concurring in the judgment.
I agree with the Court's ultimate conclusion that there is a fundamental difference, for purposes of the exclusionary rule, between live-witness testimony and other types of evidence. I perceive this distinction to be so' fundamental, however, that I would not prevent a factfinder from hearing and considering the relevant statements of any witness, except perhaps under the most remarkable of circumstances — although none such have ever been postulated that would lead me to exclude the testimony of a live witness.
*281To appreciate this position,, it is essential to bear in mind the purported justification for employing the exclusionary rule in a Fourth Amendment context: deterrence of official misconduct. See Stone v. Powell, 428 U. S. 465, 486 (1976); United States v. Janis, 428 U. S. 433, 458-459, n. 35 (1976). As an abstract intellectual proposition this can be buttressed by a plausible rationale since there is at least some comprehensible connection — albeit largely and dubiously speculative — between the exclusion of evidence and the deterrence of intentional illegality on the part of a police officer.1 But if that is the purpose of the rule, it seems to me that the appropriate inquiry in every case in which a defendant seeks the exclusion of otherwise admissible and reliable evidence is whether official conduct in reality will be measurably altered by taking such a course.
On the facts of this case the Court is, of course, correct in holding that the “[application of the exclusionary rule in this situation could not have the slightest deterrent effect on the behavior of an officer such as Biro.” Ante, at 280. Reaching this result, however, requires no judicial excursion into an area about which “philosophers have been able to argue endlessly,” 2 namely, the degree of “free will” excercised by a person when engaging in an act such as speaking.
In the history of ideas many thinkers have maintained with persuasion that there is no such thing as “free will,” in the sense that the term implies the independent ability of an actor to regulate his or her conduct. Others have steadfastly maintained the opposite, arguing that the human personality is one innately free to choose among alternatives. Still a third group *282would deny that the very term “free will” has coherent meaning. These are only a few of the many perspectives on a subject which lies at the core of our intellectual and religious heritage. While this ancient debate will undoubtedly continue, “society and the law have no choice in the matter. We must proceed ... on the scientifically unprovable assumption that human beings make choices in the regulation of their conduct and that they are influenced by society’s standards as well as by personal standards.” Blocker v. United States, 110 U. S. App. D. C. 41, 53, 288 F. 2d 853, 865 (1961) (Burger, J., concurring in result). Mr. Justice Jackson expressed this in Gregg Cartage & Storage Co. v. United States, 316 U. S. 74, 80 (1942): “[T]he practical business of government and administration of the law is obliged to proceed on more or less rough and ready judgments based on the assumption that mature and rational persons are in control of their own conduct.” And in Steward Machine Co. v. Davis, 301 U. S. 548, 590 (1937), Mr. Justice Cardozo put it thus: “Till now the law has been guided by a robust common sense which assumes the freedom of the will as a working hypothesis in the solution of its problems.”
We are nonetheless cognizant of the fact that this assumption must continually confront the inherent practical obstacle of one person’s being unable to know with certainty the content of another’s mind. We cross this barrier daily, of course, in the process of determining criminal culpability.3 Yet in criminal trials we are willing to bear the risk of error — substantially diminished by the requirement of proof beyond a reasonable doubt — in order to effectuate the common-law tradition of *283imposing punishment only upon those who can be said to be morally responsible for their acts. There is no analogue to this concern, however, in the area of Fourth Amendment exclusion, which has an admitted pragmatic purpose — based as I suggested on speculative hypotheses which ought to lead us to apply it with reasoned discrimination, not as an automatic response. In short, the results achieved from current exclusionary rule standards are bizarre enough without steering the analysis in the direction of areas which offer no reasonable hope of a comprehensible framework for inquiry.
It would be obvious nonsense to postulate that during his brief encounter in the florist shop Officer Biro was making a painstaking analysis of the extent to which Lois Hennessey’s “free will” would affect her disposition to testify against respondent at some future point. It is one thing to engage in scholastic hindsight, particularly as the dissent has done here, in which speculation proceeds from unfounded hypotheses as to the probable explanations for the decision of a live witness to come forward and testify. But it is quite another to suppose that the police officer, assuming he is contemplating illegal action, will, or would be able to, engage in a similar inquiry.
There are several reasons which support this analysis, which, I might add, is found acceptable in every other legal system in the world. Initially, I would point out that the concept of effective deterrence assumes that the police officer consciously realizes the probable consequences of a presumably impermissible course of conduct. The officer must be cognizant of at least the possibility that his actions — because of possible suppression — will undermine the chances of convicting a known criminal. I strongly suspect that in the vast majority of instances in this setting the officer accused of a Fourth Amendment violation will not be even remotely aware of the existence of a witness, as for example, where seizure of an item of evidence guides official inquiry to an eye*284witness. Of course, an officer conducting a search later held illegal may have some hope that his inquiry will lead to persons who can come forward with testimony. It is not plausible, however, that a police officer would consciously engage in illegal action simply to gain access to a witness, knowing full well that under prevailing legal doctrine the result will be the certain exclusion of whatever tangible evidence might be found.4
Even if we suppose that the officer suspects that his illegal actions will produce a lead to a witness, he faces the intractable problem of understanding how valuable that person will be to his investigation. As one philosopher has aptly stated the matter, “[t]he freedom of the will consists in the impossibility of knowing actions that still lie in the future.” L. Wittgenstein, Tractatus Logico-Philosophicus ¶ 5.1362 (Pears & McGuinness trans. 1961). In Smith v. United States, 117 U. S. App. D. C. 1, 3-4, 324 F. 2d 879, 881-882 (1963), cert. denied, 377 U. S. 954 (1964), this point was applied to the case of a live witness testifying under oath:
“The proffer of a living witness is not to be mechanically equated with the proffer of inanimate evidentiary objects illegally seized. The fact that the name of a potential witness is disclosed to police is of no evidentiary significance, per se, since the living witness is an individual human personality wffiose attributes of will, perception, memory and volition interact to determine what *285testimony he will give. The uniqueness of this human process distinguishes the evidentiary character of a [living] witness from the relative immutability of inanimate evidence.” (Emphasis added.) (Footnotes omitted.)
It can, of course, be argued, that the prospect of finding a helpful witness may play some role in a policeman’s decision to be indifferent about Fourth Amendment procedures. The answer to this point, however, is that we have never insisted on employing the exclusionary rule whenever there is some possibility, no matter how remote, of deterring police misconduct. Rather, we balance the cost to society of losing perfectly competent evidence against the prospect of incrementally enhancing Fourth Amendment values. See, e. g., Stone, 428 U. S., at 486; United States v. Calandra, 414 U. S. 338, 350-351 (1974); Alderman v. United States, 394 U. S. 165, 174-175 (1969).
Using this approach it strikes me as evident that the permanent silencing of a witness — who, after all, is appearing under oath — is not worth the high price the exclusionary rule exacts. Any rule of law which operates to keep an eyewitness to a crime — a murder, for example — from telling the jury what that person saw has a rational basis roughly comparable to the primitive rituals of human sacrifice.
I would, therefore, resolve the case of a living witness on a per se basis, holding that such testimony is always admissible, provided it meets all other traditional evidentiary requirements. At very least this solution would alleviate the burden — now squarely thrust upon courts — of determining in each instance whether the witness possessed that elusive quality characterized by the term “free will.”
Mr. Justice Marshall, with whom Mr. Justice Brennan joins, dissenting.While “reaffirm [ing]” the holding of Wong Sun v. United States, 371 U. S. 471, 485 (1963), that verbal evidence, like *286physical evidence, may be “fruit of the poisonous tree,” the Court today “significantly qualifies]” Wong Sun’s further conclusion, id., at 486, that no “ 'logical distinction’ ” can be drawn between verbal and physical evidence for purposes of the exclusionary rule. Ante, at 275. In my view, the distinction that the Court attempts to draw cannot withstand close analysis. To extend “a time-worn metaphor,” Harrison v. United States, 392 U. S. 219, 222 (1968), I do not believe that the same tree, having its roots in an unconstitutional search or seizure, can bear two different kinds of fruit, with one kind less susceptible than the other of exclusion on Fourth Amendment grounds. I therefore dissent.
The Court correctly states the question before us: whether the connection between the police officer’s concededly unconstitutional search and Hennessey’s disputed testimony was “so attenuated as to dissipate the taint,” Nardone v. United States, 308 U. S. 338, 341 (1939). See ante, at 274. In resolving questions of attenuation, courts typically scrutinize the facts of the individual case, with particular attention to such matters as the “temporal proximity” of the official illegality and the discovery of the evidence, “the presence of intervening circumstances,” and “the purpose and flagrancy of the official misconduct.” Brown v. Illinois, 422 U. S. 590, 603-604 (1975). The Court retains this general framework, but states that “[a]ttenuation analysis” should be “concerned with the differences between live-witness testimony and inanimate evidence.” Ante, at 278-279. The differences noted by the Court, however, have to a large extent already been accommodated by current doctrine. Where they have not been so accommodated, it is because the differences asserted are either illusory or of no relevance to the issue of attenuation.
One difference mentioned by the Court is that witnesses, unlike inanimate objects, “can, and often do, come forward and offer evidence entirely of their own volition.” Ante, at 276. Recognition of this obvious fact does nothing to advance *287the attenuation inquiry. We long ago held that, if knowledge of evidence is gained from a source independent of police illegality, the evidence should be admitted. Silverthorne Lumber Co. v. United States, 251 U. S. 385, 392 (1920) (Holmes, J.). This “independent source” rule would plainly apply to a witness whose identity is discovered in an illegal search but who later comes to the police for reasons unrelated to the official misconduct. In the instant case, however, as the Court recognizes, ante, at 273, there is a “ 'straight and uninterrupted' ” road between the illegal search and the disputed testimony.
Even where the road is uninterrupted, in some cases the Government may be able to show that the illegally discovered evidence would inevitably have come to light in the normal course of a legal police investigation. Assuming such evidence is admissible — a proposition that has been questioned, Fitzpatrick v. New York, 414 U. S. 1050 (1973) (White, J., dissenting from denial of certiorari) — this “inevitable discovery” rule would apply to admit the testimony of a witness who, in the absence of police misconduct, would have come forward “entirely of [his or her] own volition.” Again, however, no such situation is presented by this case, since the Court accepts the findings of the two lower courts that Hennessey's testimony would not inevitably have been discovered. Ante, at 273.
Both the independent-source and inevitable-discovery rules, moreover, can apply to physical evidence as well as to verbal evidence. The police may show, for example, that they learned from an independent source, or would inevitably have discovered through legal means, the location of an object that they also knew about as a result of illegal police activity. It may be that verbal evidence is more likely to have an independent source, because live witnesses can indeed come forward of their own volition, but this simply underscores the degree to which the Court's approach involves a form of judicial “double counting.” The Court would apparently first *288determine whether the evidence stemmed from an independent source or would inevitably have been discovered; if neither of these rules was found to apply, as here, the Court would still somehow take into account the fact that, as a general proposition (but not in the particular case), witnesses sometimes do come forward of their own volition.
The Court makes a related point that “[t]he greater the willingness of the witness to freely testify, . . . the smaller the incentive to conduct an illegal search to discover the witness.” Ante, at 276. The somewhat incredible premise of this statement is that the police in fact refrain from illegal behavior in which they would otherwise engage because they know in advance both that a witness will be willing to testify and that he or she “will be discovered by legal means.” Ibid. This reasoning surely reverses the normal sequence of events; the instances must be very few in which a witness’ willingness to testify is known before he or she is discovered. In this case, for example, the police did not even know th'at Hennessey was a potentially valuable witness, much less whether she would be willing to testify, prior to conducting the illegal search. See ante, at 279-280. When the police are certain that a witness “will be discovered by legal means,” ante, at 276 — if they ever can be certain about such a fact — they of course have no incentive to find him or her by illegal means, but the same can be said about physical objects that the police know will be discovered legally.
The only other point made by the Court is that exclusion of testimony “perpetually disable [s] a witness from testifying about relevant and material facts.”, Ante, at 277. The “perpetual ... disable [ment]” of which the Court speaks, however, applies as much to physical as to verbal evidence. When excluded, both types of evidence are lost for the duration of the particular trial, despite their being “relevant and material . . . [and] unrelated ... to the purpose of the originally *289illegal search.” Ibid. Moreover, while it is true that “often” the exclusion of testimony will be very costly to society, cmte, at 278, at least as often the exclusion of physical evidence— such as heroin in a narcotics possession case or business records in a tax case — will be as costly to the same societal interests. But other, more important societal interests, see Brown v. Illinois, 422 U. S., at 599-600; Wong Sun v. United States, 371 U. S., at 486, have led to the rule, which the Court today reaffirms, that “fruits of the poisonous tree” must be excluded despite their probative value, unless the facts of the case justify a finding of sufficient attenuation.
The facts of this case do' not justify such a finding. Although, as the Court notes, ante, at 272; see ante, at 279, four months elapsed between the illegal search and the FBI’s first contact with Hennessey, the critical evidence was provided at the time and place of the search, when the police officer questioned Hennessey and she identified respondent, ante, at 270. The time that elapsed thereafter is of no more relevance than would be a similar time period between the discovery of an object during an illegal search and its later introduction into evidence at trial. In this case, moreover, there were no intervening circumstances between Hennessey’s statement at the time of the search and her later testimony. She did not come to the authorities and ask to testify, despite being a student of police science; an FBI agent had to go to her home and interrogate her. Ante, at 272.
Finally, whatever the police officer’s purpose in the flower shop on the day of the search, the search itself. was not even of arguable legality, as was conceded by the Government below. 542 F. 2d 136, 140 n. 5 (CA2 1976). It is also undisputed that the shop had been under surveillance as part of an ongoing gambling investigation in which the local police force had actively participated; its participation included interception of at least one of respondent’s telephone conversations *290in the very month of the search. Ante, at 271-272, and n. 2. Under all of the circumstances, the connection here between the official illegality and the disputed testimony cannot be deemed “so attenuated as to dissipate the taint.” The District Court therefore properly excluded the testimony.
I would affirm the judgment of the Court of Appeals.
Empirically speaking, though, I have the gravest doubts as to whether the exclusion of evidence, in and of itself, has any direct appreciable effect on a policeman’s behavior in most situations — emergency actions in, particular. See Bivens v. Six Unknown Fed. Narcotics Agents, 403 U. S. 388, 416-417,426-427 (1971) (Burger, C. J., dissenting).
J. Sartre, Being and Nothingness 433 (Barnes trans. 1956).
A somewhat similar hurdle is presented in civil cases, which may rest decision on the standard of a “reasonable man’s” actions. In those circumstances we assume that a person is ordinarily capable of conforming conduct to an objective standard of reasonableness. Consequently, while the assumption is indulged that the person possesses control over his actions, there is generally no need to inquire into mental processes as such.
Perhaps a case might arise in which the police conducted a search only for the purpose of obtaining the names of witnesses. In such a circumstance it is possibly arguable that the exclusion of any testimony gained as a result of the search would have an effect on official behavior. This clearly did not occur here, nor can I conceive of many instances in which it would. In any event, the decision to exclude such testimony should depend on the officers’ motivation and not on the “free will” of the witnesses. I would not want to speculate, however, as to whether such an unlikely case would justify modifying a per se approach to this general problem.