and the majority does not claim
otherwise. “[T]his Court [doesn't] usually read into statutes
words that aren't there,” Romag Fasteners, Inc. v. Fossil
Group, Inc., 590 U. S. –––, ––– (2020), but that is just what
the majority has done in this case.
With so little support for its reasoning, it is more than
a little surprising that the majority forswears reliance on
precedent. See ante, at 574. Not too long ago, in Kimble
v. Marvel Entertainment, LLC, 576 U. S. 446 (2015), another
case involving a judicially created rule of patent law, the
Court applied a “superpowered form of stare decisis.” Id.,
at 458. Yet the majority refuses to wield the nearly impreg-
nable Kimble shield. Instead, it adopts a text-blind method
of statutory interpretation with which I cannot possibly
agree.
The Court's evasion of stare decisis is fully matched by the
principal dissent. That opinion sees no need to address
stare decisis because, in its view, Westinghouse has not been
a precedent for the past 69 years. According to the princi-

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580 MINER
VA SURGICAL, INC. v. HOLOGIC, INC.
Alito, J., dissenting
pal dissent, Westinghouse was abrogated by the Patent Act
of
1952. It reasons as follows: Westinghouse interpreted the
Patent Act of 1870, post, at 583 (opinion of Barrett, J.);
because that Act was superseded by the Patent Act of 1952,
we must decide whether the new Act “ratifed” Westing-
house, post, at 584; and in order to show that Westinghouse
was ratifed, the defenders of assignor estoppel must per-
suade us that “(1) the interpretation [adopted in Westing-
house was] so well settled that we can `presume Congress
knew of and endorsed it' at the time of the reenactment, and
(2) the statute [was] reenacted without material change,”
post, at 585 (quoting Jama v. Immigration and Customs En-
forcement, 543 U. S. 335, 349 (2005)).
This reasoning is unprecedented and troubling. To start,
it is quite misleading to suggest that Westinghouse was
based on an interpretation of the 1870 Patent Act. Neither
of the two statutory provisions Westinghouse mentioned said
anyth i ng that suppor ted the Cour t's decisi on, and the
Court did not claim otherwise.
1
Instead, the decision rested
on different grounds. It relied on the principle of “fair
dealing,” an analogy to the doctrine of estoppel by deed (a
feature of the law of real property), and perhaps most impor-
tantly, a body of lower court case law. 266 U. S., at 348–352.
If Westinghouse had been based on an interpretation of lan-
guage in the 1870 Act and if the 1952 Act had changed that
language, there might be a basis for fnding abrogation.
2
But that is not the situation here.
1
One provision permitted the assignment of patent rights, see Westing-
house, 266 U. S., at 348–349, and the other, which authorized the granting
of a patent to an assignee, was discussed in relation to the scope of the
doctrine of assignor estoppel, not its existence, see id., at 352.
2
By the same token, if Congress had again used that particular language
without change, there might be a basis for fnding ratifcation. See
A. Scalia & B. Garner, Reading Law: The Interpretation of Legal Texts
322 (2012) (Reading Law) (prior-construction canon applies when a “word
or phrase has been authoritatively interpreted [and] a later version of that
act perpetuat[es] the wording ”).

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as: 594 U. S. 559 (2021)
581
Alito, J., dissenting
After starting with this fawed premise, the principal dis-
sent
adopts an ill-suited standard for determining whether
one of our precedents has been abrogated by Congress. It
applies a rule of interpretation that is patently designed for
a different purpose, i. e., determining whether congressional
reenactment of a statute should be understood as a ratifca-
tion of a preexisting body of lower court case law.
3
That
was the issue in Jama, 543 U. S., at 349–352, the case the
principal dissent quotes, and that is why the rule asks
whether the interpretation in question is “well settled.”
That question makes sense as applied to a body of lower
court cases, but what does that mean with respect to one
of our precedents? I would think that endorsement by a
majority of this Court is consensus enough. Suggesting
that a rule announced in a decision of this Court can cease
to be a precedent if it is not “well settled” is very strange.
Equally strange is the question whether we can presume
that Congress “knew of ” a decision of this Court interpret-
ing a statutory provision that it reenacts. Perhaps it is hu-
bris, but we have often presumed that Congress is aware of
our decisions. See, e. g., Ryan v. Valencia Gonzales, 568
U. S. 57, 66 (2013) (“We nor ma l ly assume that, when
Congress enacts statutes, it is aware of relevant judicial
precedent” (internal quotation marks omitted)).
When we reach the fnal part of the principal dissent's test
for abrogating one of our precedents—whether the statute
in question was “reenacted without material change”—we
encounter further problems. The adjective “material” can
mean “[h]aving some logical connection with the consequen-
tial facts.” Black's Law Dictionary 1170 (11th ed. 2019).
3
This is not to say that Congress cannot pass statutes meant to incorpo-
rate this Court's interpretations of specifc statutory language. Compare
post, at 575, n. 3. It obviously can. What is unusual is not the idea that
Congress can ratify this Court's decisions but instead the application of a
test designed to assess the ratifcation of lower court decisions to assess
the abrogation of a decision of this Court.
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582 MINER
VA SURGICAL, INC. v. HOLOGIC, INC.
Alito, J., dissenting
Thus, the principal dissent can be read to say that a decision
of
this Court interpreting a statutory provision is abrogated
whenever a change in the language of the statute provides
some degree of support for a different interpretation. The
principal dissent cites no case in which we have set the abro-
gation standard so low. And this new “material change”
standard for abrogating one of our precedents is a marked
departure from the standard advocated in one of the princi-
pal dissent's chosen treatises. See Reading Law 331 (“Leg-
islative revision of law clearly established by judicial opinion
ought to be by express language or by unavoidably implied
contradiction”).
4
The principal dissent is forced to adopt this new, low
standard because it could not otherwise muster any sort of
argument for abrogation. The principal dissent cannot iden-
tify anything in the 1952 Act that does away with the judge-
made doctrine of assignor estoppel “by express language or
by unavoidably implied contradiction.” Ibid. It cites 35
U. S. C. § 282(b), which states that the invalidity of a patent
is a defense “in any action involving the validity or infringe-
ment of a patent.” See post, at 583. But as the majority
notes, the patent laws contained similar language when
Westinghouse was decided. Ante, at 571–572.
The only modifcation made by the 1952 Act that the prin-
cipal dissent claims has any logical connection with assignor
estoppel is the addition of language saying that patents gen-
erally have the attributes of personal property. See post,
at 589. That change has a bearing on whether Westinghouse
4
The principal dissent responds by noting that this passage in Reading
Law sets out the “ `express language' ” or “ `unavoidably implied contradic-
tion' ” standard as part of “an entirely different canon.” Post, at 589, n. 4.
That is precisely the point. The passage discusses the standard that
should be used when an “authoritative judicial holding ” is “cast in doubt
and subjected to challenge” by changes to a statutory scheme. Reading
Law 331. The rule applied by the principal dissent, on the other hand, is
not meant to assess congressional abrogation of our precedents.
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583
Barrett, J., dissenting
should be overruled because it undermined Westinghouse's
ana
logy to estoppel by deed, but it is not enough to show
abrogation because Westinghouse did not rely solely on that
analogy.
In sum, I do not think we can decide the question that
the petition in this case presents unless we decide whether
Westinghouse should be overruled.
5
Because the majority
and the principal dissent refuse to decide whether Westing-
house should be overruled, I would dismiss the writ as im-
providently granted. I therefore respectfully dissent.
Justice Barrett, with whom Justice Thomas and
Justice Gorsuch join, dissenting.
The Patent Act of 1952 sets forth a comprehensive scheme
for the creation and protection of patent rights. But it no-
where mentions the equitable doctrine of assignor estoppel,
which precludes inventors who fle patent applications from
later saying that the patent is invalid. To the contrary,
where the Act does address invalidity defenses, it states that
invalidity “shall” be a defense “in any action involving the
validity or infringement of a patent.” 35 U. S. C. § 282(b).
The text includes no exception for actions in which the inven-
tor is the defendant.
So why the doctrine of assignor estoppel? Because in
Westinghouse Elec. & Mfg. Co. v. Formica Insulation Co.,
266 U. S. 342 (1924), we interpreted a predecessor statute,
the Patent Act of 1870, to incorporate the doctrine. The
question before us is whether the doctrine carried over into
the Patent Act of 1952. That could have happened in one
5
Under similar circumstances in Kimble v. Marvel Entertainment, LLC,
576 U. S. 446 (2015), every Member of this Court assessed a judge-made
patent-law doctrine through the lens of stare decisis, see id., at 455–465;
id., at 470–472 (Alito, J., dissenting), even though “Congress ha[d] repeat-
edly amended . . . the specifc provision . . . on which [our earlier decision
nominally] rested,” id., at 456 (majority opinion). The principal dissent
does not even cite Kimble, let alone make any effort to reconcile its novel
approach with that in our most analogous precedent.

584 MINER
VA SURGICAL, INC. v. HOLOGIC, INC.
Barrett, J., dissenting
of two ways: (1) if Congress ratifed Westinghouse when it
reenac
ted the assignment provision in 1952, or (2) if assignor
estoppel was part of the well-settled common-law backdrop
against which Congress legislated in 1952. The Court opts
for the second theory, but in my view, neither works.
I
I will take the possibility of congressional ratifcation frst
because it follows more naturally from Westinghouse. In
that case, the Court did not present the doctrine of assignor
estoppel as a well-established background principle against
which Congress legislated when it enacted the Patent Act of
1870.
1
Nor could it have: The frst American case to apply
the doctrine was not decided until 1880. See Faulks v.
Kamp, 3 F. 898 (CC SDNY). Instead, the Westinghouse
Court identifed assignor estoppel as a rule that made sense
in light of the Act's assignment provision. After examining
the text of the provision, the Court explained: “[T]here
seems to be no reason why the principles of estoppel by deed
should not apply to assignment of a patent right in accord-
ance with the statute,” because “[i]t was manifestly intended
by Congress to surround the conveyance of patent property
with safeguards resembling those usually attaching to that
of land.” 266 U. S., at 348–349. Some lower courts had ap-
plied the doctrine to the assignment and conveyance of pat-
ents, and, giving that trend its blessing, the Court described
assignor estoppel as a sensible gloss on the assignment pro-
vision of the 1870 Act. Id., at 349–350.
2
1
I do not understand the Court to have a contrary view—its position is
that assignor estoppel had become a well-established background principle
of patent adjudication by the time Congress enacted the Patent Act of
1952. Ante, at 571–572. I address this possibility in Part II, in fra.
2
Justice Alito maintains that Westinghouse did not interpret the stat-
ute because it made no effort to parse the text. Ante, at 580 (dissenting
opinion). But whatever the decision's merits, it plainly grounded assignor
estoppel in the statute's assignment provision. See Westinghouse, 266
U. S., at 348–349.
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585
Barrett, J., dissenting
The question here is whether Congress embraced this
g
loss when it reenacted that provision in 1952. Congress
ratifes a judicial interpretation in a reenacted statute only
if two requirements are satisfed: (1) the interpretation must
be so well settled that we can “presume Congress knew of
and endorsed it” at the time of the reenactment, and (2) the
statute must be reenacted without material change. Jama
v. Immigration and Customs Enforcement, 543 U. S. 335,
349, 351 (2005); Forest Grove School Dist. v. T. A., 557 U. S.
230, 239–240 (2009); see also Lamar, Archer & Cofrin, LLP
v. Appling, 584 U. S. 709, 721–722 (2018) (noting that Con-
gress incorporates a prior judicial interpretation of a statute
when it uses “the materially same language” and the inter-
pretation is “longstanding ”); W. Eskridge, Interpreting Law:
A Primer on How to Read Statutes and the Constitution 421–
422 (2016) (“When Congress reenacts a statute, it incorpo-
rates settled interpretations of the reenacted statute. The
rule is inapplicable when there is no settled standard Con-
gress could have known or the reenactment makes a material
change in the text” (footnote omitted)). So here, respond-
ents must persuade us that (1) as of 1952, Westinghouse's
construction of the assignment provision in the Patent Act
of 1870 was well settled, and (2) the assignment provision in
the Patent Act of 1952 is materially identical to the 1870
version. Jama, 543 U. S., at 349. They cannot clear either
hurdle.
3
3
Justice Alito suggests that the reenactment canon has no application
when this Court, as opposed to lower courts, has interpreted the prior
version of a statute. Ante, at 581–582. He is mistaken. As a leading
treatise explains, the canon has its most obvious application when a “court
of last resort” interprets a statute, though it “applies as well to uniform
holdings of lower courts and even to well-established agency interpreta-
tions.” A. Scalia & B. Garner, Reading Law: The Interpretation of Legal
Texts 323–324 (2012) (footnote omitted); see also C. Nelson, Statutory In-
terpretation 479 (2011) (noting that the canon applies when “[t]he Supreme
Court (or a critical mass of lower courts, or an agency that Congress has
put in charge of administering the statute) adopt[s] a prominent interpre-
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A
W
estinghouse's construction of the assignment provision
in the Patent Act of 1870 was far from well settled in 1952.
Indeed, it is diffcult to describe Westinghouse itself as much
more than a “mild endorsement of assignor estoppel.” Brief
for Petitioner 20. While accepting the doctrine, the Court
simultaneously declined to apply it. Westinghouse, 266
U. S., at 355 (holding that an assignor could use prior art to
narrow the patent claims); cf. Brief for United States as
Amicus Curiae 10 (“This Court has never actually applied
assignor estoppel in a case before it”). Still, if Westinghouse
had been the last word on assignor estoppel, one might argue
that it set forth a statement of a stable principle.
But Westinghouse was not the last word on assignor
estoppel. The next time we considered the doctrine, we
backpedaled. See Scott Paper Co. v. Marcalus Mfg. Co., 326
U. S. 249 (1945). Deeming Westinghouse's analysis a “logi-
cal embarrassment,” we carved out an exception plainly in-
consistent with the general rule of assignor estoppel: that it
does not apply to an assignor who contests a patent's validity
by invoking an expired patent. 326 U. S., at 253, 256–257.
We also cast doubt on the continuing validity of the doctrine,
tation of one of the statute's provisions”); Forest Grove School Dist. v.
T. A., 557 U. S. 230, 239 (2009) (applying the canon to a statute that we had
previously interpreted and that Congress had reenacted without material
change); Shapiro v. United States, 335 U. S. 1, 16, 20 (1948) (same). To be
sure, there is a practical difference between the canon's application to this
Court and lower courts: It takes a “uniform and suffciently numerous”
body of lower court decisions to satisfy the presumption of congressional
notice, whereas a single decision of this Court can be enough. Scalia,
Reading Law, at 325; see, e. g., Forest Grove, 557 U. S., at 239; Manhattan
Properties, Inc. v. Irving Trust Co., 291 U. S. 320, 336 (1934); cf. BP p.l.c.
v. Mayor and City Council of Baltimore, 593 U. S. 230, 244 (2021). But
if a decision of this Court is oblique or thrown into doubt by subsequent
precedent—both of which are true here—we have no basis for presuming
that Congress was on notice of and endorsed our position. See Forest
Grove, 557 U. S., at 239.
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587
Barrett, J., dissenting
expressly declining to address the extent to which assignor
estoppel
“may be deemed to have survived the [Westing-
house] decision or to be restricted by it.” Id., at 254. The
dissent, for its part, claimed that the Court had “repudiated
judicially” assignor estoppel altogether. Id., at 264 (opinion
of Frankfurter, J.); see also Edward Katzinger Co. v. Chicago
Metallic Mfg. Co., 329 U. S. 394, 400 (1947) (describing Scott
Paper as holding that an assignor could “challenge the valid-
ity” of a patent and “defeat an action for infringement”).
So when Congress reenacted the Patent Act in 1952, as-
signor estoppel was far from well settled—if anything, it was
on life support. Respondents could muster only three cases
in the seven years after Scott Paper (before Congress
reenacted the Patent Act) even loosely suggesting support
for the doctrine. See Brief for Respondents 6. Indeed,
rather than embracing assignor estoppel after Scott Paper,
courts questioned the doctrine's validity. See, e. g., Doug-
lass v. United States Appliance Corp., 177 F. 2d 98, 101 (CA9
1949). Scholars did the same; some, like the dissent in Scott
Paper, concluded that we had “wipe[d] out estoppel by as-
signment.” Lechner, Estoppel Against Patent Assignors—
The Scott Paper Company Case, 28 J. Pat. Off. Soc. 325, 330
(1946). Others concluded that the law was a mess. As one
scholar put it: Westinghouse's rule “has become so unsettled
during the past forty years of judicial consideration that,
today, some courts apparently consider the rule to be no
longer valid, others fnd no weakening of the rule, while still
other courts apply the rule only after considerable specula-
tion as to its continued validity.” Cooper, Estoppel To Chal-
lenge Patent Validity: The Case of Private Good Faith vs.
Public Policy, 18 W. Res. L. Rev. 1122, 1123 (1967) (footnotes
omitted; emphasis added).
Post-1952 judicial decisions addressing assignor estoppel
supply yet more evidence that the status of the doctrine was
(at best) uncertain when Congress reenacted the Patent Act.
In Lear, Inc. v. Adkins, 395 U. S. 653 (1969), for example, we

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Barrett, J., dissenting
disavowed the closely related doctrine of licensee estoppel,
noti
ng along the way that the exception articulated in Scott
Paper had “undermined the very basis of the `general rule' ”
of assignor estoppel. 395 U. S., at 666. And following Lear,
several lower courts concluded that assignor estoppel was
not just unsettled but, like licensee estoppel, dead. See,
e. g., Coastal Dynamics Corp. v. Symbolic Displays, Inc., 469
F. 2d 79 (CA9 1972).
Tellingly, respondents could not come up with even one
case applying assignor estoppel in the nearly 20-year period
from Lear until the Federal Circuit resurrected the doctrine
in 1988. See Diamond Scientifc Co. v. Ambico, Inc., 848
F. 2d 1220, 1224–1225. And when the Federal Circuit resur-
rected the doctrine, even it acknowledged that we had left
the vitality of assignor estoppel unsettled: “Although the
Supreme Court has examined th[e] doctrine . . . its opinions
have hardly been defnite or defnitive.” Id., at 1222.
Given all this, it is hard to see how we can “presume Con-
gress knew of and endorsed” the doctrine when it adopted a
new version of the Patent Act in 1952. Jama, 543 U. S., at
349. That is so even if a technical parsing of Scott Paper
fnds that it left assignor estoppel not quite dead, but
ever-so-faintly breathing. In this circumstance, it would be
strange to conclude that the vitality of assignor estoppel was
so “unquestioned,” 543 U. S., at 349, that Congress would
have “regard[ed] the point as settled law,” A. Scalia &
B. Garner, Reading Law: The Interpretation of Legal Texts
325 (2012). On the contrary, the law was anything but clear.
Today's opinion confrms as much. Although the Court
endorses assignor estoppel, it does not apply the doctrine as
Westinghouse described it. Westinghouse stated the “rule”
of assignor estoppel this way: “[A]n assignor of a patent right
is estopped to attack the utility, novelty or validity of a pat-
ented invention which he has assigned or granted as against
any one claiming the right under his assignment or grant.”
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589
Barrett, J., dissenting
much more narrowly: “The doctrine applies only when an
i
nventor says one thing (explicitly or implicitly) in assigning
a patent and the opposite in litigating against the patent's
owner.” Ante, at 566. This version of assignor estoppel
does not appear in Westinghouse—nor, to my knowledge, in
any other judicial decision. The new version might be pref-
erable to the old, but if Congress truly had ratifed Westing-
house, it would have endorsed the Westinghouse version.
B
The reenactment canon has a second requirement: The re-
enacted statute must be materially identical to the one
previously interpreted.
4
This poses another stumbling
block for assignor estoppel, because the assignment provi-
sion of the 1952 Act contains a signifcant sentence that the
1870 Act did not: “Subject to the provisions of this title, pat-
ents shall have the attributes of personal property.”
5
Pat-
4
Justice Alito argues that the “material change” standard requires
“express language” or “unavoidably implied contradiction.” Ante, at 582
(internal quotation marks omitted). He draws this heightened standard
from an entirely different canon—the presumption that a later enacted
statute does not impliedly repeal a former one. See ibid.; Scalia, Reading
Law, at 327–333 (describing the “Presumption Against Implied Repeal”).
As for the canon applicable here—the reenactment canon—our precedents
do not support Justice Alito's proposed standard. See, e. g., Lorillard
v. Pons, 434 U. S. 575, 580 (1978) (“Congress is presumed to be aware of
[a] judicial interpretation of a statute and to adopt that interpretation
when it re-enacts a statute without change”); Forest Grove, 557 U. S., at
239–240 (same); see also Lamar, Archer & Cofrin, LLP v. Appling, 584
U. S. 709, 721 (2018) (Congress is presumed to be aware of a prior judicial
interpretation of a statute when it uses “the materially same language” in
a subsequent statute). The implied repeal canon addresses a different
situation: when a court interprets two different and arguably conficting
statutory provisions. Scalia, Reading Law, at 331.
5
The Patent Act of 1870 provides, in relevant part:
“[E]very patent or any interest therein shall be assignable in law, by an
instrument in writing; and the patentee or his assigns or legal representa-
tives may, in like manner, grant and convey an exclusive right under his
patent to the whole or any specifed part of the United States; and said
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Barrett, J., dissenting
ent Act of 1952, § 261, 66 Stat. 810 (emphasis added). Given
the
reasoning of Westinghouse, this is a material change.
Westinghouse grounded its approval of assignor estoppel
in the idea that patents are like real property. After ob-
serving that the assignment provision required an assign-
ment to be made in writing and recorded in the Patent Offce
within three months, the Court stated that “there seems to
be no reason why the principles of estoppel by deed should
not apply to assignment of a patent right in accordance with
the statute.” 266 U. S., at 348–349. After all, the purpose
of the writing-and-recording requirement is to “furnish writ-
ten and recorded evidence of title and to protect the pur-
chaser of the title as recorded for value without notice.” Id.,
at 349. Thus, the Court concluded, “[i]t was manifestly in-
tended by Congress to surround the conveyance of patent
property with safeguards” that resemble “those usually at-
taching to that of land.” Ibid.
But this analogy was inapt from the start. Even Westing-
house admitted that deeds and patents differed in an impor-
tant respect: “A tract of land is easily determined by survey.
Not so the scope of a patent right for an invention.” Id., at
350. Moreover, unlike the grantor of a deed, who guaran-
tees the quality of title, an assignor of a patent cannot war-
rant a patent's validity. The validity of a patent involves a
factual and legal inquiry “predicated on factors as to which
reasonable men can differ widely.” Lear, 395 U. S., at 670;
see Stanford, Diamond Scientifc Co. v. Ambico, Inc.: En-
forcing Patent Assignor Estoppel, 26 Houston L. Rev. 761,
766 (1989) (“Since validity is never contractually transferred
in an assignment, there is no theoretical basis for assignor
estoppel by deed” (footnote omitted)).
assignment, grant, or conveyance shall be void as against any subsequent
purchaser or mortgagee for a valuable consideration, without notice, un-
less it is recorded in the patent offce within three months from the date
thereof.” § 36, 16 Stat. 203; see Westinghouse, 266 U. S., at 348 (citing to
the revised version of the 1870 Act, “§ 4898”).

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In any event, however persuasive we found the patent-
deed
analogy in Westinghouse, the Patent Act of 1952
unraveled it. When Congress reenacted the assignment
provision, it specifed that patents have “the attributes of
personal property.” 66 Stat. 810 (emphasis added). Be-
cause this language is inconsistent with the premise on which
Westinghouse rested, it undercuts the argument that Con-
gress ratifed Westinghouse in the new assignment provision.
Cf. Holder v. Martinez Gutierrez, 566 U. S. 583, 593 (2012)
(concluding that a reenacted provision did not ratify a prior
judicial construction where the provision lacked the word on
which the earlier construction was based).
Respondents try to brush off the change as insignifcant,
asserting that this “sentence has no bearing on assignor
estoppel anyway, because estoppel by deed, upon which as-
signor estoppel is based, can apply to real or personal prop-
erty.” Brief for Respondents 20 (internal quotation marks
omitted). But whatever the scope of estoppel by deed—
respondents do not elaborate—Westingho use expressly
rested on “[t]he analogy between estoppel in conveyances of
land and estoppel in assignments of a patent right.” 266
U. S., at 350 (emphasis added). In the one case, a “grantor
purports to convey the right to exclude others . . . from a
defned tract of land, and in the other, from a described and
limited feld of the useful arts.” Ibid. By making clear
that patents have “the attributes of personal property,” Con-
gress directly undermined an interpretation that treated
patents like deeds conveying land.
II
If Congress did not ratify our gloss on the assignment pro-
vision in the 1952 Act, how else might assignor estoppel be
part of the statute? The Court comes at the interpretive
problem from a different angle: It holds that by 1952, as-
signor estoppel had become “a background principle of pat-
ent adjudication” against which Congress legislated. Ante,
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at 572. On this theory, Westinghouse is important not for
its
interpretation of the assignment provision but for the eq-
uitable principle that it endorsed. In my view, this theory
also fails because Westinghouse proved to be a false start for
the doctrine.
We have said that “where a common-law principle is well
established, . . . courts may take it as given that Congress
has legislated with an expectation that the principle will
apply” absent statutory cues to the contrary. Astoria Fed.
Sav. & Loan Assn. v. Solimino, 501 U. S. 104, 108 (1991);
see also Eskridge, Interpreting Law, at 348 (“[C]ourts will
assume that legislatures act against the background of the
common law”). So, for example, a federal statute of limita-
tions ordinarily is subject to equitable tolling even when the
text is silent because “Congress must be presumed to draft
limitations periods in light of this background principle.”
Young v. United States, 535 U. S. 43, 49–50 (2002); see also
Nelson, Statutory Interpretation, at 629 (“[C]ourts fre-
quently understand federal statutes to come with some
unstated qualifcations or embellishments suggested by prin-
ciples of general jurisprudence”).
The Court says that assignor estoppel works this way too.
Ante, at 571. True, the Patent Act provides that “[i]nvalid-
ity of the patent shall be a defense[ ] in any action involving ”
infringement. Ibid. (quoting 35 U. S. C. § 282(b); internal
quotation marks and some alterations omitted). Yet, the
Court reasons, this apparently absolute language does not
“foreclose applying in patent cases a whole host of common-
law preclusion doctrines,” including “equitable estoppel, col-
lateral estoppel, res judicata, and law of the case.” Ante, at
571–572. Assignor estoppel, the Court says, falls in that
same category. Id., at 572. According to the Court, Wes-
tinghouse confrmed a trend that had already begun in the
lower courts and has continued unabated since, giving as-
signor estoppel a place in the pantheon of well-established
common-law principles.
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593
Barrett, J., dissenting
I disagree. The common-law pedigree of assignor estop-
pel
differs markedly from that of the preclusion doctrines
with which the Court groups it. Some of those doctrines
have been around for nearly a thousand years, see, e. g., Bur-
sak, Note, Preclusions, 91 N. Y. U. L. Rev. 1651, 1663 (2016)
(“Res judicata had migrated to England no later than the
early 1100s”); Millar, The Historical Relation of Estoppel by
Record to Res Judicata, 35 Ill. L. Rev. 41, 44–45 (1940) (res
judicata and collateral estoppel go back to at least the 1200s),
and were far more settled in early American courts, see, e. g.,
Washington, Alexandria, & Georgetown Steam-Packet Co.
v. Sickles, 24 How. 333, 341 (1861) (“The authority of the res
judicata . . . is derived by us from the Roman law and the
Canonists”); Hopkins v. Lee, 6 Wheat. 109, 114 (1821) (stating
that collateral estoppel has “found its way into every system
of jurisprudence”). They are frmly rooted in our jurispru-
dence now.
Assignor estoppel, by contrast, has far from this kind of
“impeccable historic pedigree.” Kirtsaeng v. John Wiley &
Sons, Inc., 568 U. S. 519, 538 (2013). It is more recent and
far shakier. It was introduced into patent law in the late
19th century—about a hundred years after Congress enacted
the frst patent laws and a decade after Congress passed the
1870 Act. See Faulks v. Kamp, 3 F. 898 (CC SDNY 1880);
ante, at 568 (opinion of the Court). And after its introduc-
tion, it lacked staying power. Westinghouse proved to be
the “high-water mark of the doctrine in this Court.” Brief
for Petitioner 20. As I have already explained, lower
courts, commentators, the Scott Paper dissent, and even the
Court itself in Lear regarded Scott Paper as having gutted
the doctrine of assignor estoppel. See supra, at 586–588.
It is therefore diffcult for me to see how, in 1952, assignor
estoppel constituted a “long-established and familiar princi-
pl[e]” like res judicata. Isbrandtsen Co. v. Johnson, 343
U. S. 779, 783 (1952). At most, the doctrine of assignor
estoppel was in a confused state by 1952—a far cry from the
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high bar that we have required to incorporate “well-settled”
common-law
background principles into a statute. See, e. g.,
Kirtsaeng, 568 U. S., at 538 (noting the frst-sale doctrine's
“impeccable historic pedigree”); Lozano v. Montoya Alvarez,
572 U. S. 1, 10 (2014) (emphasizing that equitable tolling is a
“long-established feature of American jurisprudence”). In-
deed, whatever one might have said when the Court decided
Westinghouse in 1924, Scott Paper cut sharply in the other
direction. And such “contradictory signals are not typically
the stuff of which background rules of common law are
made.” B&B Hardware, Inc. v. Hargis Industries, Inc., 575
U. S. 138, 163 (2015) (Thomas, J., dissenting).
III
Respondents insist that assignor estoppel promotes fair
dealing, while petitioner protests that the supposedly equita-
ble doctrine is actually inequitable in practice. If we had
authority to develop federal common law on the subject, we
could take sides in that debate. But no one contends that
we do. This case turns on whether the Patent Act of 1952
incorporates the doctrine, and because it does not, I respect-
fully dissent.
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