author: Justice Stevens Justice O’Connor,

text:
    KANSAS v. COLORADO
    No. 105,
    Orig.
    Argued March 19, 2001
    Decided June 11, 2001

      Stevens, J., delivered the opinion of the Court, Parts I, IV, and V of which were unanimous, and Parts II and III of which were joined by Rehnquist, C. J., and Kennedy, Soutek, Ginsbukg, and Breyer, JJ. O’Connor, J., filed an opinion concurring in part and dissenting in part, in which Scalia and Thomas, JJ., joined, post, p. 20.

      John B. Draper, Special Assistant Attorney General of Kansas, argued the cause for plaintiff. With him on the briefs were Carla J. Stovall, Attorney General, John W. Campbell, Chief Deputy Attorney General, John M. Cassidy, Assistant Attorney General, Leland E. Rolfs, Special Assistant Attorney General, and Andrew S. Montgomery.

      Jeffrey P. Minear argued the cause for intervenor United States. With him on the briefs were Solicitor General Waxman, Assistant Attorney General Schiffer, Deputy Solicitor General Kneedler, and Patricia Weiss.

      David W. Robbins, Special Assistant Attorney General of Colorado, argued the cause for defendant. With him on the briefs were Ken Salazar, Attorney General, Carol D. Angel, Senior Assistant Attorney General, and Dennis M. Montgomery, Special Assistant Attorney General.
   Justice Stevens

delivered the opinion of the Court.

The Arkansas River rises in the mountains of Colorado just east of the Continental Divide, descends for about 280 miles to the Kansas border, then flows through that State, Oklahoma, and Arkansas and empties into the Mississippi River. On May 20, 1901, Kansas first invoked this Court’s original jurisdiction to seek a remedy for Colorado’s diversion of water from the Arkansas River. See Kansas v. Colorado, 185 U. S. 125, 126 (1902) (statement of case). In opinions written during the past century, most recently in Kansas v. Colorado, 514 U. S. 673, 675-678 (1995), we have described the history and the importance of the river. For present purposes it suffices to note that two of those cases, Kansas v. Colorado, 206 U. S. 46 (1907), and Colorado v. Kansas, 320 U. S. 383 (1943), led to the negotiation of the Arkansas River Compact (Compact), an agreement between Kansas and Colorado that in turn was approved by Congress in 1949. See 63 Stat. 145. The case before us today involves a claim by Kansas for damages based on Colorado’s violations of that Compact.

The Compact was designed to “[s]ettle existing disputes and remove causes of future controversy” between the two States and their citizens concerning waters of the Arkansas River and to “[ejquitably divide and apportion” those waters and the benefits arising from construction and operation of the federal project known as the “John Martin Reservoir.” Arkansas River Compact, Art. I, reprinted in App. to Brief for Kansas A-l, A-2. Article IV-D of the Compact provides:

“This Compact is not intended to impede or prevent future beneficial development of the Arkansas River basin in Colorado and Kansas by Federal or State agencies, by private enterprise, or by combinations thereof, which may involve construction of dams, reservoir, and other works for the purpose of water utilization and control, as well as the improved or prolonged functioning of existing works: Provided, that the waters of the Arkansas River, as defined in Article III, shall not be materially depleted in usable quantity or availability for use to the water users in Colorado and Kansas under this Compact by such future development or construction.” Id., at A-5.

It is the proviso to that paragraph that is of special relevance to this case.

In 1986, we granted Kansas leave to file a complaint alleging three violations of the Compact by Colorado. See 514 U. S., at 679-680. After taking evidence in the liability phase of the proceeding, Special Master Arthur L. Little-worth filed his first report, in which he recommended that two of the claims be denied, but that the Court find, that post-Compact increases in groundwater well pumping in Colorado had materially depleted the waters of the river in violation of Article IV-D. See id., at 680. We overruled Colorado’s exceptions to that recommendation, including an argument that Kansas was guilty of laches. Id., at 687-689. We remanded the ease to the Special Master to determine: an appropriate remedy for the violations of Article IV-D. Id., at 694. ,

After further proceedings the Special Master filed a second report recommending an award of damages. Colorado filed exceptions to that report, arguing that the Eleventh Amendment barred an award based on losses incurred by Kansas citizens, and that the report improperly recommended the recovery of prejudgment interest on an un-liquidated claim. We overruled those exceptions without prejudice to their renewal after the Special Master made a more specific recommendation for a remedy. 522 U. S. 1073 (1998). He did so in his third report, and we are now confronted with exceptions filed by both States.

In the third report, the Special Master recommends that damages be measured by Kansas’ losses, rather than Colorado’s profits, attributable to Compact violations after 1950; that the damages be paid in money rather than water; and that the damages should include prejudgment interest from 1969 to the date of judgment. Colorado has filed four objections to the report. It contends (1) that the recommended award of damages would violate the Eleventh Amendment to the United States Constitution; (2) that the damages award should not include prejudgment interest; (3) that the amount of interest awarded is excessive; and (4) that the Special Master improperly credited flawed expert testimony, with the result that Kansas’ crop production losses were improperly calculated. On the other hand, Kansas has filed an objection submitting that prejudgment interest should be paid from 1950, rather than 1969. The United States, which intervened because of its interest in the operation of flood control projects in Colorado, submits that both States’ objections should be overruled.

1 — 1

We have decided that a State may recover monetary damages from another State in an original action, without running afoul of the Eleventh Amendment. See, e.g., Texas v. New Mexico, 482 U. S. 124, 130 (1987) (“The Court has recognized the propriety of money judgments against a State in an original action, and specifically in a case involving a compact. In proper original actions, the Eleventh Amendment is no barrier, for by its terms, it applies only to suits by citizens against a State” (citations omitted)); see also Maryland v. Louisiana, 451 U. S. 725, 745, n. 21 (1981); South Dakota v. North Carolina, 192 U. S. 286, 317-321 (1904). Colorado contends, however, that the Eleventh Amendment precludes any such recovery based on losses sustained by individual water users in Kansas.

It is firmly established, and undisputed in this litigation, that the text of the Eleventh Amendment would bar a direct action against Colorado by citizens of Kansas. Moreover, we have several times held that a State may not invoke our original jurisdiction when it is merely acting as an agent or trustee for one or more of its citizens. For example, in New Hampshire v. Louisiana, 108 U. S. 76 (1883), we refused to assume jurisdiction over an action to recover payment, on defaulted bonds that had been formally assigned to the state plaintiffs but remained beneficially owned by private individuals. And, in North Dakota v. Minnesota, 263 U. S. 365 (1923), we held that, while the plaintiff State could obtain an injunction against the improper operation of Minnesota’s drainage ditches, the Eleventh Amendment precluded an award of damages based on injuries to individual farmers, where the damages claim was financed by contributions from the farmers and the State had committed to dividing any recovery among the farmers “in proportion to the amount of [their] loss.” Id., at 375.

Those cases make it clear that a “State is not permitted to enter a controversy as a nominal party in order to forward the claims of individual citizens.” Maryland v. Louisiana, 451 U. S., at 737; see also New Hampshire v. Louisiana, 108 U. S., at 89 (Eleventh Amendment applies and acts to bar jurisdiction where “the State and the attorney-general' are only nominal actors in the proceeding”). The “governing principle” is that in order to invoke our original jurisdiction, “the State must show a direct interest of its own and not merely seek recovery for the benefit of individuals who are the real parties in interest.” Oklahoma ex rel. Johnson v. Cook, 304 U. S. 387, 396 (1938).

Kansas has unquestionably made such a showing. Indeed, the present proceeding is but one of several in which Kansas’ own interest in preventing upstream diversions from the Arkansas River has justified an exercise of our original jurisdiction. In Cook we even offered as an example of proper original jurisdiction one of the prior original suits between Kansas and Colorado, see id., at 393-394 (citing Kansas v. Colorado, 206 U. S. 46 (1907)), and in Texas v. New Mexico we held that enforcement of an interstate water compact by means of a recovery of money damages can be within a State’s proper pursuit of the “general public interest” in an original action, 482 U. S., at 132, n. 7.

Moreover, the record in this case plainly discloses that the State of Kansas has been in full control of this litigation since its inception. Its right to control the disposition of any recovery of damages is entirely unencumbered. The injury to individual farmers is but one component of the formula adopted by the Special Master to quantify the damages caused by Colorado’s violation of its contractual obligations. In short, there is simply nothing in the record to suggest that the State of Kansas is merely a “nominal party” to this litigation or that the individual farmers are “the real parties in interest.”

When a State properly invokes our jurisdiction to seek redress for a wrong perpetrated against it by a sister State, neither the measure of damages that we ultimately determine to be proper nor our method for calculating those damages can retrospectively negate our jurisdiction. Nor would our jurisdiction to order a damages remedy be affected by Kansas’ postjudgment decisions concerning the use of the money recovered from Colorado. As we have previously recognized, it is the State’s prerogative either to deposit the proceeds of any judgment in “the general coffers of the State” or to use them to “benefit those who were hurt.” Ibid.

We overrule Colorado’s first exception.

II

Colorado next excepts to the Special Master’s conclusion that the damages award should include prejudgment interest despite the fact that Kansas’ claim is unliquidated. At one point in time, the fact that the claim was unliquidated would have been of substantial importance. As a general matter, early common-law cases drew a distinction between liquidated and unliquidated claims and refused to allow interest on the latter. See, e.g., Comment, Prejudgment Interest: Survey and Suggestion, 77 Nw. U. L. Rev. 192,196, and n. 26 (1982) (discussing history and collecting sources). This rule seems to have rested upon a belief that there was something inherently unfair about requiring debtors to pay interest when they were unable to halt its accrual by handing over to their creditors a fixed and unassailable amount. See, e. g., id., at 196.

This common-law distinction has long since lost its hold on the legal imagination. Beginning in the early part of the last century, numerous courts and commentators have rejected the distinction for failing to acknowledge the compensatory nature of interest awards. This Court allied itself with the evolving consensus in 1933, when we expressed the opinion that the distinction between cases of liquidated and unliquidated damages “is not a sound one.” Funkhouser v. J B. Preston Co., 290 U. S. 163, 168 (1933). The analysis supporting that conclusion gave no doubt as to our reasoning: ‘‘Whether the case is of the one class or the other, the injured party has suffered a loss which may be regarded as not fully compensated if he is confined to the amount found to be recoverable as of the time of breach and nothing is added for the delay in obtaining the award of damages.” Ibid. Our cases since 1933 have consistently acknowledged that a monetary award does not fully compensate for an injury unless it includes an interest component. See, e. g., Milwaukee v. Cement Div., National Gypsum Co., 515 U. S. 189, 195 (1995) (“The essential rationale for awarding prejudgment interest is to ensure that an injured party is fully compensated for its loss”); West Virginia v. United States, 479 U. S. 305, 310-311, n. 2 (1987); General Motors Corp. v. Devex Corp., 461 U. S. 648, 655-656, n. 10 (1983).

Relying on our cases, the Special Master “concluded that the unliquidated nature of Kansas’ money damages does not, in and of itself, bar an award of prejudgment interest.” Second Report, § XV, reprinted in App. to Third Report 43. In reaching that conclusion, the Special Master was fully cognizant of both the displaced common-law rule and the subsequent doctrinal evolution. In addition, he gave careful consideration to equitable considerations that might mitigate against an award of interest, concluding that “considerations of fairness,” Board of Comm’rs of Jackson Cty. v. United States, 308 U. S. 343, 352 (1939), supported the award of at least some prejudgment interest in this case.

We find no fault in the Special Master’s analysis of either our prior cases or the equities of this matter. While we will deal with the amount of prejudgment interest below, to answer Colorado’s second objection it is sufficient to conclude that the Special Master was correct in determining that the unliquidated nature of the damages does not preclude an award of prejudgment interest.

Colorado’s second exception is overruled.

III

Colorado’s third exception takes issue with both the rate of interest adopted by the Special Master and the date from which he recommended that interest begin to accrue. As to the second of these two concerns, Colorado submits that, if any prejudgment interest is to be awarded, it should begin to accrue in 1985 (when Kansas filed its complaint in this action), rather than in 1969 (when, the Special Master concluded, Colorado knew or should have known that it was violating the Compact). On the other hand, Kansas has entered an exception, arguing that the accrual of interest should begin in 1950. We first address the rate question, then the timing issue.

A

The Special Master credited the testimony of Kansas’ three experts who calculated the interest rates that they thought necessary to provide full compensation for the damages caused by Colorado’s violations of the Compact in the years since 1950. As a result of inflation and changing market conditions those rates varied from year to year. In their calculation of the damages suffered by Kansas farmers, the experts used the interest rates that were applicable to individuals in the relevant years rather than the (lower) rates available to States.

Colorado argues that the lower rates should have been used because it is the State, rather than the individual farmers, that is maintaining the action and will receive any award of damages. But if, as we have already decided, see Part I, supra, it is permissible for the State to measure a portion of its damages by losses suffered by individual farmers, it necessarily follows that the courts are free to utilize whatever interest rate will most accurately measure those losses. The money in question in this portion of the damages award is revenue that would — but for Colorado’s actions — have been earned by individual farmers. Thus, the Special Master correctly concluded that the economic consequences of Colorado’s breach could best be remedied by an interest award that mirrors the cost of any additional borrowing the farmers may have been forced to undertake in order to compensate for lost revenue.

B

Although the Special Master rejected Colorado’s submission that there is a categorical bar to the award of prejudgment interest on unliquidated claims, he concluded that such interest should not “be awarded according to [any] rigid theory of compensation for money withheld,” but rather should respond to “ ‘considerations of fairness.’ ” Third Report 97 (quoting Jackson Cty., 308 U. S., at 352). Kansas argues that our decisions subsequent to Jackson County have effectively foreclosed the equities-balancing approach that the Special Master adopted. There is some merit to Kansas’ position. See National Gypsum Co., 515 U. S., at 193 (affirming a decision of the Court of Appeals that had read our cases as “disapproving of a ‘balancing of the equities’ as a method of deciding whether to allow prejudgment interest”).

However, despite the clear direction indicated by some of our earlier opinions, we cannot say that by 1949 our case law had developed sufficiently to put Colorado on notice that, upon a violation of the Compact, we would automatically award prejudgment interest from the time of injury. Given the state of the law at that time, Colorado may well have believed that we would balance the equities in order to achieve a just and equitable remedy, rather than automatically imposing prejudgment interest in order to achieve full compensation. See Jackson Cty., 808 U. S., at 352 (prejudgment interest award limited by “considerations of fairness”); Miller v. Robertson, 266 U. S. 243, 258 (1924) (“[W]hen necessary in order to arrive at fair compensation, the court in the exercise of a sound discretion may include interest or its equivalent as an element of damages” on un-liquidated claims); Restatement of Contracts §337, p. 542 (1932) (prejudgment interest on unliquidated claims “may be allowed in the discretion of the court, if justice requires it”). While we are confident that, when it signed the Compact, Colorado was on notice that it might be subject to prejudgment interest if such interest was necessary to fashion an equitable remedy, we are unable to conclude with sufficient certainty that Colorado was on notice that such interest would be imposed as a matter of course. We, therefore, believe that the Special Master acted properly in carefully analyzing the facts of the case and in only awarding as much prejudgment interest as was required by a balancing of the equities.

We also agree with the Special Master that the equities in this case do not support an award of prejudgment interest from the date of the first violation of the Compact, but rather favor an award beginning on a later date. In reaching this conclusion, the Special Master appropriately considered several factors. In particular, he relied on the fact that in the early years after the Compact was signed, no one had any thought that the pact was being violated. Third Report 106. In addition, he considered the long interval that passed between the original injuries and these proceedings, as well as the dramatic impact of compounding interest over many years. Id., at 99-101; see also n. 3, supra.

In its exception, Kansas argues that the Special Master’s reasoning would be appropriate if damages were being awarded as a form of punishment, but does not justify a refusal to provide full compensation to an injured party. Moreover, Kansas argues, a rule that rewards ignorance might discourage diligence in making sure that there is full compliance with the terms of the Compact. Kansas’ argument is consistent with a “rigid theory of compensation for money withheld,” but, for the reasons discussed above, we are persuaded that the Special Master correctly declined to adopt such a theory. The equitable considerations identified by the Special Master fully justify his view that in this case it would be inappropriate to award prejudgment interest for any years before either party was aware of the excessive pumping in Colorado.

In its third exception, Colorado argues that, if prejudgment interest is to be awarded at all, the equities are best balanced by limiting such interest to the time after the complaint was filed, rather than the time after which Colorado knew or should have known that it was violating the Compact. Specifically, Colorado suggests that prejudgment interest should begin to accrue in 1985 rather than 1969. The choice between the two dates is surely debatable; it is a matter over which reasonable people can — and do — disagree. After examining the equities for ourselves, however, a majority of the Court has decided that the later date is the more appropriate.

When we overruled Colorado’s objections to the Special Master’s first report, we held that Kansas was not guilty of inexcusable delay in failing to complain more promptly about post-Compact well pumping. 514 U. S., at 687-689. In saying that the delay was not inexcusable, we recognized that the nature and extent Of Colorado’s violations continued to be unclear even in the years after which it became obvious that the Compact was being violated. Id., at 688-689. That conclusion is something of a two-edged sword, however. While Kansas’ delay was understandable given the amorphous nature of its claims, there is no doubt that the interests of both States would have been served if the claim had been advanced promptly after its basis became known. Once it became obvious that a violation of the Compact had occurred, it was equally clear that the proceedings necessary to evaluate the significance of the violations would be complex and protracted. Despite the diligence of the parties and the Special Master, over 15 years have elapsed since the complaint was filed. Given the uncertainty over the scope of damages that prevailed during the period between 1968 and 1985 and the fact that it was uniquely in Kansas’ power to begin the process by which those damages would be quantified, Colorado’s request that we deny prejudgment interest for that period is reasonable.

For these reasons, we overrule Kansas’ exception. We also overrule Colorado’s third exception insofar as it challenges the interest rates recommended by the Special Master, but we sustain that objection insofar as it challenges the award of interest for the years prior to 1985.

IV

Colorado’s final objection challenges the Special Master’s determination of the value of the crop losses attributable to the Compact violations, the largest component of Kansas’ damages claim. The Special. Master accomplished the calculation by estimating the amount of farmland affected by Colorado’s violations, the crops planted on that farmland, the price of those crops, and the difference in yield between what the affected land would have produced with the additional water and what the land actually produced with the water it received.

The parties were in agreement concerning most of the facts bearing on the Special Master’s calculation. They agreed that water was in short supply in the affected area each year, 178 Tr. 127-128; they agreed on the amount of the shortage that resulted from Colorado’s violations, ibid.; and they generally agreed on which crops were planted on the affected farmland as well as the prices of those crops in the relevant years. See Third Report 46.

The only issue on which the parties disagreed was the exact effect of the diverted water on the crop yields for the farmland in question. On that score, both Kansas and Colorado accepted the general notion that “[u]p to the point where crops no longer can make use of additional water, more water produces more crop yield.” Id., at 47. But they parted ways on the question of precisely how much additional yield would have been produced with the missing water. Kansas’ experts relied upon the hypothesis of a generally linear relationship between water available for use and increased crop yields. With figures drawn from a number of studies, Professor Norman Whittlesey, Kansas’ principal expert, developed quantitative estimates of the lost yield, per unit of water, for the various crops grown on the affected farmland. Although Colorado’s expert initially attempted to propose his own model, he ultimately abandoned his position when confronted with flaws in his data. 197 Tr. 44-46.

Its own expert having recanted his alternative proposal for calculating the effects of the diverted water on crop yield, Colorado attempts to poke holes in Kansas’ methodology through a speculative application of abstract economic theory. Kansas’ numbers (for crop losses due to diverted water) cannot be correct, Colorado argues, because if they were, it would have been economically profitable for the affected farmers to drill wells and obtain water from underground sources rather than suffer the reduced yield from the shortage of surface water. Brief for Colorado 41-49. Because Kansas farmers did not install wells, Colorado concludes, we can know that the diverted water was not as valuable as Kansas’ experts claim.

The Special Master did not question Colorado’s assertion that digging wells would, in retrospect, actually have been profitable for Kansas farmers, but he declined to employ Colorado’s argument as a basis for rejecting Kansas’ expert testimony on the extent of crop losses. His thoughtful analysis is worth quoting in full:

“Given the hindsight of present day economists, it might have been profitable for everyone to drill supplemental wells .... However, there are many reasons why this may not have been done, and the failure to drill wells does not by itself indicate that Kansas’ estimate of crop losses is too high. The favorable economics of drilling wells may not have been understood at the time. Quality information regarding costs and returns was not readily available. [211 Tr.] 31. Some farmers, for reasons of age or otherwise, may not have wanted to go into long-term debt. Some farmers may not have had the available capital, or the credit to borrow. Many farmers were ‘cash poor.’ Id. at 32. Some farmers may have been averse to risk. Some farmers may have been tenants, and the landlord may not have been willing to undertake the necessary investment. Some farms may have been small in terms of total acreage, or the acreage spread out over space, so that it was not feasible or practical to consider a well investment. [208 Tr.] 37-39. Capital for well investments, with three to ten year repayment periods, was less available than for long-term investments. [211 Tr.] 32.” Third Report 60-61.

We agree with the Special Master that accepting Colorado’s argument requires a good deal of speculation, not only about the comparative advantages of wells as opposed to irrigation, but also about the ability of the farmers fully to understand or to implement different choices without the benefit of expert hindsight. Given Colorado’s inability to mount an effective challenge to Kansas’ experts on their own terms and its complete failure to provide a plausible alternative estimate of the crop damage that resulted from its violations of the Compact, we conclude that its attack on Kansas’ conclusions is unpersuasive.

Colorado’s fourth exception is overruled.

V

We remand the ease to the Special Master for preparation of a final judgment consistent with this opinion.

It is so ordered.

Justice O’Connor,

with whom Justice Scalia and Justice Thomas join, concurring in part and dissenting in part.

I agree with the Court’s disposition of this case as to Colorado’s first and fourth exceptions to the Special Master’s Third Report, concerning the award and determination of damages. I therefore join Parts I, IV, and V of the Court’s opinion. I do not concur in Parts II and III of the Court’s opinion because I believe that the award of prejudgment interest to Kansas, coming over half a century after the Arkansas River Compact’s (hereinafter Compact) negotiation and approval, is clearly improper under our precedents.

We are dealing with an interstate compact apportioning the flow of a river between two States. A compact is a contract. It represents a bargained-for exchange between its signatories and “remains a legal document that must be construed and applied in accordance with its terms.” Texas v. New Mexico, 482 U. S. 124, 128 (1987); see also Petty v. Tennessee-Missouri Bridge Comm’n, 859 U. S. 275, 285 (1959) (Frankfurter, J., dissenting) (“A Compact is, after all, a contract”). It is a fundamental tenet of contract law that parties to a contract are deemed to have contracted with reference to principles of law existing at the time the contract was made. See, e. g., Norfolk & Western R. Co. v. Train Dispatchers, 499 U. S. 117, 129-130 (1991); Farmers and Merchants Bank of Monroe v. Federal Reserve Bank of Richmond, 262 U. S. 649, 660 (1923); see generally 11 Williston on Contracts § 30:19 (4th ed. 1999). The basic question before the Court is thus one of “the fair intendment of the contract itself.” Virginia v. West Virginia, 238 U. S. 202, 233 (1915). Specifically, the question is whether, at the time the Compact was negotiated and approved, Colorado and Kansas could fairly be said to have intended, or at least to have expected or assumed, that Colorado might be exposing itself to liability for prejudgment interest in the event of the Compact’s breach. Cf. id., at 232-236 (awarding interest to Virginia in a suit against West Virginia for breach of a contract to assume “an equitable proportion” of Virginia’s interest-bearing public debt upon finding that “there is no escape from the conclusion that there was a contract duty on the part of West Virginia to provide for accruing interest as a part of the equitable proportion assumed”).

I fail to see how Colorado and Kansas could have contemplated that prejudgment interest would be awarded. The “venerable . . . rule” at common law was that prejudgment interest was unavailable on claims for unliquidated or, even more significantly, unascertainable damages. Milwaukee v. Cement Div., National Gypsum Co., 515 U. S. 189, 197 (1995). Contrary to the Court’s suggestion, see ante, at 9-11, 13-14, that rule had not been abandoned by the period between 1943 and 1949, the years of the Compact’s negotiation and ultimate approval by Congress. By that time, the state of the law in general regarding awards of prejudgment interest for unliquidated claims was uncertain at best, as the Court itself recognizes. See ante, at 9-11, and n. 3; cf. ante, at 13-14; see also Funkhouser v. J. B. Preston Co., 290 U. S. 163, 168 (1933) (noting “the numerous, and not harmonious, decisions upon the allowance of interest in the case of unliq-uidated claims,” and that “the rule with respect to unliqui-dated claims has been in evolution”). To be sure, we had by then, along with other courts, criticized the common law rule that prejudgment interest was recoverable on claims for liquidated, but not for unliquidated, damages. See ibid. But in the absence of a statute providing for such interest, many courts, including our own, still denied and would continue to deny prejudgment interest on claims for unliquidated and unascertainable damages in a great many, and probably most, circumstances. See, e. g., Board of Comm’rs of Jackson Cty. v. United States, 308 U. S. 343, 353 (1939); Blau v. Lehman, 368 U. S. 403, 414 (1962); Lineman v. Schmid, 32 Cal. 2d 204, 211, 195 P. 2d 408, 412 (1948) (Although “there is authority to the effect that the distinction formerly existing between liquidated and unliquidated demands is practically obliterated, . . . further reading . . . discloses, with citation of many cases, that the general rule is almost uniformly adhered to, namely, that interest is not allowable where the damages depend upon no fixed standard and cannot be made certain except by accord, verdict or decree”); D. Dobbs, Remedies § 3.5, p. 165 (1973) (“Most courts, in the absence of a statute to the contrary, would not award interest on unliquidated pecuniary claims, the amount of which could not be ascertained or computed, even in theory, without a trial”); see generally C. McCormick, Law of Damages §51, p. 210 (1935) (explaining evolution of rule in America); see also 1 D. Dobbs, Law of Remedies § 3.6(1), p. 336 (2d ed. 1993) (“The most significant limitation on the recovery of prejudgment interest is the general rule that, apart from statute, prejudgment interest is not recoverable on claims that are neither liquidated as a dollar sum nor ascertainable by fixed standards” (footnotes omitted)).

Awards of such interest on claims for unliquidated and unascertainable damages for breach of a contract appear to have been rarer still. See, e. g., Williams v. Idaho Potato Starch Co., 73 Idaho 13, 24, 245 P. 2d 1045, 1051-1052 (1952); Meyer v. Strom, 37 Wash. 2d 818, 829-830, 226 P. 2d 218, 224 (1951); Lineman v. Schmid, supra, at 207-213, 195 P. 2d, at 410-413; see also 3 Williston on Contracts §1413, p. 2508 (1920) (“Interest is not generally allowed . .. where market rates or prices furnish no definite or exact test of the amount due” (footnote omitted)); 1 T. Sedgwick, Measure of Damages §312, p. 614 (9th ed. 1912) (“Generally speaking, no interest can be recovered for breach of a contract, where the damages are in their nature unliquidated, until the amount is ascertained” (footnote omitted)). In fact, at the time, they were not allowed in either Colorado or Kansas. See, e. g., Clark v. Giacomini, 85 Colo. 530, 536-537, 277 P. 306, 308 (1929); Denver Horse Imp, Co. v. Schafer, 58 Colo. 376, 390, 147 P. 367, 372 (1915); Roe v. Snattinger, 91 Kan. 567, 568, 138 P. 581, 582 (1914); Evans v. Moseley, 84 Kan. 322, 332-333, 114 R 374, 378 (1911).

Finally, and most important to this case, an award of prejudgment interest on unliquidated and unascertainable damages for breach of an interstate compact was unheard of at the time of the Compact’s negotiation and approval. Unlike cases involving bonds or other instruments of credit, see, e. g., Virginia v. West Virginia, supra, at 232-236; South Dakota v. North Carolina, 192 U. S. 286, 317-321 (1904), monetary damages in cases of this sort, involving the apportionment of water between States, are notoriously difficult to ascertain. Indeed, despite 15 years of litigation over the Compact, and resort to a great deal of data, expert testimony, complicated methodologies, and sophisticated analyses on the subject, the final value of Kansas’ damages still has yet to be determined. See ante, at 9, n. 2; see also Third Report §§ III to X (detailing and analyzing the numerous variables and data elements necessary to arrive at a determination of Kansas’ damages). It thus is not surprising that, until 1987, we had never even suggested that monetary damages could be recovered from a State as a remedy for its violation of an interstate compact apportioning the flow of an interstate stream. And when we first allowed such damages in Texas v. New Mexico, 482 U. S. 124 (1987), we did so partially at the behest of New Mexico, the breaching State. See id., at 129-132. How, then, can one say that, at the time the Compact was negotiated and approved, its signatories could fairly be said to have intended, or at least could reasonably be said to have expected or assumed, that Kansas might recover prejudgment interest on damages caused by Colorado’s breach? The necessary predicate to such a recovery was neither recognized nor even contemplated by this Court or, apparently, by the state parties to original actions of this sort, until some 40 years thence.

In light of this history, it seems inescapable that any participant in the drafting and negotiation of the Compact would, if asked at the time, have reacted with marked surprise to the notion that the Compact rendered its signatories liable for an award of prejudgment interest such as that sanctioned by the Court today. As both the Compact itself and the parties’ post-Compact course of dealing make clear, the “fair intendment” of the Compact very probably was simply for the in-kind recovery of water as a remedy for its breach. The Compact says nothing about the availability of prejudgment interest on money damages as part of any remedy or, for that matter, about the availability of money damages as a remedy in the first instance. It contemplates the delivery of water from Colorado to Kansas, pure and simple. See Arkansas River Compact, reprinted in App. to Brief for Kansas A-l. When Kansas filed its complaint in this matter, “it sought only a decree commanding Colorado To deliver the waters of the Arkansas River in accordance with the provisions of the Arkansas River Compact.’” Third Report § XI, at 98. Cf. Colorado v. Kansas, 320 U. S. 383, 391 (1943) (discussing Kansas’ prayer for relief in the form of “an apportionment in second feet or acre feet”). Not until our decision in Texas v. New Mexico, supra, did Kansas amend its complaint to include a claim for monetary damages. See Third Report §XI, at 98. Neither Kansas nor Colorado appears ever to have anticipated or assumed, much less expected, that the Compact might result in a monetary award of prejudgment interest over half a century after its signing.

The Court ignores all- of this in awarding prejudgment interest to Kansas, seizing instead upon the compensatory rationale behind the criticism of the common law rule and awards of prejudgment interest on unliquidated claims for damages in general. See ante, at 10-11. I do not dispute that awards of interest are compensatory in nature or that, as a general matter, “a monetary award does not fully compensate for an injury unless it includes an interest component.” Ante, at 10; see also National Gypsum Co., 515 U. S., at 195, n. 7. But, as the Court itself recognizes, see ante, at 11, our precedents make clear that, at least today and in the absence of a governing statute, awards of prejudgment interest on unliquidated claims for damages are governed not by any “rigid theory of compensation for money withheld,” but rather by “considerations of fairness.” Blau, 368 U. S., at 414 (internal quotation marks and citation omitted); see, e. g., General Motors Corp. v. Devex Corp., 461 U. S. 648, 651-653, and n. 5 (1983); Funkhouser, 290 U. S., at 168-169. This is especially so where, as here, we are dealing with suits by one governmental body against another. See West Virginia v. United States, 479 U. S. 305, 309-312 (1987); Board of Comm’rs of Jackson Cty., 308 U. S., at 349-353.

There is nothing fair about awarding prejudgment interest as a remedy for the Compact’s breach when all available evidence suggests that the signatories to the Compact neither intended nor contemplated such an unconventional remedy. Many compacts between States are old; suits involving compacts concerning water rights are late in starting and are invariably long pending; and, because statutes of limitation or the doctrine of laches is rarely available to preclude the steady buildup of prejudgment interest, the amount of such interest can become quite large, as Kansas’ claim for approximately $41 million illustrates. See ante, at 9, n. 2. One would think that, particularly in such circumstances, even the most rudimentary conception of fairness would dictate that the Court ought not to interpret a contract between two States as exposing one of them to liability under a novel legal principle some 50 years later without some indication that the States might have contemplated such exposure in conjunction with the contractual rights and duties expressed in their compact. Contrary to the Court’s apparent belief, see ante, at 11 — 12, n. 4, nothing about such a contextualized historical approach would create an across-the-board incentive for the continued breach of interstate compacts entered into before 1987, especially given the prospect of large and uncertain damages awards. Had Kansas and Colorado anticipated or even suspected what the Court today effects, they almost certainly would have negotiated a provision in the Compact to address the situation. States in the future very likely will do so in the wake of the Court’s decision, which creates a very different backdrop from the one against which Kansas and Colorado operated. In the absence of such a provision, however, “the loss [as to interest] should remain where it has fallen.” Board of Comm'rs of Jackson Cty., supra, at 353; see Third Report § XI, at 101 (“Prejudgment interest here neither takes from those who benefitted, nor goes to those who were injured”).

For the foregoing reasons, I respectfully dissent from the Court’s award of pre judgment interest.

       That Amendment provides: “The Judicial power of the United States shall not be construed to extend to any suit in law or equity, commenced or prosecuted against one of the United States by Citizens of another State, or by Citizens or Subjects of any Foreign State.”

       Though final damages have not yet been calculated, the importance of this issue is illustrated by breaking down the damages claimed by Kansas. Of $62,369,173 in damages so claimed, $9,218,305 represents direct and indirect losses in actual dollars when the damage occurred. Of the remaining $53,150,868, about $12 million constitutes an adjustment for inflation (a type of interest that Colorado concedes is appropriate) while the remaining amount (approximately $41 million) represents additional interest intended to compensate for lost investment opportunities. Third Report of Special Master 87-88 (hereinafter Third Report). The magnitude of prejudgment interest ultimately awarded in this case will, of .course, turn on the date from which interest accrues. See Part III-B, infra.

       For sources from the early part of the century criticizing, qualifying, or rejecting the distinction, see, e. g., Faber v. New York, 222 N. Y. 255, 262, 118 N. E. 609, 610-611 (1918); Bernhard, v. Rochester German Ins. Co., 79 Conn. 388, 398, 65 A. 134, 138 (1906); Restatement of Contracts §337, p. 542 (1932); C. McCormick, Law of Damages §51, p. 210 (1935); 1 T. Sedgwick, Measure of Damages §315 (9th ed. 1912); cf. 3 S. Williston, Law of Contracts §1413, p. 2508 (1920) (“The disinclination to allow interest on claim of uncertain amount seems based on practice rather than theoretical grounds”). For a thorough modern treatment of the issue, see 1 D. Dobbs, Law of Remedies § 3.6(3) (2d ed. 1993).

       Justice O’Connor argues that the state of the law was insufficiently evolved by 1949 for Colorado to have had notice that the courts might award prejudgment interest if it violated its obligations under the Compact. See post, at 21-25 (opinion concurring in part and dissenting in part). Though the law was indeed in flux at that time, this Court had already made it clear that it put no stock in the traditional common-law prohibition, see Funkhouser v. J. B. Preston Co., 290 U. S. 163, 168 (1933), and had stated explicitly that such interest may accrue when “considerations of fairness” demand it, see Board of Comm’rs of Jackson Cty. v. United States, 308 U. S. 343, 352 (1939). The contemporary Restatement of Contracts was in accord. See Restatement of Contracts § 337(b), at 542 (“Where the contract that is broken [is not for a set or easily ascertainable amount of money], interest may be allowed in the discretion of the court, if justice requires it, on the amount that would have been just compensation if it had been paid when performance was due”). Under those circumstances, we think it is clear that, in 1949, an informed contracting party would not have concluded that an agreement’s silence on the issue deprived a reviewing court of the authority to award compensatory interest if the party willfully violated its contractual obligations. Moreover, under Justice O’Connor’s reasoning, States who entered into interstate compacts before 1987, see post, at 23, would retain a perpetual incentive to breach their contractual obligations and reap the benefits of such a breach with the full knowledge that the courts lack the authority to order a fully compensatory remedy.

       Justice O’Connor, Justice Scalia, and Justice Thomas would not allow any prejudgment interest. See post, at 20. Justice Kennedy and The Chief Justice are of the opinion that prejudgment interest should run from the date of the filing of the complaint. Justice Souter, Justice Ginsburg, Justice Breyer, and the author of this opinion agree with the Special Master’s view that interest should run from the time when Colorado knew or should have known that it was violating the Compact. In order to produce a majority for a judgment, the four Justices who agree with the Special Master have voted to endorse the position expressed in the text.

       As the Special Master noted, “Colorado experts did not dispute, in general, the linear relationship between [water usage] and crop yield.... However, they were of the view that the particular linear crop yield coefficients used by Kansas were not sufficiently reliable to determine the increase in yields that would have occurred if there had been no depletions of headgate deliveries to the [affected] lands.” Third Report 47. Colorado suggests that Kansas’ model, based as it is upon academic studies, does not adequately account for reductions in crop yield from such real-world conditions as “weather, disease, and pests.” Brief for Colorado 44, n. 12. But, as the Special Master correctly noted, Kansas’ experts reduced the predicted crop yield by 25% in order to account for such possibilities. Third Report 51 (“The 25% reduction was calculated to adjust the controlled experimental data to ‘realistic long-term type conditions’ in western Kansas, including high temperatures, winds, insects, and other stressful conditions”).

       Professor Whittlesey served for 20 years as a full professor and agricultural economist at Washington State University. His publications, many of which concern the kind of issues presented by this case, fill 14 pages on his curriculum vitae. See Kan. Exh. 891.

       We also agree with the Special Master’s decision to disregard the Colorado expert’s comparison of the numbers produced by Kansas’ model with numbers drawn from the literature on the various crops planted on the affected farmland. As Colorado admits, see Brief for Colorado 46, the water values in the literature were not based on “a ‘short-short rim’ situation, that is, an intra-seasonal transaction in which no capital costs were involved, and only additional harvesting and irrigation costs would be required.” Third Report 63. Because the circumstances in Kansas involved short-short run situations, and because such short-short run situations generally involve higher values for water, values derived from other contexts are of limited use in evaluating Kansas’ model. See ibid.


source: Caselaw Access Project

author: Justice Scalia Justice Stevens,

license: Public Domain

text:
    KYLLO v. UNITED STATES
    No. 99-8508.
    Argued February 20, 2001
    Decided June 11, 2001

      Scaua, J., delivered the opinion of the Court, in which Souter, Thomas, Ginsburg, and Breyer, JJ., joined. Stevens, J., filed a dissenting opinion, in which Rehnquist, C. J., and O’Connor and Kennedy, JJ., joined, post, p. 41.

      Kenneth Lerner, by appointment of the Court, 531 U. S. 955, argued the cause and filed briefs for petitioner.

      Deputy Solicitor General Dreeben argued the cause for the United States. With him on the brief were former Solicitor General Waxman, Assistant Attorney General Robinson, Irving L. Gornstein, and Deborah Watson

      Briefs of amici curiae Urging reversal were filed for the Liberty Project by Julie M. Carpenter; and for the National Association of Criminal Defense Lawyers et al. by James J. Tomkovicz, Lisa B. Kemler, and Steven R. Shapiro.

   Justice Scalia

This case presents the question whether the use of a thermal-imaging device aimed at a private home from a public street to detect relative amounts of heat within the home constitutes a “search” within the meaning of the Fourth Amendment.

I

In 1991 Agent William Elliott of the United States Department of the Interior came to suspect that marijuana was being grown in the home belonging to petitioner Danny Kyllo, part of a triplex on Rhododendron Drive in Florence, Oregon. Indoor marijuana growth typically requires high-intensity lamps. In order to determine whether an amount of heat was emanating from petitioner’s home consistent with the use of such lamps, at 3:20 a.m. on January 16,1992, Agent Elliott and Dan Haas used an Agema Thermovision 210 thermal imager to scan the triplex. Thermal imagers detect infrared radiation, which virtually all objects emit but which is not visible to the naked eye. The imager converts radiation into images based on relative warmth — black is cool, white is hot, shades of gray connote relative differences; in that respect, it operates somewhat like a video camera showing heat images. The scan of Kyllo’s home took only a few minutes and was performed from the passenger seat of Agent Elliott’s vehicle across the street from the front of the house and also from the street in back of the house. The scan showed that the roof over the garage and a. side wall of petitioner’s home were relatively hot compared to the rest of the home and substantially warmer than neighboring homes in the triplex. Agent Elliott concluded that petitioner was using halide lights to grow marijuana in his house, which indeed he was. Based on tips from informants, utility bills, and the thermal imaging, a Federal Magistrate Judge issued a warrant authorizing a search of petitioner’s home, and the agents found an indoor growing operation involving more than 100 plants. Petitioner was indicted on one count of manufacturing marijuana, in violation of 21 U. S. C. § 841(a)(1). He unsuccessfully moved to suppress the evidence seized from his home and then entered a conditional guilty plea.

The Court of Appeals for the Ninth Circuit remanded the case for an evidentiary hearing regarding the intrusiveness of thermal imaging. On remand the District Court found that the Agema 210 “is a non-intrusive device which emits no rays or beams and shows a crude visual image of the heat being radiated from the outside of the house”; it “did not show any people or activity within the walls of the structure”; “[t]he device used cannot penetrate walls or windows to reveal conversations or human activities”; and “[n]o intimate details of the home were observed.” Supp. App. to Pet. for Cert. 89-40. Based on these findings, the District Court upheld the validity of the warrant that relied in part upon the thermal imaging, and reaffirmed its denial of the motion to suppress. A divided Court of Appeals initially reversed, 140 F. 3d 1249 (1998), but that opinion was withdrawn and the panel (after a change in composition) affirmed, 190 F. 3d 1041 (1999), with Judge Noonan dissenting. The court held that petitioner had shown no subjective expectation of privacy because he had made no attempt to conceal the heat escaping from his home, id., at 1046, and even if he had, there was no objectively reasonable expectation of privacy because the imager “did not expose any intimate details of Kyllo’s life,” only “amorphous ‘hot spots’ on the roof and exterior wall,” id., at 1047. We granted certiorari. 530 U. S. 1305 (2000).

II

The Fourth Amendment provides that [t]he right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated.” “At the very core” of the Fourth Amendment “stands the right of a man to retreat into his own home and there be free from unreasonable governmental intrusion.” Silverman v. United States, 365 U. S. 505, 511 (1961). With few exceptions, the question whether a warrantless search of a home is reasonable and hence constitutional must be answered no. See Illinois v. Rodriguez, 497 U. S. 177, 181 (1990); Payton v. New York, 445 U. S. 573, 586 (1980).

On the other hand, the antecedent question whether or not a Fourth Amendment “search” has occurred is not so simple under our precedent. The permissibility of ordinary visual surveillance of a home used to be clear because, well into the 20th century, our Fourth Amendment jurisprudence was tied to common-law trespass. See, e.g., Goldman v. United States, 316 U. S. 129, 134-136 (1942); Olmstead v. United States, 277 U. S. 438, 464-466 (1928). Cf. Silverman v. United States, supra, at 510-512 (technical trespass not necessary for Fourth Amendment violation; it suffices if there is “actual intrusion into a constitutionally protected area”). Visual surveillance was unquestionably lawful because “‘the eye cannot by the laws of England be guilty of a trespass.’ ” Boyd v. United States, 116 U. S. 616, 628 (1886) (quoting Entick v. Carrington, 19 How. St. Tr. 1029, 95 Eng. Rep. 807 (K. B. 1765)). We have since decoupled violation of a person’s Fourth Amendment rights from trespassory violation of his property, see Rakas v. Illinois, 439 U. S. 128, 143 (1978), but the lawfulness of warrantless visual surveillance of a home has still been preserved. As we observed in California v. Ciraolo, 476 U. S. 207, 213 (1986), “[t]he Fourth Amendment protection of the home has never been extended to require law enforcement officers to shield their eyes when passing by a home on public thoroughfares.”

One might think that the new validating rationale would be that examining the portion of a house that is in plain public view, while it is a “search” despite the absence of trespass, is not an “unreasonable” one under the Fourth Amendment. See Minnesota v. Carter, 525 U. S. 83, 104 (1998) (Breyer, J., concurring in judgment). But in fact we have held that visual observation is no “search” at all— perhaps in order to preserve somewhat more intact our doctrine that warrantless searches are presumptively unconstitutional. See Dow Chemical Co. v. United States, 476 U. S. 227, 234-235, 239 (1986). In assessing when a search is not a search, we have applied somewhat in reverse the principle first enunciated in Katz v. United States, 389 U. S. 347 (1967). Katz involved eavesdropping by means of an electronic listening device placed on the outside of a telephone booth — a location not within the catalog (“persons, houses, papers, and effects”) that the Fourth Amendment protects against unreasonable searches. We held that the Fourth Amendment nonetheless protected Katz from the warrantless eavesdropping because he “justifiably relied” upon the privacy of the telephone booth. Id., at 353. As Justice Harlan’s oft-quoted concurrence described it, a Fourth Amendment search occurs when the government violates a subjective expectation of privacy that society recognizes as reasonable. See id., at 361. We have subsequently applied this principle to hold that a Fourth Amendment search does not occur — even when the explicitly protected location of a house is concerned — unless “the individual manifested a subjective expectation of privacy in the object of the challenged search,” and “society [is] willing to recognize that expectation as reasonable.” Ciraolo, supra, at 211. We have applied this test in holding that it is not a search for the police to use a pen register at the phone company to determine what numbers were dialed in a private home, Smith v. Maryland, 442 U. S. 735, 743-744 (1979), and we have applied the test on two different occasions in holding that aerial surveillance of private homes and surrounding areas does not constitute a search, Ciraolo, supra; Florida, v. Riley, 488 U. S. 445 (1989).

The present case involves officers on a public street engaged in more than naked-eye surveillance of a home. We have previously reserved judgment as to how much technological enhancement of ordinary perception from such a vantage point, if any, is too much. While we upheld enhanced aerial photography of an industrial complex in Dow Chemical, we noted that we found “it important that this is not an area immediately adjacent to a private home, where privacy expectations are most heightened,” 476 U. S., at 237, n. 4 (emphasis in original).

III

It would be foolish to contend that the degree of privacy secured to citizens by the Fourth Amendment has been entirely unaffected by the advance of technology. For example, as the cases discussed above make clear, the technology enabling human flight has exposed to public view (and hence, we have said, to official observation) uncovered portions of the house and its curtilage that once were private. See Ciraolo, supra, at 215. The question we confront today is what limits there are upon this power of technology to shrink the realm of guaranteed privacy.

The Katz test — whether the individual has an expectation of privacy that society is prepared to recognize as reasonable — has often been criticized as circular, and hence subjective and unpredictable. See 1 W. LaFave, Search and Seizure § 2.1(d), pp. 393-394 (3d ed. 1996); Posner, The Uncertain Protection of Privacy by the Supreme Court, 1979 S. Ct. Rev. 173, 188; Carter, supra, at 97 (Scalia, J., concurring). But see Rakas, supra, at 143-144, n. 12. While it may be difficult to refine Katz when the search of areas such as telephone booths, automobiles, or even the eurtilage and uncovered portions of residences is at issue, in the case of the search of the interior of homes — the prototypical and hence most commonly litigated area of protected privacy— there is a ready criterion, with roots deep in the common law, of the minimal expectation of privacy that exists, and that is acknowledged to be reasonable. To withdraw protection of this minimum expectation would be to permit police technology to erode the privacy guaranteed by the Fourth Amendment. We think that obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained without physical “intrusion into a constitutionally protected area,” Silverman, 365 U. S., at 512, constitutes a search— at least where (as here) the technology in question is not in general public use. This assures preservation of that degree of privacy against government that existed when the Fourth Amendment was adopted. On the basis of this criterion, the information obtained by the thermal imager in this ease was the product of a search.

The Government maintains, however, that the thermal imaging must be upheld because it detected “only heat radiating from the external surface of the house,” Brief for United States 26. The dissent makes this its leading point, see post, at 41, contending that there is a fundamental difference between what it calls “off-the-wall” observations and “through-the-wall surveillance.” But just as a thermal imager captures only heat emanating from a house, so also a powerful directional microphone picks up only sound emanating from a house — and a satellite capable of scanning from many miles away would pick up only visible light emanating from a house. We rejected such a mechanical interpretation of the Fourth Amendment in Katz, where the eavesdropping device picked up only sound waves that reached the exterior of the phone booth. Reversing that approach would leave the homeowner at the mercy of advancing technology— including imaging technology that could discern all human activity in the home. While the technology used in the present case was relatively crude, the rule we adopt must take account of more sophisticated systems that are already in use or in development. The dissent’s reliance on the distinction between “off-the-wall” and “through-the-wall” observation is entirely incompatible with the dissent’s belief, which we discuss below, that thermal-imaging observations of the intimate details of a home are impermissible. The most sophisticated thermal-imaging devices continue to measure heat “off-the-wall” rather than “through-the-wall”; the dissent’s disapproval of those more sophisticated thermal-imaging devices, see post, at 49, is an acknowledgment that there is no substance to this distinction. As for the dissent’s extraordinary assertion that anything learned through “an inference” cannot be a search, see post, at 44, that would validate even the “through-the-wall” technologies that the dissent purports to disapprove. Surely the dissent does not believe that the through-the-wall radar or ultrasound technology produces an 8-by-10 Kodak glossy that needs no analysis (i. e., the making of inferences). And, of course, the novel proposition that inference insulates a search is blatantly contrary to United States v. Karo, 468 U. S. 705 (1984), where the police “inferred” from the activation of a beeper that a certain can of ether was in the home. The police activity was held to be a search, and the search was held unlawful.

The Government also contends that the thermal imaging was constitutional because it did not “detect private activities occurring in private areas,” Brief for United States 22. It points out that in Dow Chemical we observed that the enhanced aerial photography did not reveal any “intimate details.” 476 U. S., at 238. Dow Chemical, however, involved enhanced aerial photography of an industrial complex, which does not share the Fourth Amendment sanctity of the home. The Fourth Amendment’s protection of the home has never been tied to measurement of the quality or quantity of information obtained. In Silverman, for example, we made clear that any physical invasion of the structure of the home, “by even a fraction of an inch,” was too much, 365 U. S., at 512, and there is certainly no exception to the warrant requirement for the officer who barely cracks open the front door and sees nothing but the non-intimate rug on the vestibule floor. In the home, our cases show, all details are intimate details, because the entire area is held safe from prying government eyes. Thus, in Karo, supra, the only thing detected was a can of ether in the home; and in Arizona v. Hicks, 480 U. S. 321 (1987), the only thing detected by a physical search that went beyond what officers lawfully present could observe in “plain view” was; the registration number of a phonograph turntable. These were intimate details because they were details of the home, just as was the detail of how warm — or even how relatively warm — Kyllo was heating his residence.

Limiting the prohibition of thermal imaging to “intimate details” would not only be wrong in principle; it would be impractical in application, failing to provide “a workable accommodation between the needs of law enforcement and the interests protected by the Fourth Amendment,” Oliver v. United States, 466 U. S. 170, 181 (1984). To begin with, there is no necessary connection between the sophistication of the surveillance equipment and the “intimacy” of the details that it observes — which means that one cannot say (and the police cannot be assured) that use of the relatively crude equipment at issue here will always be lawful. The Agema Thermovision 210 might disclose, for example, at what hour each night the lady of the house takes her daily sauna and bath — a detáil that many would consider “intimate”; and a much more sophisticated system might detect nothing more intimate than the fact that someone left a closet light on. We could not, in other words, develop a rule approving only that through-the-wall surveillance which identifies objects no smaller than 36 by 36 inches, but would have to develop a jurisprudence specifying which home activities are “intimate” and which are not. And even when (if ever) that jurisprudence were fully developed, no police officer would be able to know in advance whether his through-the-wall surveillance picks up “intimate” details— and thus would be unable to know in advance whether it is constitutional.

The dissent’s proposed standard — whether the technology offers the “functional equivalent of actual presence in the area being searched,” post, at 47 — would seem quite similar to our own at first blush. The dissent concludes that Katz was such a case, but then inexplicably asserts that if the same listening device only revealed the volume of the conversation, the surveillance would be permissible, post, at 49-50. Yet if, without technology, the police could not discern volume without being actually present in the phone booth, Justice Stevens should conclude a search has occurred. Cf. Karo, 468 U. S., at 735 (Stevens, J., concurring in part and dissenting in part) (“I find little comfort in the Court’s notion that no invasion of privacy occurs until a listener obtains some significant information by use of the device. ... A bathtub is a less private area when the plumber is present even if his back is turned”). The same should hold for the interior heat of the home if only a person present in the home could discern the heat. Thus the driving force of the dissent, despite its recitation of the above standard, appears to be a distinction among different types of information — whether the “homeowner would even care if anybody noticed,” post, at 50. The dissent offers no practical guidance for the application of this standard, and for reasons already discussed, we believe there can be none. The people in their houses, as well as the police, deserve more precision.

We have said that the Fourth Amendment draws “a firm line at the entrance to the house,” Payton, 445 U. S., at 590. That line, we think, must be not only firm but also bright— which requires clear specification of those methods of surveillance that require a warrant. While it is certainly possible to conclude from the videotape of the thermal imaging that occurred in this case that no “significant” compromise of the homeowner’s privacy has occurred, we must take the long view, from the original meaning of the Fourth Amendment forward.

“The Fourth Amendment is to be construed in the light of what was deemed an unreasonable search and seizure when it was adopted, and in a manner which will conserve public interests as well as the interests and rights of individual citizens.” Carroll v. United States, 267 U. S. 132, 149 (1925).

Where, as here, the Government uses a device that is not in general public use, to explore details of the home that would previously have been unknowable without physical intrusion, the surveillance is a “search” and is presumptively unreasonable without a warrant.

Since we hold the Thermovision imaging to have been an unlawful search, it will remain for the District Court to determine whether, without the evidence it provided, the search warrant issued in this case was supported by probable cause — and if not, whether there is any other basis for supporting admission of the evidence that the search pursuant to the warrant produced.

* * *

The judgment of the Court of Appeals is reversed; the case is remanded for further proceedings consistent with this opinion.

It is so ordered.

Justice Stevens,

with whom The Chief Justice, Justice O’Connor, and Justice Kennedy join, dissenting.

There is, in my judgment, a distinction of constitutional magnitude between “ through-the-wall surveillance” that gives the observer or listener direct access to information in a private area, on the one hand, and the thought processes used to draw inferences from information in the public domain, on the other hand. The Court has crafted a rule that purports to deal with direct observations of the inside of the home, but the case before us merely involves indirect deductions from “off-the-wall” surveillance, that is, observations of the exterior of the home. Those observations were made with a fairly primitive thermal imager that gathered data exposed on the outside of petitioner’s home but did not invade any constitutionally protected interest in privacy. Moreover, I believe that the supposedly “bright-line” rule the Court has created in response to its concerns about future technological developments is unnecessary, unwise, and inconsistent with the Fourth Amendment.

There is no need for the Court to craft a new rule to decide this case, as it is controlled by established principles from our Fourth Amendment jurisprudence. One of those core principles, of course, is that “searches and seizures inside a home without a warrant are presumptively unreasonable.”1 Payton v. New York, 445 U. S. 573, 586 (1980) (emphasis added). But it is equally well settled that searches and seizures of property in plain view are presumptively reasonable. See id., at 586-587. Whether that property is residential or commercial, the basic principle is the same: ‘“What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection.’ ” California v. Ciraolo, 476 U. S. 207, 213 (1986) (quoting Katz v. United States, 389 U. S. 347, 351 (1967)); see Florida v. Riley, 488 U. S. 445, 449-450 (1989); California v. Greenwood, 486 U. S. 35, 40-41 (1988); Dow Chemical Co. v. United States, 476 U. S. 227, 235-236 (1986); Air Pollution Variance Bd. of Colo. v. Western Alfalfa Cory., 416 U. S. 861, 865 (1974). That is the principle implicated here.

While the Court “take[s] the long view” and decides this case based largely on the potential of yet-to-be-developed technology that might allow “through-the-wall surveillance,” ante, at 38-40; see ante, at 36, n. 3, this case involves nothing more than off-the-wall surveillance by law enforcement officers to gather information exposed to the general public from the outside of petitioner’s home. All that the infrared camera did in this case was passively measure heat emitted from the exterior surfaces of petitioner’s home; all that those measurements showed were relative differences in emission levels, vaguely indicating that some areas of the roof and outside walls were warmer than others. As still images from the infrared scans show, see Appendix, infra, no details regarding the interior of petitioner’s home were revealed. Unlike an x-ray scan, or other possible “through-the-wall” techniques, the detection of infrared radiation emanating from the home did not accomplish “an unauthorized physical penetration into the premises,” Silverman v. United States, 365 U. S. 505, 509 (1961), nor did it “obtain information that it could not have obtained by observation from outside the curtilage of the house,” United States v. Karo, 468 U. S. 705, 715 (1984).

Indeed, the ordinary use of the senses might enable a neighbor or passerby to notice the heat emanating from a building, particularly if it is vented, as was the case here. Additionally, any member of the public might notice that one part of a house is warmer than another part or a nearby building if, for example, rainwater evaporates or snow melts at different rates across its surfaces. Such use of the senses would not convert into an unreasonable search if, instead, an adjoining neighbor allowed an officer onto her property to verify her perceptions with a sensitive thermometer. Nor, in my view, does such observation become an unreasonable search if made from a distance with the aid of a device that merely discloses that the exterior of one house, or one area of the house, is much warmer than another. Nothing more occurred in this case.

Thus, the notion that heat emissions from the outside of a dwelling are a private matter implicating the protections of the Fourth Amendment (the text of which guarantees the right of people “to be secure in their ... houses” against unreasonable searches and seizures (emphasis added)) is not only unprecedented but also quite difficult to take seriously. Heat waves, like aromas that are generated in a kitchen, or in a laboratory or opium den, enter the public domain if and when they leave a building. A subjective expectation that they would remain private is not only implausible but also surely not “one that society is prepared to recognize as ‘reasonable.’ ” Katz, 389 U. S., at 361 (Harlan, J., concurring).

To be sure, the homeowner has a reasonable expectation of privacy concerning what takes place within the home, and the Fourth Amendment’s protection against physical invasions of the home should apply to their functional equivalent. But the equipment in this case did not penetrate the walls of petitioner’s home, and while it did pick up “details of the home” that were exposed to the public, ante, at 38, it did not obtain “any information regarding the interior of the home,” ante, at 34 (emphasis added). In the Court’s own words, based on what the thermal imager “showed” regarding the outside of petitioner’s home, the officers “concluded” that petitioner was engaging in illegal activity inside the home. Ante, at 30. It would be quite absurd to characterize their thought processes as “searches,” regardless of whether they inferred (rightly) that petitioner was growing marijuana in his house, or (wrongly) that “the lady of the house [was taking] her daily sauna and bath.” Ante, at 38. In either case, the only conclusions the officers reached concerning the interior of the home were at least as indirect as those that might have been inferred from the contents of discarded garbage, see California v. Greenwood, 486 U. S. 35 (1988), or pen register data, see Smith v. Maryland, 442 U. S. 735 (1979), or, as in this case, subpoenaed utility records, see 190 F. 3d 1041, 1043 (CA9 1999). For the first time in its history, the Court assumes that an inference can amount to a Fourth Amendment violation. See ante, at 36-37.

Notwithstanding the implications of today’s decision, there is a strong public interest in avoiding constitutional litigation over the monitoring of emissions from homes, and over the inferences drawn from such monitoring. Just as “the police cannot reasonably be expected to avert their eyes from evidence of criminal activity that could have been observed by any member of the public,” Greenwood, 486 U. S., at 41, so too public officials should not have to avert their senses or their equipment from detecting emissions in the public domain such as excessive heat, traces of smoke, suspicious odors, odorless gases, airborne particulates, or radioactive emissions, any of which could identify hazards to the community. In my judgment, monitoring such emissions with “sense-enhancing technology,” ante, at 34, and drawing useful conclusions from such monitoring, is an entirely reasonable public service.

On the other hand, the countervailing privacy interest is at best trivial. After all, homes generally are insulated to keep heát in, rather than to prevent the detection of heat going out, and it does not seem to me that society will suffer from a rule requiring the rare homeowner who both intends to engage in uncommon activities that produce extraordinary amounts of heat, and wishes to conceal that production from outsiders, to make sure that the surrounding area is well insulated. Cf. United States v. Jacobsen, 466 U. S. 109, 122 (1984) (“The concept of an interest in privacy that society is prepared to recognize as reasonable is, by its very nature, critically different from the mere expectation, however well justified, that certain facts will not come to the attention of the authorities”). The interest in concealing the heat escaping from one’s house pales in significance to “the chief evil against which the wording of the Fourth Amendment is directed,” the “physical entry of the home,” United States v. United States Dist. Court for Eastern Dist. of Mich., 407 U. S. 297, 313 (1972), and it is hard to believe that it is an interest the Framers sought to protect in our Constitution.

Since what was involved in this case was nothing more than drawing inferences from off-the-wall surveillance, rather than any “through-the-wall” surveillance, the officers’ conduct did not amount to a search and was perfectly reasonable.

II

Instead of trying to answer the question whether the use of the thermal imager in this case was even arguably unreasonable, the Court has fashioned a rule that is intended to provide essential guidance for the day when “more sophisticated systems” gain the “ability to 'see’ through walls and other opaque barriers.” Ante, at 36, and n. 3. The newly minted rule encompasses “obtaining [1] by sense-enhancing technology [2] any information regarding the interior of the home [3] that could not otherwise have been obtained without physical intrusion into a constitutionally protected area ... [4] at least where (as here) the technology in question is not in general public use.” Ante, at 34 (internal quotation marks omitted). In my judgment, the Court’s new rule is at once too broad and too narrow, and is not justified by the Court’s explanation for its adoption. As I have suggested, I would not erect a constitutional impediment to the use of sense-enhancing technology unless it provides its user with the functional equivalent of actual presence in the area being searched.

Despite the Court’s attempt to draw a line that is “not only firm but also bright,” ante, at 40, the contours of its new rule are uncertain because its protection apparently dissipates as soon as the relevant technology is “in general public use,” ante, at 34. Yet how much use is general public use is not even hinted at by the Court’s opinion, which makes the somewhat doubtful assumption that the thermal imager used in this .case does not satisfy that criterion. In any event, putting aside its lack of clarity, this criterion is somewhat perverse because it seems likely that the threat to privacy will grow, rather than recede, as the use of intrusive equipment becomes more readily available.

It is clear, however, that the category of “sense-enhancing technology” covered by the new rule, ibid., is far too broad. It would, for example, embrace potential mechanical substitutes for dogs trained to react when they sniff narcotics. But in United States v. Place, 462 U. S. 696, 707 (1983), we held that a dog sniff that “discloses only the presence or absence of narcotics” does “not constitute a ‘search’ within the meaning of the Fourth Amendment,” and it must follow that sense-enhancing equipment that identifies nothing but illegal activity is not a search either. Nevertheless, the use of such a device would be unconstitutional under the Court’s rule, as would the use of other new devices that might detect the odor of deadly bacteria or chemicals for making a new type of high explosive, even if the devices (like the dog sniffs) are “so limited both in the manner in which” they obtain information and “in the content of the information” they reveal. Ibid. If nothing more than that sort of information could be obtained by using the devices in a public place to monitor emissions from a house, then their use would be no more objectionable than the use of the thermal imager in this case.

The application of the Court’s new rule to “any information regarding the interior of the home,” ante, at 34, is also unnecessarily broad. If it takes sensitive equipment to detect an odor that identifies criminal conduct and nothing else, the fact that the odor emanates from the interior of a home should not provide it with constitutional protection. See supra, at 47 and this page. The criterion, moreover, is too sweeping in that information “regarding” the interior of a home apparently is not just information obtained through its walls, but also information concerning the outside of the building that could lead to (however many) inferences “regarding” what might be inside. Under that expansive view, I suppose, an officer using an infrared camera to observe a man silently entering the side door of a house at night carrying a pizza might conclude that its interior is now occupied by someone who likes pizza, and by doing so the officer would be guilty of conducting an unconstitutional “search” of the home.

Because the new rule applies to information regarding the “interior” of the home, it is too narrow as well as too broad. Clearly, a rule that is designed to protect individuals from the overly intrusive use of sense-enhancing equipment should not be limited to a home. If such equipment did provide its user with the functional equivalent of access to a private place — such as, for example, the telephone booth involved in Katz, or an office building — then the rule should apply to such an area as well as to a home. See Katz, 389 U. S., at 351 (“[T]he Fourth Amendment protects people, not places”).

The final requirement of the Court’s new, rule, that the information “could not otherwise have been obtained without physical intrusion into a constitutionally protected area,” ante, at 34 (internal quotation marks omitted), also extends too far as the Court applies it. As noted, the Court effectively treats the mental process of analyzing data obtained from external sources as the equivalent of a physical intrusion into the home. See supra, at 44. As I have explained, however, the process of drawing inferences from data in the public domain should not be characterized as a search.

The two reasons advanced by the Court as justifications for the adoption of its new rule are both unpersuasive. First, the Court suggests that its rule is compelled by our holding in Katz, because in that case, as in this, the surveillance consisted of nothing more than the monitoring of waves emanating from a private area into the public domain. See ante, at 35. Yet there are critical differences between the cases. In Katz, the electronic listening device attached to the outside of the phone booth allowed the officers to pick up the content of the conversation inside the booth, making them the functional equivalent of intruders because they gathered information that was otherwise available only to someone inside the private area; it would be as if, in this case, the thermal imager presented a view of the heat-generating activity inside petitioner’s home. By contrast, the thermal imager here disclosed only the relative amounts of heat radiating from the house; it would be as if, in Katz, the listening device disclosed only the relative volume of sound leaving the booth, which presumably was discernible in the public domain. Surely, there is a significant difference between the general and well-settled expectation that strangers will not have direct access to the contents of private communications, on the one hand, and the rather theoretical expectation that an occasional homeowner would even care if anybody noticed the relative amounts of heat emanating from the walls of his house, on the other. It is pure hyperbole for the Court to suggest that refusing to extend the holding of Katz to this case would leave the homeowner at the mercy of “technology that could discern all human activity in the home.” Ante, at 35-36.

Second, the Court argues that the permissibility of “through-the-wall surveillance” cannot depend on a distinction between observing “intimate details” such as “the lady of the house [taking] her daily sauna and bath,” and noticing only “the nonintimate rug on the vestibule floor” or “objects no smaller than 36 by 36 inches.” Ante, at 37, 38-39. This entire argument assumes, of course, that the thermal imager in this case could or did perform “through-the-wall surveillance” that could identify any detail “that would previously have been unknowable without physical intrusion.” Ante, at 39-40. In fact, the device could not, see n. 1, supra, and did not, see Appendix, infra, enable its user to identify either the lady of the house, the rug on the vestibule floor, or anything else inside the house, whether smaller or larger than 36 by 36 inches. Indeed, the vague thermal images of petitioner’s home that are reproduced in the Appendix were submitted by him to the District Court as part of an expert report raising the question whether the device could even take “accurate, consistent infrared images” of the outside of his house. Defendant’s Exh. 107, p. 4. But even if the device could reliably show extraordinary differences in the amounts of heat leaving his home, drawing the inference that there was something suspicious occurring inside the residence — a conclusion that officers far less gifted than Sherlock Holmes would readily draw — does not qualify as “through-the-wall surveillance,” much less a Fourth Amendment violation.

Ill

Although the Court is properly and commendably concerned about the threats to privacy that may flow from advances in the technology available to the law enforcement profession, it has unfortunately failed to heed the tried and true counsel of judicial restraint. Instead of concentrating on the rather mundane issue that is actually presented by the case before it, the Court has endeavored to craft an all-encompassing rule for the future. It would be far wiser to give legislators an unimpeded opportunity to grapple with these emerging issues rather than to shackle them with prematurely devised constitutional constraints.

I respectfully dissent.

[Appendix to opinion of Stevens, J., follows this page.]

APPENDIX TO OPINION OF STEVENS, J.

(Images and text reproduced from defendant’s exhibit 107)

Top left: Infrared image of a video frame from the videotape submitted as evidence in this case. The thermogram indicates the suspect house as it appeared with the Gain and contrast in its default setting. Only the outline of the house is visible. The camera used was the Thermovision 210.

•Top Right: Infrared image of a subsequent videoframe taken from the videotape. The gain and contrast settings have been increased in order to make the walls and roof of the structure appear hotter than what it actually is.

Bottom Left: Infrared image of the opposite side of the suspects house. The thermogram is also taken from the same videotape. The camera settings are in the default mode and the outline of the house is barely visible. Only the hot electrical transformer and the street light are identifiable. Bottom Right: The same image, but with the gain and contrast increased. This change in camera settings cause any object to appear hotter than what it actually is. The arrow indicates the overloading of an area immediately around a hot object in this case the electrical transformer and the streetlight. This overloading of the image is an inherent design flaw in the camera itself.

       When the Fourth Amendment was adopted, as now, to “search” meant “[t]o look over or through for the purpose of finding something; to explore; to examine by inspection; as, to search the house for a book; to search the wood for a thief.” N. Webster, An American Dictionary of the English Language 66 (1828) (reprint 6th ed. 1989).

       The dissent’s repeated assertion that the thermal imaging did not obtain information regarding the interior of the home, post, at 43,44 (opinion of Stevens, J.), is simply inaccurate. A thermal imager reveals the relative heat of various rooms in the home. The dissent may not find that information particularly private or important, see post, at 43-44, 45, 49-50, but there is no basis for saying it is not information regarding the interior of the home. The dissent’s comparison of the thermal imaging to various circumstances in which outside observers might be able to perceive, without technology, the heat of the home — for example, by observing snowmelt on the roof, post, at 43 — is quite irrelevant. The fact that equivalent information could sometimes be obtained by other means does not make lawful the use of means that violate the Fourth Amendment. The police might, for example, learn how many people are in a particular house by setting up year-round surveillance; but that does not make breaking and entering to find out the same information lawful. In any event, on the night of January 16, 1992, no outside observer could have discerned the relative heat of Kyllo’s home without thermal imaging.

       The ability to “see” through walls and other opaque barriers is a clear, and scientifically feasible, goal of law enforcement research and development. The National Law Enforcement and Corrections Technology Center, a program within the United States Department of Justice, features on its Internet Website projects that include a “Radar-Based Through-the-Wall Surveillance System,” “Handheld Ultrasound Through the Wall Surveillance,” and a “Radar Flashlight” that “will enable law enforcement officers to detect individuals through interior building walls.” www.nlectc.org/techproj/ (visited May 3, 2001). Some devices may emit low levels of radiation that travel “through-the-wall,” but others, such as more sophisticated thermal-imaging devices, are entirely passive, or “off-the-wall” as the dissent puts it.

       The dissent asserts, post, at 44-45, n. 3, that we have misunderstood its point, which is not that inference insulates a search, but that inference alone is not a search. If we misunderstood the point, it was only in a good-faith effort to render the point germane to the case at hand. The issue in this case is not the police’s allegedly unlawful inferencing, but their allegedly unlawful thermal-imaging measurement of the emanations from a house. We say such measurement is a search; the dissent says it is not, because an inference is not a search. We took that to mean that, since the technologically enhanced emanations had to be the basis of inferences before anything inside the house could be known, the use of the emanations could not be a search. But the dissent certainly knows better than we what it intends. And if it means only that an inference is not a search, we certainly agree. That has no bearing, however, upon whether hi-tech measurement of emanations from a house is a search.

       The Government cites our statement in California v. Ciraolo, 476 U. S. 207 (1986), noting apparent agreement with the State of California that aerial surveillance of a house’s curtilage could become ‘“invasive”’ if ‘“modern technology’” revealed “‘those intimate associations, objects or activities otherwise imperceptible to police or fellow citizens.’” Id., at 215, n. 3 (quoting Brief for State of California 14-15). We think the Court’s focus in this secondhand dictum was not upon intimacy but upon otherwise-imperceptibility, which is precisely the principle we vindicate today.

       The dissent argues that we have injected potential uncertainty into the constitutional analysis by noting that whether or not the technology is in general public use may be a factor. See post, at 47. That quarrel, however, is not with us but with this Court’s precedent. See Ciraolo, supra, at 215 (“In an age where private and commercial flight in the public airways is routine, it is unreasonable for respondent to expect that his marijuana plants were constitutionally protected from being observed with the naked eye from an altitude of 1,000 feet”). Given that we can quite confidently say that thermal imaging is not “routine,” we decline in this case to reexamine that factor.

       After an evidentiary hearing, the District Court found:
      “[T]he use of the thermal imaging device here was not an intrusion into Kyllo’s home. No intimate details of the home were observed, and there was no intrusion upon the privacy of the individuals within the home. The device used cannot penetrate walls or windows to reveal conversations or human activities. The device recorded only the heat being emitted from the home.” Supp. App. to Pet. for Cert. 40.

       Thus, for example, we have found consistent with the Fourth Amendment, even absent a warrant, the search and seizure of garbage left for collection outside the curtilage of a home, California v. Greenwood, 486 U. S. 85 (1988); the aerial surveillance of a fenced-in backyard from an altitude of 1,000 feet, California v. Ciraolo, 476 U. S. 207 (1986); the aerial observation of a partially exposed interior of a residential greenhouse from 400 feet above, Florida v. Riley, 488 U. S. 445 (1989); the aerial photography of an industrial complex from several thousand feet above, Dow Chemical Co. v. United States, 476 U. S. 227 (1986); and the observation of smoke emanating from chimney stacks, Air Pollution Variance Bd. of Colo. v. Western Alfalfa Corp., 416 U. S. 861 (1974).

       Although the Court credits us with the “novel proposition that inference insulates a search,” ante, at 36, our point simply is that an inference cannot be a search, contrary to the Court’s reasoning. See supra this page. Thus, the Court’s use of United States v. Karo, 468 U. S. 705 (1984), to refute a point we do not make underscores the fact that the Court has no real answer (either in logic or in law) to the point we do make. Of course, Karo itself does not provide any support for the Court’s view thát inferences can amount to unconstitutional searches. The illegality in that case was “the monitoring of a beeper in a private residence” to obtain information that “could not have [been] obtained by observation from outside,” id,., at 714-715, rather than any thought processes that flowed from such monitoring.

       This view comports with that of all the Courts of Appeals that have resolved the issue. See 190 F. 3d 1041 (CA9 1999); United States v. Robinson, 62 F. 3d 1325 (CA11 1995) (upholding warrantless use of thermal imager); United States v. Myers, 46 F. 3d 668 (CA7 1995) (same); United States v. Ishmael, 48 F. 3d 850 (CA5 1995) (same); United States v. Pinson, 24 F. 3d 1056 (CA8 1994) (same). But see United States v. Cusumano, 67 F. 3d 1497 (CA10 1995) (warrantless use of thermal imager violated Fourth Amendment), vacated and decided on other grounds, 83 F. 3d 1247 (CA10 1996) (en banc).

       The record describes a device that numbers close to a thousand manufactured units; that has a predecessor numbering in the neighborhood of 4,000 to 5,000 units; that competes with a similar product numbering from 5,000 to 6,000 units; and that is “readily available to the public” for commercial, personal, or law enforcement purposes, and is just an 800-number away from being rented from “half a dozen national companies” by anyone who wants one. App. 18. Since, by virtue of the Court’s new rule, the issue is one of first impression, perhaps it should order an evidentiary hearing to determine whether these facts suffice to establish “general public use.”

       The use of the latter device would be constitutional given Smith v. Maryland, 442 U. S. 785, 741 (1979), which upheld the use of pen registers to record numbers dialed on a phone because, unlike “the listening device employed in Katz . . . pen registers do not acquire the contents of communications.”