(Slip Opinion)

OCTOBER TERM, 2020

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Syllabus
NOTE: Where it is feasible, a syllabus (headnote) will be released, as is
being done in connection with this case, at the time the opinion is issued.
The syllabus constitutes no part of the opinion of the Court but has been
prepared by the Reporter of Decisions for the convenience of the reader.
See United States v. Detroit Timber & Lumber Co., 200 U. S. 321, 337.

SUPREME COURT OF THE UNITED STATES
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 amoisture-permeable applicator head to destroy targeted cells in theuterine lining. Truckai filed a patent application and later assigned
the application, along with any future continuation applications, to his
company, Novacept, Inc. The PTO issued a patent for the device. Novacept, along with its portfolio of patents and patent applications, waseventually 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 Minerva Endometrial Ablation System, the new device uses amoisture-impermeable applicator head to remove cells in the uterine
lining. The PTO issued a patent, and the FDA approved the device forcommercial sale. Meanwhile, Hologic filed 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 applicatorheads generally, 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 response, 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 affirmed in relevant part. Minerva now asks this Court to abandon or narrow assignor

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
Syllabus
estoppel.

Held: Assignor estoppel is well grounded in centuries-old fairness principles, 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. 5–17.
(a) Courts have long applied the doctrine of assignor estoppel to deal
with inconsistent representations about a patent’s validity. The doctrine got its start in late 18th-century England and crossed the Atlantic about a hundred years later. This Court first 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,” Westinghouse 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 invalidity 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. 5–9.
(b) The Court rejects Minerva’s contention that assignor estoppel
should be abandoned. Minerva’s first argument on that score—that
Congress abrogated the doctrine in the Patent Act of 1952—is unpersuasive. Minerva relies on statutory language providing 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 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 conflict withthis Court’s precedents. See, e.g., SCA Hygiene Products Aktiebolag v.
First Quality Baby Products, LLC, 580 U. S. ___, ___. 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-Westinghousedecisions have already interred assignor estoppel. In Scott Paper Co.
v. Marcalus Mfg. Co., 326 U. S. 249, the Court did nothing more thandecline to apply assignor estoppel in a novel and extreme circumstance. The Court did not question—indeed, it restated—the “basic

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Syllabus
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 patent 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 estoppel, but suggested that the doctrine needed to stay attached to its equitable moorings.
Finally, the Court rejects Minerva’s claim that contemporary patent
policy—specifically, the need to weed out bad patents—supports overthrowing assignor estoppel. Assignor estoppel reflects a demand forconsistency in dealing with others. When a person sells his patentrights, he makes an (at least) implicit representation to the buyer thatthe patent at issue is valid. In later raising an invalidity defense, theassignor disavows that implied warranty. By saying one thing andthen saying another, the assignor wants to profit 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. 9–14.
(c) Assignor estoppel comes with limits: it applies only when its underlying principle of fair dealing comes into play. That principle demands consistency in representations about a patent’s validity. Whenan assignor warrants that a patent claim is valid, his later denial ofvalidity breaches norms of equitable dealing. But when the assignor
has made neither explicit nor implicit representations in conflict withan invalidity defense, then there is no unfairness in its assertion—andso there is no ground for applying assignor estoppel. One example ofnon-contradiction is when an assignment occurs before an inventor canpossibly make a warranty of validity as to specific patent claims. Thatsituation 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 alater legal development renders irrelevant the warranty given at thetime of assignment. Third, and most relevant here, a post-assignmentchange 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, theassignee may return 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 hemade no such representation, then he can challenge the new claims in

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
Syllabus
litigation: Because there is no inconsistency in his positions, there isno estoppel.
The Federal Circuit failed to recognize these boundaries. Minerva
argued that estoppel should not apply because it was challenging aclaim 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 making the assignment. And without such a prior inconsistent representation, there is no basis for estoppel. The judgment of the Federal
Circuit is therefore vacated, and the case is remanded for the Court ofAppeals to address whether Hologic’s new claim is materially broader
than the ones Truckai assigned. Pp. 14–17.

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., filed a
dissenting opinion. BARRETT, J., filed a dissenting opinion, in which
THOMAS and GORSUCH, JJ., joined.

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Opinion of the Court
NOTICE: This opinion is subject to formal revision before publication in the
preliminary print of the United States Reports. Readers are requested to
notify the Reporter of Decisions, Supreme Court of the United States, Washington, D. C. 20543, of any typographical or other formal errors, in order that
corrections may be made before the preliminary print goes to press.

SUPREME COURT OF THE UNITED STATES_________________

No. 20–440_________________MINERVA SURGICAL, INC., PETITIONER v.
HOLOGIC, INC., ET AL.
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF
APPEALS FOR THE FEDERAL CIRCUIT[June 29, 2021]

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 “wellsettled” 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
contend 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 contradicts explicit or implicit representations he made in assigning the patent.
I
Inventors look to the patent system to obtain valuable
rights. A typical patent application, filed with the U. S. Patent and Trademark Office (PTO), includes a written description and drawing of the invention and one or moreclaims particularly setting out the invention’s scope. See

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Opinion of the Court

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, andnon-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 expires (currently, 20 years after the application date). See §154.
The invention sparking this lawsuit is a device to treat
abnormal uterine bleeding, a medical condition affecting
many millions of women. Csaba Truckai, a founder of thecompany Novacept, Inc., invented the device—called theNovaSure System—in the late 1990s. He soon afterward
filed a patent application, and assigned his interest in theapplication—as well as in any future “continuation applications”—to Novacept. 1 The NovaSure System, as describedin Truckai’s patent application, uses an applicator head todestroy targeted cells in the uterine lining. To avoid unintended burning or ablation (tissue removal), the head is“moisture permeable,” meaning that it conducts fluid 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 benefits from theNovaSure System patent. In 2004, Novacept sold its assets,
including its portfolio of patents and patent applications, to——————
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

another company (netting Truckai individually about $8
million). And in another sale, in 2007, respondent Hologic,
Inc. acquired all patent rights in the NovaSure System. Today, Hologic sells that device throughout the United States.
Not through with inventing, Truckai founded in 2008 petitioner Minerva Surgical, Inc. There, he developed a supposedly improved device to treat abnormal uterine bleeding. Called the Minerva Endometrial Ablation System, thedevice (like the NovaSure System) uses an applicator headto remove cells in the uterine lining. But the new device,relying on a different way to avoid unwanted ablation, is“moisture impermeable”: It does not remove any fluid during treatment. The PTO issued a patent for the device, andin 2015 the FDA approved it for commercial sale.
Meanwhile, in 2013, Hologic filed a continuation application requesting to add claims to its patent for the NovaSure
System. Aware of Truckai’s activities, Hologic drafted oneof those claims to encompass applicator heads generally,
without 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 infringe. But more relevant here, it also asserted that Hologic’s amended patent is invalid. The essential problem,
according to Minerva, is that the new, broad claim about
applicator heads does not match the invention’s description,
which addresses their water-permeability. See DefendantMinerva’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 isclaimed by the patent application must be the same” aswhat is described). In response, Hologic invoked the doctrine of assignor estoppel. Because Truckai assigned theoriginal patent application, Hologic argued, he and Minerva(essentially, his alter-ego) could not impeach the patent’s

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court

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. 3d507, 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 upheld the judgment, focusing on assignor estoppel. The court
first “decline[d] Minerva’s invitation to ‘abandon [that] doctrine.’ ” 957 F. 3d 1256, 1267 (2020). Citing both thisCourt’s precedents and equitable principles, the court affirmed the doctrine’s “continued vitality.” Id., at 1268. An
assignor, the court stated, “should not be permitted to sellsomething and later to assert that what was sold is worthless, 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 worthless.”
Ibid. It would “work an injustice,” the court reasoned, to“allow the assignor to make that representation at the timeof assignment (to his advantage) and later to repudiate it(again to his advantage).” Ibid. (quoting Diamond Scientific 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 assignment,” it would “be unfair to block Truckai (or Minerva)
from challenging the breadth of those claims.” 957 F. 3d, at1268. 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 Scientific, 848
F. 2d, at 1226).
We granted certiorari, 592 U. S. ___ (2021), to consider
the important issues raised in the Federal Circuit’s judgment. Assignor estoppel, we now hold, is well grounded inCite as: 594 U. S. ____ (2021)

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Opinion of the Court

centuries-old fairness principles, and the Federal Circuit
was right to uphold it. But the court failed to recognize thedoctrine’s proper limits. The equitable basis of assignor estoppel defines 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 fromthe one here) begins with an inventor who both applies forand 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 infringement, 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 wasnever valid. That kind of about-face is what assignor estoppel operates to prevent—or, in legalese, estop. As one of theearly American courts to use the doctrine held: The assignor is not “at liberty to urge [invalidity] in a suit uponhis 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 representation” that the patent he is assigning is “not worthless . . . deprive[s] him of the ability to challenge later the[patent’s] validity.” Diamond Scientific, 848 F. 2d, at 1224.
Assignor estoppel got its start in late 18th-century England and crossed the Atlantic about a hundred years later.
In the first recorded case, Lord Kenyon found that a patentassignor “was by his own oath and deed estopped” in an infringement suit from “attempt[ing] to deny his having had

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court

any title to convey.” Oldham v. Langmead (1789), as described 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, 100Eng. 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 toprevent a conveyor of land from later asserting that he hadlacked good title at the time of sale. See 2 E. Coke, The
First Part of the Institutes 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 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 thatthe patent is not good.” Chambers v. Crichley, 33 Beav. 374,
376, 55 Eng. Rep. 412 (1864); see Walton v. Lavater, 8 C. B.
N. S. 162, 187, 141 Eng. Rep. 1127, 1137 (C. P. 1860) (“Thedefendant, who has received a large sum for the sale of thispatent, ought not to be allowed to raise any question as toits validity”). The earliest American decision applying thedoctrine dates from 1880. See Faulks v. Kamp, 3 F. 898 (CCSDNY). Within a decade or two, the doctrine was “so wellestablished and generally accepted that citation of authority is useless.” Griffith v. Shaw, 89 F. 313, 315 (CC SD Iowa
1893); see 2 W. Robinson, Law of Patents for Useful Inventions §787 (1890) (collecting cases).
This Court first considered—and unanimously approved—assignor estoppel in 1924, in Westinghouse v. Formica. Speaking through Chief Justice Taft, the Court initially invoked the doctrine’s uniform acceptance in thelower courts. The first decision applying assignor estoppel,
the Court recounted, was soon “followed by a myriad.” 266
U. S., at 349. “[L]ater cases in nearly all the Circuit Courtsof Appeal” were “to the same point” as the first, adding up

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Opinion of the Court

to a full “forty-five years of judicial consideration and conclusion.” Ibid. Such a “well settled” rule, in the Court’sview, should “not [be] lightly disturb[ed].” Ibid. And so it
was not disturbed, 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 efficacy”; but “the assignor can not beheard to question” the assignee’s rights in what was conveyed. Ibid.
Westinghouse, like its precursor decisions, grounded assignor estoppel in a principle of fairness. “If one lawfully
conveys to another a patented right,” the Court reasoned,“fair dealing should prevent him from derogating from thetitle he has assigned.” Id., at 350. After all, the “grantor
purports to convey the right to exclude others”; how can helater say, given that representation, that the grantee in factpossesses no such right? Ibid. The Court supported thatview of equity by referring to estoppel by deed. See supra,
at 6. Under that doctrine, the Court explained, “a grantorof 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 deedshould not apply to [the] assignment of a patent right.” Id.,
at 348, 350. In the latter context too, the Court held, theassignor could not fairly “attack” the validity of a right hehad formerly sold. Id., at 349.
After thus endorsing assignor estoppel, the Court madeclear 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 therole in patent suits of prior art—the set of earlier inventions

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court(and other information) used to decide whether the specified invention is novel and non-obvious enough to merit apatent. Id., at 350. “Of course,” the Court said, the assignor
cannot use prior art in an infringement suit “to destroy thepatent,” 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.” Id., at 350–351. “Otherwise,” theCourt explained, a judge “would be denied” the “most satisfactory means” of “reaching a just conclusion” about the patent’s scope—a conclusion needed to resolve the infringement charge. Id., at 350–351. “The distinction” thus
established, the Court thought, “may be a nice one, but
seems to be workable.” Id., at 351. And, indeed, the Court
applied it to decide the case at hand for the assignor, findingthat he had not infringed the properly narrowed claim. 2
Finally, the Court left for another day several other questions about the contours of assignor estoppel. One concerned privity: When was an assignor so closely affiliatedwith 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 thatmost 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 thatcase. The Court saw a possible distinction between the two.
——————
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 ambiguous,” after consideration of text and canons, will a court think aboutnarrowing 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 affirming the assignor
estoppel doctrine, the Court made clear that it did not always bar assignors from effectively defending against infringement suits.

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In a patent application, the Court began, the inventor“swor[e] to” a particular “specification.” Id., at 352. But theexact rights at issue were at that point “inchoate”—not “certainly defined.” Ibid. And afterward, the Court (presciently) 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 estoppel. 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 has been already. Brief for Petitioner 17. We rejectthat view. The doctrine has lasted for many years, and wecontinue to accept the fairness principle at its core. Minerva’s back-up contention is that assignor estoppel “should
be constrained.” Id., at 41. On that score, we find that theFederal Circuit has applied the doctrine too expansively.
Today, we clarify the scope of assignor estoppel, includingin the way Westinghouse suggested.
A
In its quest to abolish assignor estoppel, Minerva lodges
three main arguments. The first 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 forthe 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 challenges” and so keeps bad patents alive. Id., at 38. (Theprincipal dissent essentially endorses the first two arguments, but not the third. See post, at 1, 4–6, 11 (opinion of
BARRETT, J.).)

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On the first point, we do not agree that the Patent Act of1952 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] noroom for assignor 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 foreclose applying in patent cases a whole
host of common-law preclusion doctrines—not just assignor
estoppel, but equitable estoppel, collateral estoppel, res judicata, and law of the case. That broad result would conflictwith this Court’s precedents. See, e.g., SCA Hygiene Products Aktiebolag v. First Quality Baby Products, LLC, 580
U. S. ___, ___ (2017) (slip op., at 16) (recognizing equitableestoppel in a patent suit). And it would subvert congressional design. For Congress “legislate[s] against a background of common-law adjudicatory principles,” and it “expect[s]” those principles to “apply except 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 estoppel 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 sincethat time.
We likewise do not accept Minerva’s view that two of ourpost-Westinghouse decisions have already interred assignor
estoppel. According to Minerva (quoting the case’s dissent),

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Scott Paper Co. v. Marcalus Mfg. Co. “eliminated any justification for assignor estoppel and ‘repudiated’ the doctrine.”
Reply Brief 13 (quoting 326 U. S. 249, 264 (1945) (Frankfurter, J., dissenting)). And if that were not enough, Minerva 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 police 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 anovel and extreme circumstance. The petitioner in Scott
Paper made the same ask Minerva does here: to abolish theWestinghouse rule. The Court expressly declined that request. See 326 U. S., at 254. And it restated the “basic
principle” animating assignor estoppel, describing it as “oneof good faith, that one who has sold his invention may not,to the detriment of the purchaser, deny the existence of thatwhich he has sold.” Id., at 251. The Court, to be sure, declined to apply the doctrine in the case before it. There,
estoppel would have prevented the assignor from making adevice on which the patent had expired—a device, in otherwords, that had already entered the public domain. The
Court could not find any precedent for applying estoppel inthat situation. See id., at 254. And the Court thought thatdoing so would carry the doctrine too far, reasoning that the
public’s interest in using an already-public invention outweighs the “interest in private good faith.” Id., at 256–257.
But the Court did not question—again, it reaffirmed—theprinciple of fairness on which assignor estoppel rests inmore common cases, where the assignee is not claiming tocontrol 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,

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court

the Court considered and toppled a different patent estoppel doctrine. Called licensee estoppel, it barred (as its namesuggests) a patent licensee from contesting the validity ofthe patent on a device he was paying to use. Minerva’s
basic claim is that as goes one patent estoppel rule, so goesanother. Brief for Petitioner 23–25. But Lear did not purport to decide the fate of the separate assignor estoppel doctrine. To the contrary, the Court stated that the patent
holder’s “equities” in the assignment context “were far morecompelling than those presented in the typical licensing arrangement.” 395 U. S., at 664. And so they are. As explained earlier, assignor estoppel rests on the idea that theassignor has made an explicit or implicit representation
about the patent’s validity, and received some kind of payment in return. See supra, at 5–6. No rationale of that kind
supports licensee estoppel. The licensee is a buyer of patentrights, not a seller. He has given no assurances of the patent’s worth. All he has done is purchase the right to use apatented device—which, if the patent is invalid, he need nothave done. Lear’s refusal to bar a licensee’s claim of invalidity showed that the Court was alert to “the important
public interest in permitting full and free competition in theuse of ideas”—and so would not always apply patent estoppel doctrines. 395 U. S., at 670. But that does not mean, atthe other extreme, always rejecting those doctrines. Learcounseled careful attention to the equities at stake in discrete patent contexts—and expressly distinguished assignor from licensee estoppel.
In sum, Scott Paper and Lear left Westinghouse rightabout where they found it—as a bounded doctrine designed
to prevent an inventor from first selling a patent and thencontending that the thing sold is worthless. Westinghouse
saw that about-face as unfair; Scott Paper and Lear neverquestioned that view. At the same time, Westinghouse realized that assignor estoppel has limits: Even in approving
the doctrine, the Court made clear that not every assignor

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Opinion of the Court

defense in every case would fall within its scope. See supra,
at 7–8. Scott Paper and Lear adopted a similar stance.
They maintained assignor estoppel, but suggested (if in different 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 ofcommon-law rules.
Finally, we do not think, as Minerva claims, that contemporary patent policy—specifically, the need to weed out badpatents—supports overthrowing assignor estoppel. In rejecting 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 assignor estoppel justified on the fairness grounds that courts
applying the doctrine have always given. Assignor estoppel, like many estoppel rules, reflects a demand for consistency in dealing with others. See H. Herman, The Lawof 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
patent rights, he makes an (at least) implicit representation
to the buyer that the patent at issue is valid—that it will
actually give the buyer his sought-for monopoly. 3 In later——————
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—astatement attesting that he is “the original inventor” of the “claimed invention.” §115(b)(2); see supra, at 2. 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 presenting 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 theinventor then assigns those claims to another, he effectively incorporates
that assurance.

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court

raising 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) andlater to repudiate it (again to his advantage).” Diamond
Scientific, 848 F. 2d, at 1224; see supra, at 4. By saying one
thing and then saying another, the assignor wants to profit
doubly—by gaining both the price of assigning the patentand 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 outweigh any loss to the public from leaving an invalidity defense to someone other than the assignor. 4
B
Still, our endorsement of assignor estoppel comes withlimits—true to the doctrine’s reason for being. Just as we
guarded the doctrine’s boundaries in the past, see supra, at
7–8, 11–13, so too we do so today. Assignor estoppel should
apply only when its underlying principle of fair dealingcomes into play. That principle, as explained above, demands consistency in representations about a patent’s validity: What creates the unfairness is contradiction. Whenan assignor warrants that a patent is valid, his later denialof validity breaches norms of equitable dealing. And theoriginal warranty need not be express; as we have explained, the assignment of specific patent claims carries
with it an implied assurance. See supra, at 13. But when——————
4 Even beyond promoting fairness, assignor estoppel furthers some patent policy goals. Assignors are especially likely infringers because of
their knowledge of the relevant technology. By preventing them fromraising an invalidity defense in an infringement suit, the doctrine gives
assignees confidence 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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Opinion of the Court

the assignor has made neither explicit nor implicit representations in conflict with an invalidity defense, then there
is no unfairness in its assertion. And so there is no groundfor applying assignor estoppel.
One example of non-contradiction is when the assignment occurs before an inventor can possibly make a warranty of validity as to specific patent claims. Consider acommon employment arrangement. An employee assigns
to his employer patent rights in any future inventions hedevelops 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 notcome into being. See Lemley, Rethinking Assignor Estoppel, 54 Houston L. Rev. 513, 525–527 (2016). And so theemployee’s transfer of rights cannot estop him from alleging
a patent’s invalidity in later litigation.
A second example is when a later legal development renders irrelevant the warranty given at the time of assignment. Suppose an inventor conveys a patent for value, withthe 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 fromsaying so.
Most relevant here, another post-assignment development—a change in patent claims—can remove the rationale for applying assignor estoppel. Westinghouse itself
anticipated 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
defined than that of a granted patent.” 266 U. S., at 352–
353; see supra, at 9. That is because the assignee, once he

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
Opinion of the Court

is the owner of the application, may return to the PTO to“enlarge[ ]” 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 liberally dropped hints—and the equitable basis for
assignor estoppel—point all in one direction. Assumingthat the new claims are materially broader than the oldclaims, the assignor did not warrant to the new claims’ validity. And if he made no such representation, then he canchallenge the new claims in litigation: Because there is noinconsistency in his positions, there is no estoppel. The limits of the assignor’s estoppel go only so far as, and not beyond, what he represented in assigning the patent application.
The Federal Circuit, in both its opinion below and priordecisions, has failed to recognize those boundaries. Minerva (recall, Truckai’s alter-ego) argued to the court thatestoppel should not apply because it was challenging aclaim 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 Hologic had expanded the assigned
claims: Even if so, Minerva could not contest the newclaim’s validity. 957 F. 3d, at 1268 (quoting Diamond Scientific, 848 F. 2d, at 1226); see supra, at 4. 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 warranted 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

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Opinion of the Court

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 conflict with his later invalidity
defense—and so will determine whether assignor estoppel
applies.
IV
This Court recognized assignor estoppel a century ago,
and we reaffirm that judgment today. But as the Court recognized from the beginning, the doctrine is not limitless. Its
boundaries reflect its equitable basis: to prevent an assignor from warranting one thing and later alleging another. Assignor estoppel applies when an invalidity defense
in an infringement suit conflicts with an explicit or implicit
representation made in assigning patent rights. But absent
that kind of inconsistency, an invalidity defense raises noconcern of fair dealing—so assignor estoppel has no place.
For these reasons, we vacate the judgment of the Federal
Circuit and remand the case for further proceedings consistent with this opinion.
It is so ordered.

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1

ALITO, J., dissenting

SUPREME COURT OF THE UNITED STATES_________________

No. 20–440_________________MINERVA SURGICAL, INC., PETITIONER v.
HOLOGIC, INC., ET AL.
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF
APPEALS FOR THE FEDERAL CIRCUIT[June 29, 2021]

JUSTICE ALITO, dissenting.
We granted review in this case to decide whether the doctrine 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 assignor estoppel, should be overruled. Both the majority andthe principal dissent go to great lengths to avoid that question, but in my judgment, their efforts are unsuccessful.
The majority says it has no need to invoke precedent, seeante, at 13, 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) (slip op., at 3), but thatis just what the majority has done in this case.
With so little support for its reasoning, it is more than alittle surprising that the majority forswears reliance on
precedent. See ante, at 13. Not too long ago, in Kimble v.
Marvel Entertainment, LLC, 576 U. S. 446 (2015), anothercase involving a judicially created rule of patent law, theCourt applied a “superpowered form of stare decisis.” Id.,

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
ALITO, J., dissenting

at 458. Yet the majority refuses to wield the nearly impregnable Kimble shield. Instead, it adopts a text-blind methodof 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 addressstare decisis because, in its view, Westinghouse has not beena precedent for the past 69 years. According to the principal
dissent, Westinghouse was abrogated by the Patent Act of1952. It reasons as follows: Westinghouse interpreted the
Patent Act of 1870, post, at 1 (opinion of BARRETT, J.); because that Act was superseded by the Patent Act of 1952,
we must decide whether the new Act “ratified” Westinghouse, post, at 1; and in order to show that Westinghouse
was ratified, the defenders of assignor estoppel must persuade us that “(1) the interpretation [adopted in Westinghouse 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 2–3 (quoting Jama v. Immigration and Customs
Enforcement, 543 U. S. 335, 349 (2005)).
This reasoning is unprecedented and troubling. To start,
it is quite misleading to suggest that Westinghouse wasbased on an interpretation of the 1870 Patent Act. Neitherof the two statutory provisions Westinghouse mentioned
said anything that supported the Court’s decision, 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 importantly, a body of lower court case law. 266 U. S., at 348–
352. If Westinghouse had been based on an interpretation——————
1 One provision permitted the assignment of patent rights, see Westinghouse, 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 scopeof the doctrine of assignor estoppel, not its existence, see id., at 352.

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3

ALITO, J., dissentingof language in the 1870 Act and if the 1952 Act had changed
that language, there might be a basis for finding abrogation. 2 But that is not the situation here.
After starting with this flawed premise, the principal dissent 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 fora different purpose, i.e., determining whether congressional
reenactment of a statute should be understood as a ratification of a preexisting body of lower court case law. 3 That
was the issue in Jama, 543 U. S., at 349–352, the case theprincipal 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 arule announced in a decision of this Court can cease to be aprecedent 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 interpreting a statutory provision that it reenacts. Perhaps it is hubris, but we have often presumed that Congress is aware ofour decisions. See, e.g., Ryan v. Valencia Gonzales, 568——————
2 By the same token, if Congress had again used that particular language without change, there might be a basis for finding ratification. 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”).
3 This is not to say that Congress cannot pass statutes meant to incorporate this Court’s interpretations of specific statutory language. Compare post, at 3, n. 3. It obviously can. What is unusual is not the ideathat Congress can ratify this Court’s decisions but instead the application of a test designed to assess the ratification of lower court decisions
to assess the abrogation of a decision of this Court.

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
ALITO, J., dissenting

U. S. 57, 66 (2013) (“We normally assume that, when Congress enacts statutes, it is aware of relevant judicial precedent” (internal quotation marks omitted)).
When we reach the final 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 consequential facts.” Black’s Law Dictionary 1170 (11th ed.
2019). Thus, the principal dissent can be read to say that adecision of this Court interpreting a statutory provision isabrogated whenever a change in the language of the statuteprovides some degree of support for a different interpretation. The principal dissent cites no case in which we have
set the abrogation standard so low. And this new “materialchange” standard for abrogating one of our precedents is amarked departure from the standard advocated in one ofthe principal dissent’s chosen treatises. See Reading Law
331 (“Legislative 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 ofargument for abrogation. The principal dissent cannot
identify anything in the 1952 Act that does away with thejudge-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——————
4 The principal dissent responds by noting that this passage in Reading
Law sets out the “ ‘express language’ ” or “ ‘unavoidably implied contradiction’ ” standard as part of “an entirely different canon.” Post, at 7, n. 4.
That is precisely the point. The passage discusses the standard thatshould 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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ALITO, J., dissenting

a patent is a defense “in any action involving the validity or
infringement of a patent.” See post, at 1. But as the majority notes, the patent laws contained similar language when
Westinghouse was decided. Ante, at 10.
The only modification made by the 1952 Act that the principal dissent claims has any logical connection with assignor estoppel is the addition of language saying that patents generally have the attributes of personal property.
See post, at 7. That change has a bearing on whether Westinghouse should be overruled because it undermined Westinghouse’s analogy to estoppel by deed, but it is not enoughto show abrogation because Westinghouse did not rely solely
on that analogy.
In sum, I do not think we can decide the question that thepetition in this case presents unless we decide whether
Westinghouse should be overruled. 5 Because the majority
and the principal dissent refuse to decide whether Westinghouse should be overruled, I would dismiss the writ as improvidently granted. I therefore respectfully dissent.

——————
5 Under similar circumstances in Kimble v. Marvel Entertainment,
LLC, 576 U. S. 446 (2015), every Member of this Court assessed a judgemade patent-law doctrine through the lens of stare decisis, see id., at455–465; id., at 470–472 (ALITO, J., dissenting), even though “Congressha[d] repeatedly amended . . . the specific provision . . . on which [ourearlier decision nominally] rested,” id., at 456 (majority opinion). The
principal dissent does not even cite Kimble, let alone make any effort toreconcile its novel approach with that in our most analogous precedent.

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1

BARRETT, J., dissenting

SUPREME COURT OF THE UNITED STATES_________________

No. 20–440_________________MINERVA SURGICAL, INC., PETITIONER v.
HOLOGIC, INC., ET AL.
ON WRIT OF CERTIORARI TO THE UNITED STATES COURT OF
APPEALS FOR THE FEDERAL CIRCUIT[June 29, 2021]

JUSTICE BARRETT, with whom JUSTICE THOMAS andJUSTICE GORSUCH join, dissenting.
The Patent Act of 1952 sets forth a comprehensive
scheme for the creation and protection of patent rights. But
it nowhere mentions the equitable doctrine of assignor estoppel, which precludes inventors who file patent applications from later saying that the patent is invalid. To thecontrary, 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 inventor is the defendant.
So why the doctrine of assignor estoppel? Because inWestinghouse 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 of
two ways: (1) if Congress ratified Westinghouse when it
reenacted 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.

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
BARRETT, J., dissentingI
I will take the possibility of congressional ratification
first 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 first Americancase to apply the doctrine was not decided until 1880. See
Faulks v. Kamp, 3 F. 898 (CC SDNY). Instead, the Westinghouse Court identified assignor estoppel as a rule thatmade 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 rightin accordance with the statute,” because “[i]t was manifestly intended by Congress to surround the conveyance ofpatent property with safeguards resembling those usuallyattaching to that of land.” 266 U. S., at 348–349. Somelower courts had applied the doctrine to the assignment andconveyance of patents, and, giving that trend its blessing,
the Court described assignor estoppel as a sensible gloss on
the assignment provision of the 1870 Act. Id., at 349–350. 2
The question here is whether Congress embraced thisgloss when it reenacted that provision in 1952. Congress
ratifies a judicial interpretation in a reenacted statute only
if two requirements are satisfied: (1) the interpretationmust be so well settled that we can “presume Congress——————
1 I do not understand the Court to have a contrary view—its position isthat assignor estoppel had become a well-established background principle of patent adjudication by the time Congress enacted the Patent Act
of 1952. Ante, at 10. I address this possibility in Part II, infra.
2 JUSTICE ALITO maintains that Westinghouse did not interpret thestatute because it made no effort to parse the text. Ante, at 2–3 (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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BARRETT, J., dissenting

knew of and endorsed it” at the time of the reenactment,and (2) the statute must be reenacted without materialchange. 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. ___, ___ (2018) (slip op.,at 11) (noting that Congress incorporates a prior judicial interpretation of a statute when it uses “the materially samelanguage” and the interpretation 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 incorporates settled interpretations of
the reenacted statute. The rule is inapplicable when there
is no settled standard Congress could have known or the reenactment makes a material change in the text” (footnote
omitted)). So here, respondents 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 theprior version of a statute. Ante, at 3–4. 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 interpretations.” A. Scalia & B. Garner, Reading Law: The Interpretationof Legal Texts 323–324 (2012) (footnote omitted); see also C. Nelson,
Statutory Interpretation 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] aprominent interpretation of one of the statute’s provisions”); Forest Grove
School Dist. v. T. A., 557 U. S. 230, 239 (2009) (applying the canon to astatute 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

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
BARRETT, J., dissenting

A
Westinghouse’s construction of the assignment provisionin the Patent Act of 1870 was far from well settled in 1952.
Indeed, it is difficult to describe Westinghouse itself as
much more than a “mild endorsement of assignor estoppel.”
Brief for Petitioner 20. While accepting the doctrine, theCourt 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 Statesas 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 “logical embarrassment,” we carved out an exception plainly inconsistent 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 thedoctrine, expressly declining to address the extent to whichassignor estoppel “may be deemed to have survived the[Westinghouse] decision or to be restricted by it.” Id., at 254.
——————
the canon’s application to this Court and lower courts: It takes a “uniform
and sufficiently numerous” body of lower court decisions to satisfy thepresumption of congressional notice, whereas a single decision of thisCourt 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. ___, ___ (2021) (slip op., at 11). 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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BARRETT, J., dissenting

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 validity” of a patent and “defeat an action for infringement”).
So when Congress reenacted the Patent Act in 1952, assignor estoppel was far from well settled—if anything, it
was on life support. Respondents could muster only threecases 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.,
Douglass 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 assignment.” 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 therule to be no longer valid, others find no weakening of therule, while still other courts apply the rule only after considerable speculation as to its continued validity.” Cooper,
Estoppel To Challenge 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 PatentAct. In Lear, Inc. v. Adkins, 395 U. S. 653 (1969), for example, we disavowed the closely related doctrine of licensee estoppel, noting along the way that the exception articulated

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MINERVA SURGICAL, INC. v. HOLOGIC, INC.
BARRETT, J., dissenting

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 Scientific Co. v. Ambico, Inc., 848
F. 2d 1220, 1224–1225. And when the Federal Circuit resurrected the doctrine, even it acknowledged that we had leftthe vitality of assignor estoppel unsettled: “Although theSupreme Court has examined th[e] doctrine . . . its opinions
have hardly been definite or definitive.” Id., at 1222.
Given all this, it is hard to see how we can “presume Congress knew of and endorsed” the doctrine when it adopted a
new version of the Patent Act in 1952. Jama, 543 U. S., at349. That is so even if a technical parsing of Scott Paper
finds that it left assignor estoppel not quite dead, but everso-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 confirms as much. Although the Court
endorses assignor estoppel, it does not apply the doctrine asWestinghouse described it. Westinghouse stated the “rule”
of assignor estoppel this way: “[A]n assignor of a patentright is estopped to attack the utility, novelty or validity ofa patented invention which he has assigned or granted asagainst any one claiming the right under his assignment or
grant.” 266 U. S., at 349. The Court describes assignor
estoppel much more narrowly: “The doctrine applies only

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BARRETT, J., dissenting

when an inventor says one thing (explicitly or implicitly) inassigning a patent and the opposite in litigating against thepatent’s owner.” Ante, at 5. This version of assignor
estoppel does not appear in Westinghouse—nor, to myknowledge, in any other judicial decision. The new version
might be preferable to the old, but if Congress truly had
ratified Westinghouse, it would have endorsed theWestinghouse version.
B
The reenactment canon has a second requirement: The
reenacted statute must be materially identical to the one
previously interpreted. 4 This poses another stumbling
block for assignor estoppel, because the assignment provision of the 1952 Act contains a significant sentence that the1870 Act did not: “Subject to the provisions of this title, patents shall have the attributes of personal property.” 5 Patent Act of 1952, §261, 66 Stat. 810 (emphasis added).
——————
4 JUSTICE ALITO argues that the “material change” standard requires“express language” or “unavoidably implied contradiction.” Ante, at 4
(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 beaware 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. ___, ___ (2018) (slip op., at 11) (Congress is presumed to
be aware of a prior judicial interpretation of a statute when it uses “thematerially same language” in a subsequent statute). The implied repeal
canon addresses a different situation: when a court interprets two different and arguably conflicting 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

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BARRETT, J., dissenting

Given the reasoning of Westinghouse, this is a materialchange.
Westinghouse grounded its approval of assignor estoppel
in the idea that patents are like real property. After observing that the assignment provision required an assignment
to be made in writing and recorded in the Patent Officewithin 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 withthe statute.” 266 U. S., at 348–349. After all, the purposeof the writing-and-recording requirement is to “furnish
written and recorded evidence of title and to protect thepurchaser of the title as recorded for value without notice.”Id., at 349. Thus, the Court concluded, “[i]t was manifestly
intended by Congress to surround the conveyance of patent
property with safeguards” that resemble “those usually attaching to that of land.” Ibid.
But this analogy was inapt from the start. Even Westinghouse admitted that deeds and patents differed in an important 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, whoguarantees the quality of title, an assignor of a patent cannot warrant 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 Scientific Co. v. Ambico, Inc.:
Enforcing Patent Assignor Estoppel, 26 Houston L. Rev.
——————
an instrument in writing; and the patentee or his assigns or legal representatives may, in like manner, grant and convey an exclusive right under his patent to the whole or any specified part of the United States;and said assignment, grant, or conveyance shall be void as against any
subsequent purchaser or mortgagee for a valuable consideration, without
notice, unless it is recorded in the patent office within three months fromthe 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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761, 766 (1989) (“Since validity is never contractually transferred in an assignment, there is no theoretical basis for assignor estoppel by deed” (footnote omitted)).
In any event, however persuasive we found the patentdeed analogy in Westinghouse, the Patent Act of 1952 unraveled it. When Congress reenacted the assignment provision, it specified that patents have “the attributes of personal property.” 66 Stat. 810 (emphasis added). Because
this language is inconsistent with the premise on which
Westinghouse rested, it undercuts the argument that Congress ratified 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 theword on which the earlier construction was based).
Respondents try to brush off the change as insignificant,asserting that this “sentence has no bearing on assignor estoppel anyway, because estoppel by deed, upon which assignor estoppel is based, can apply to real or personal property.” Brief for Respondents 20 (internal quotation marks
omitted). But whatever the scope of estoppel by deed—respondents do not elaborate—Westinghouse expressly rested
on “[t]he analogy between estoppel in conveyances of landand 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 defined tract of land, and in the other, from a described andlimited field of the useful arts.” Ibid. By making clear thatpatents have “the attributes of personal property,” Congress directly undermined an interpretation that treatedpatents like deeds conveying land.
II
If Congress did not ratify our gloss on the assignment
provision in the 1952 Act, how else might assignor estoppel
be part of the statute? The Court comes at the interpretive

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BARRETT, J., dissenting

problem from a different angle: It holds that by 1952, assignor estoppel had become “a background principle of patent adjudication” against which Congress legislated. Ante,at 10. On this theory, Westinghouse is important not for its
interpretation of the assignment provision but for the equitable 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 wellestablished, . . . courts may take it as given that Congresshas 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 willassume that legislatures act against the background of thecommon law”). So, for example, a federal statute of limitations 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 frequently understand federal statutes to come with some unstated qualifications or embellishments suggested by principles of general jurisprudence”).
The Court says that assignor estoppel works this way too.
Ante, at 10. True, the Patent Act provides that “[i]nvalidityof 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, theCourt reasons, this apparently absolute language does not“foreclose applying in patent cases a whole host of commonlaw preclusion doctrines,” including “equitable estoppel,
collateral estoppel, res judicata, and law of the case.” Ante,at 10. Assignor estoppel, the Court says, falls in that same
category. Ibid. According to the Court, Westinghouse confirmed a trend that had already begun in the lower courts

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and has continued unabated since, giving assignor estoppel
a place in the pantheon of well-established common-law
principles.
I disagree. The common-law pedigree of assignor estoppel 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.,
Bursak, Note, Preclusions, 91 N. Y. U. L. Rev. 1651, 1663
(2016) (“Res judicata had migrated to England no later thanthe 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 atleast 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) (“Theauthority 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 “foundits way into every system of jurisprudence”). They arefirmly rooted in our jurisprudence 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 andfar shakier. It was introduced into patent law in the late
19th century—about a hundred years after Congress enacted the first patent laws and a decade after Congress
passed the 1870 Act. See Faulks v. Kamp, 3 F. 898 (CCSDNY 1880); ante, at 6 (opinion of the Court). And after its
introduction, 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 4–6.
It is therefore difficult for me to see how, in 1952, assignor

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BARRETT, J., dissenting

estoppel constituted a “long-established and familiar principl[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 thehigh 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 first-sale doctrine’s“impeccable historic pedigree”); Lozano v. Montoya Alvarez,
572 U. S. 1, 10 (2014) (emphasizing that equitable tolling isa “long-established feature of American jurisprudence”).
Indeed, whatever one might have said when the Court decided Westinghouse in 1924, Scott Paper cut sharply in theother direction. And such “contradictory signals are nottypically 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 fairdealing, while petitioner protests that the supposedly equitable 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 respectfully dissent.