(Slip Opinion) OCTOBER TERM, 2020 1 

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 a 

moisture-permeable applicator head to destroy targeted cells in the 

uterine 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, was 

eventually 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 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 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 applicator 

heads 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 
2 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 with 

this 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-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 
Cite as: 594 U. S. ____ (2021) 3 

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 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 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. 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 conflict 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 

possibly make a warranty of validity as to specific patent claims. That 

situation 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 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 he 

made no such representation, then he can challenge the new claims in 
4 MINERVA SURGICAL, INC. v. HOLOGIC, INC.

Syllabus

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 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 of 

Appeals 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.
Cite as: 594 U. S. ____ (2021) 1 

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 “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 

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 more 

claims particularly setting out the invention’s scope. See 
2 MINERVA SURGICAL, INC. v. HOLOGIC, INC.

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, 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 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 the 

company Novacept, Inc., invented the device—called the 

NovaSure System—in the late 1990s. He soon afterward 

filed a patent application, and assigned his interest in the 

application—as well as in any future “continuation applications”—to Novacept.1 The NovaSure System, as described 

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 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 the

NovaSure System patent. In 2004, Novacept sold its assets, 

including its portfolio of patents and patent applications, to 

—————— 1A 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.
Cite as: 594 U. S. ____ (2021) 3 

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, the 

device (like the NovaSure System) uses an applicator head 

to 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, and 

in 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 one 

of 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 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 

(essentially, his alter-ego) could not impeach the patent’s 
4 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. 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 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 this 

Court’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 sell 

something 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 time 

of 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, 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 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) 5 

Opinion of the Court

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 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 from 

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 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 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 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 patent 

assignor “was by his own oath and deed estopped” in an infringement suit from “attempt[ing] to deny his having had 
6 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, 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 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 that 

the 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) (“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 American 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.” 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 the 

lower 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 Courts 

of Appeal” were “to the same point” as the first, adding up 
Cite as: 594 U. S. ____ (2021) 7 

Opinion of the Court

to a full “forty-five 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 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 be 

heard 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 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 6. 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 inventions 
8 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 a 

patent. Id., at 350. “Of course,” the Court said, the assignor 

cannot use prior art in an infringement suit “to destroy 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.” Id., at 350–351. “Otherwise,” the 

Court 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, finding 

that 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 affiliated 

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. 

—————— 2The 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 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 affirming the assignor 

estoppel doctrine, the Court made clear that it did not always bar assignors from effectively defending against infringement suits.
Cite as: 594 U. S. ____ (2021) 9 

Opinion of the Court

In a patent application, the Court began, the inventor 

“swor[e] to” a particular “specification.” Id., at 352. But the 

exact 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 reject 

that view. The doctrine has lasted for many years, and we 

continue 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 the 

Federal 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 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 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 challenges” and so keeps bad patents alive. Id., at 38. (The 

principal 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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Opinion of the Court

On the first 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 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 conflict 

with 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 equitable 

estoppel 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 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), 
Cite as: 594 U. S. ____ (2021) 11

Opinion of the Court

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 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 request. See 326 U. S., at 254. And it restated the “basic 

principle” 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, declined 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 find 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 outweighs the “interest in private good faith.” Id., at 256–257. 

But the Court did not question—again, it reaffirmed—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, 
12 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 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 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 more 

compelling 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 the 

assignor 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 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 important 

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 doctrines. Lear 

counseled 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 right 

about where they found it—as a bounded doctrine designed 

to prevent an inventor from first 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 realized that assignor estoppel has limits: Even in approving 

the doctrine, the Court made clear that not every assignor 
Cite as: 594 U. S. ____ (2021) 13

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 of 

common-law rules.

Finally, we do not think, as Minerva claims, that contemporary patent policy—specifically, the need to weed out bad 

patents—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 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 

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 

—————— 3Recognizing 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 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 the 

inventor then assigns those claims to another, he effectively incorporates 

that assurance.
14 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) and 

later 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 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 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 with 

limits—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 dealing 

comes into play. That principle, as explained above, demands consistency in representations about a patent’s validity: 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 

original 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 

—————— 4Even 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 from 

raising 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.
Cite as: 594 U. S. ____ (2021) 15

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 ground 

for 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 a 

common employment arrangement. An employee assigns 

to his employer 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.

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, 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 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 
16 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. Assuming 

that the new claims are materially broader than the old 

claims, the assignor 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 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 prior 

decisions, has failed to recognize those boundaries. Minerva (recall, Truckai’s alter-ego) argued to the court that 

estoppel should 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 Hologic had expanded the assigned 

claims: Even if so, Minerva could not contest the new 

claim’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 
Cite as: 594 U. S. ____ (2021) 17

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 no 

concern 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.
Cite as: 594 U. S. ____ (2021) 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 and 

the 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, see 

ante, 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 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 13. 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., 
2 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 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 principal 

dissent, Westinghouse was abrogated by the Patent Act of 

1952. 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 was 

based on an interpretation of the 1870 Patent Act. Neither 

of 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 

—————— 1One 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 scope 

of the doctrine of assignor estoppel, not its existence, see id., at 352.
Cite as: 594 U. S. ____ (2021) 3 

ALITO, J., dissenting

of 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 for 

a 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 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 interpreting a statutory provision that it reenacts. Perhaps it is hubris, but we have often presumed that Congress is aware of 

our decisions. See, e.g., Ryan v. Valencia Gonzales, 568 

—————— 2By 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”). 3This 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 idea 

that 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.
4 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 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 abrogation 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 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 of 

argument for abrogation. The principal dissent cannot 

identify 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 

—————— 4The 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 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.
Cite as: 594 U. S. ____ (2021) 5 

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 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 Westinghouse should be overruled, I would dismiss the writ as improvidently granted. I therefore respectfully dissent.

—————— 5Under 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., at 

455–465; id., at 470–472 (ALITO, J., dissenting), even though “Congress 

ha[d] repeatedly amended . . . the specific 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.
Cite as: 594 U. S. ____ (2021) 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 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 nowhere mentions the equitable doctrine of assignor estoppel, which precludes inventors who file 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 inventor 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 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.
2 MINERVA SURGICAL, INC. v. HOLOGIC, INC.

BARRETT, J., dissenting

I 

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 American 

case 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 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 accordance 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 applied the doctrine to the assignment and 

conveyance 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 this 

gloss when it reenacted that provision in 1952. Congress 

ratifies a judicial interpretation in a reenacted statute only 

if two requirements are satisfied: (1) the interpretation

must be so well settled that we can “presume Congress 

—————— 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 10. I address this possibility in Part II, infra. 2 JUSTICE ALITO maintains that Westinghouse did not interpret the 

statute 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.
Cite as: 594 U. S. ____ (2021) 3 

BARRETT, J., dissenting

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. ___, ___ (2018) (slip op., 

at 11) (noting that Congress incorporates a prior judicial interpretation of a statute when it uses “the materially same 

language” 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 the 

prior 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 Interpretation 

of 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] a 

prominent 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 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 
4 MINERVA SURGICAL, INC. v. HOLOGIC, INC.

BARRETT, J., dissenting

A 

Westinghouse’s construction of the assignment provision 

in 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, 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 “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 the 

doctrine, expressly declining to address the extent to which 

assignor 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 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. ___, ___ (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. 
Cite as: 594 U. S. ____ (2021) 5 

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 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.,

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 the 

rule to be no longer valid, others find no weakening of the 

rule, 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 Patent 

Act. 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 
6 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 left 

the vitality of assignor estoppel unsettled: “Although the 

Supreme 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., at 

349. 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 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 patented invention which he has assigned or granted as 

against 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 
Cite as: 594 U. S. ____ (2021) 7 

BARRETT, J., dissenting

when an inventor says one thing (explicitly or implicitly) in 

assigning a patent and the opposite in litigating against the 

patent’s owner.” Ante, at 5. This version of assignor 

estoppel does not appear in Westinghouse—nor, to my 

knowledge, 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 the 

Westinghouse 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 the 

1870 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 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. ___, ___ (2018) (slip op., at 11) (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 conflicting statutory provisions. Scalia, Reading Law, 

at 331. 5The Patent Act of 1870 provides, in relevant part:

“[E]very patent or any interest therein shall be assignable in law, by 
8 MINERVA SURGICAL, INC. v. HOLOGIC, INC.

BARRETT, J., dissenting

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 observing that the assignment provision required an assignment 

to be made in writing and recorded in the Patent Office 

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 

written and recorded evidence of title and to protect the 

purchaser 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, who 

guarantees 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 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”).
Cite as: 594 U. S. ____ (2021) 9 

BARRETT, J., dissenting

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 the 

word 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 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 defined tract of land, and in the other, from a described and 

limited field of the useful arts.” Ibid. By making clear that 

patents have “the attributes of personal property,” Congress directly undermined an interpretation that treated 

patents 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 
10 MINERVA SURGICAL, INC. v. HOLOGIC, INC.

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 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 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]nvalidity 

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 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
Cite as: 594 U. S. ____ (2021) 11

BARRETT, J., dissenting

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 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 

firmly 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 and 

far 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 (CC 

SDNY 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 
12 MINERVA SURGICAL, INC. v. HOLOGIC, INC.

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 the 

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 first-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”). 

Indeed, 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 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.