"There is no species of reasoning more common, more useful, and even necessary to human life, than that which is derived from the testimony of men, and the reports of eyewitnesses and spectators." David Hume (An Enquiry Concerning Human Understanding, Pg. 74)
The conditions that have to be met for us to gain knowledge from the testimony of others are the same conditions that must be met for us to gain knowledge from any other source of belief. There must be adequate grounds to support our judgement that that our belief is in fact true.
Consider the traditional "Justified True
Belief"definition of knowledge:-
[K1] (A knows that p) iff ( (p is true) &
 (A believes that p) &
 (A is justified in believing that p))
Applying this concept of knowledge to
the case of testimony, we have:-
[K2] (A knows that p on the basis of testimony from B) iff
( (p is true) &
 (A believes that p) &
 (A is suitably justified in believing that p
on the basis of testimony from B))
This then gives us a start for identifying the conditions for us to gain knowledge from testimony. Because this definition of knowledge requires that p be true, if B is testifying falsely then A can not gain knowledge. Even if A comes to believe that p on the basis of B's testimony, A would acquire a false belief and not knowledge that p. So the first condition that must be met for us to gain knowledge from the testimony of others, is that when B testifies that p, p must be true.
In other words, to be informed by testimony is to believe that what the testimony asserts is true. And this belief can turn out to be knowledge when the testimony is in fact true. It should be noted in passing that knowledge obtained purely by testimony is "thin". What A comes to believe from the testimony of B is simply that p. More specifically A does not learn the reasons that B may have for believing that p is true. A need not even understand p. So A does not inherit the justification that B may have that qualifies B's belief as knowledge. The question therefore becomes -- "Under what conditions can A be suitably justified in believing that p on the basis of the testimony of B?" For this we need a concept of "suitable justification".
Traditional theories of epistemological justification maintain that gaining knowledge from any source (other than Foundationalism's self-evident awareness, not an issue here) must be a matter of inductive inference. This is called the "prior grounding" model of justification. To elevate A's belief that p into A's knowledge that p, A must have sufficient supporting evidence to make it likely that p.
In the case of testimony, however, it would initially seem obvious that A would almost never have any evidence beyond B's testimony. Consider your situation when you read a newspaper article. You have no evidence supporting the truth of what the reporter has told you, and no way of checking the reporter's history of veracity. Our informants are human, fallible, and with complex interests responsive to other things besides truth. We all recognize that they are sensitive to their own personal conception of self interest. Hence, such reasoning would suggest, A would (normally) have no suitable justification for believing that p just on the basis of the testimony of B.
But this kind of thinking focuses too closely on a single individual instance of a transmission of p from B to A. The foreground focus of attention highlights the plight of A, who must rely upon the word of B without benefit of knowing him/her. Whereas it is the background, which this sort of intuitive thinking leaves out of focus, that supplies the enormous supportive foundation for A's belief in B's veracity. We need to view any single act of testimony as but one iteration of an ongoing process that takes place through time. Any one instance of B's assertion that p is a single frame in an ever evolving process which has shaped and guided our own participation in it. This grounding, being implicit in our linguistic and social practices, provides our suitable justification for accepting testimony. As long as there is no reason to suspect such abnormal features as mistake, delusion, or deception, B can conventionally be presumed not only to be asserting what he believes, but also to be communicating the truth. By testimony, B gives us not only a piece of his mind, but a glimpse of the world as he knows it.
Many thinkers on this issue call this prior presumption of adequate grounding a matter of "trust" -- suggesting that testimony only succeeds in transmitting knowledge if there is trust (by A of B's veracity). But I think that this is to ignore the major role of language in causing people to have beliefs, and in generating knowledge. We expect communicators to be cooperative, not as the outcome of a statistical weighing, but as a presupposition of fruitful linguistic and social exchange. This expectation constitutes our testimonial "trust" -- it is a necessary part of employing language that hearers take speakers as believing what they assert is true.
Behind the hearers' response is a vast history of largely (although certainly not completely) successful communication. Cooperation between speaker and hearer is almost always the enlightened rational choice. The liar, in order to succeed in his lie, at least requires us to understand him. And that understanding relies on a history of proper (correct) use of language. Statements get their meaning from the standard practice of intending and taking them to be true. McDowell draws attention to this feature of communication by reminding us of the evolutionary survival value of the capacity to spread important news by making meaningful signals(1). The tail flash of the white-tailed deer at the sight of a prowling wolf is surely not intended to signal the flasher's intention to alarm, but simply to pass on the beneficial results of one deer's perceptual knowledge to other individuals. Like both perception and inference, testimony sometimes turns out to be unreliable. But if the fallibility of our senses does not annul the reliability of perception, and the fallibility of inductive inference can be met with a naturalistic or pragmatic response, it would be logically inconsistent to refuse the award of "knowledge" to true beliefs based on testimony simply because some understandable utterances could be false.
Certainly, there is no doubting that A is at risk if B's testimony is false. What matters, however, is A's perception of the relative magnitude of the risks involved. From the fact that A goes along with B's testimony, it does not follow that A accepts ("believes") it. First, if A does not need to rely upon the information immediately, A has little incentive to question it. Second, even if A does rely on it immediately, this still does not require A's unqualified endorsement. If A must act upon B's testimony, and if the success of that act depends upon the truth of that testimony, then A is often immediately able to confirm (or disconfirm) both the asserted facts and the reliability of B as informant. Consider, for example, how you respond to the advice of others according to the magnitude of the decision you need to make, and your perception of the expertise of the advisor. Getting directions to the mall is one thing. Buying a house another. Your standards of what constitute "suitable justification" for knowing that B's testimony is accurate will change dramatically. The "trust" that we extend for testimony is not directed at the character of the informant. It is directed at the reliability of this single instance of testimony, within an informationally rich background context of similar linguistic exchanges and social constraints on truthfulness. I am not disrespecting you, nor doubting your word, when I check on your claim that this house is free from significant defect. Nor do I have specific reason to doubt you. What I have is both an unusually high cost if you are wrong, and the belief that there are limits on even an expert's good judgment on these matters.
What also needs to be considered is that B is also at risk if the testimony is false. There are powerful social constraints on B to be truthful and reliable. The force of these constraints vary, of course, according to such factors as the community's sensitivity to deception or error, the costs to A once an error is detected, and the rapidity and extent of communication about these findings. Although such behavioural constraints are not usually viewed as "suitably justifying" evidence, because we are not normally consciously aware of them, they do retain the crucial mark of evidence - in the presence of these constraints, it is much more likely that p is true than otherwise.
Moreover, our presuppositions about the likely veracity of our informants are not uniform. There is room for A to filter B's testimony through the rest of A's belief-set. The alternatives are not simply acceptance (belief that p) or rejection (belief that not-p) of the testimony. We bestow different degrees of belief in testimony. Testimony is in this sense "impure". Its believability varies depending upon what we already know about similar cases and how risk-significant the information is. It is obvious that we do draw all manner of distinctions among sources of testimony. Consider how differently we treat a news story if the source is a grocery store tabloid versus the New York Times, or it is gossip about some starlet versus a local weather report.
For all that there is unquestionably adequate grounds (normally) for assuming the veracity of B, there is one consideration that tells against the "prior grounding" model of justification. The epistemic responsibilities implied by a requirement for an inductively supported belief become "too absurdly enormous to be discharged by a single individual with limited time, expertise, and cognitive equipment."(2) We do normally have this enormous background set of beliefs supporting the acceptance of B's testimony. Such an enormous background, in fact, that we generally do not initially consider B's credibility. On the contrary, we believe implicitly in the truth of testimony, unless we have positive grounds for doubt or disbelief. This simple fact argues against the Coherence account of justification, and for the Contextualist account.
The position of the "default and challenge" approach to justification is that any assertion is creditworthy until shown otherwise; whereas the "prior grounding" approach requires that specific evidence for its reliability. Coherentism adopts the "prior grounding" model of justification because it demands that p must cohere with the rest of A's beliefs set in order for p to qualify as knowledge for A. Contextualism, on the other hand, adopts the "default and challenge" model of justification and grants that A is prima facie justified in believing that p simply in virtue of understanding that B testifies that p. From the Contextualist understanding, therefore, there is no necessity that A be consciously aware of, or even implicitly consider, the "absurdly enormous" amount of background grounding we have been discussing. (Which is not to suggest, I should emphasize, that the necessary grounding does not need to exist.)
Finally, lets consider for a moment
Nozick's truth-tracking theory of knowledge. Nozick's Subjunctive Conditional
definition of knowledge can function as a check on our discussion so far because
it is an entirely externalist view of knowledge, and is therefore completely
insulated against any of the problems inherent in an internalist conception of
"suitable justification". Adapting Nozick's theory to our discussion of
knowledge from testimony would mean that --
[K3] (A knows that p on the basis of testimony from B) iff
( (p is true) &
 (A believes that p) &
[3a] (if p were false, A would not believe it) &
[3b] (if p were true under other circumstances,
A would still believe it))
If we assume (as we established in the beginning) that B intends to be truthful with his testimony, then it is clear that A would know p based on B's testimony because (3a) if p were false, B would not testify that p, and A would then not come to believe that p; and (3b) if p were true under other circumstances, B would still testify that p and A would still come to believe that p. This means that our internalist discussion of the suitable justification for A gaining knowledge that p based on the testimony of B is consistent with Nozick's externalist understanding of knowledge.
Whether our belief based on testimony counts as "knowledge" is ultimately a subjective evaluation. Along that continuum between "silly wild-assed guess" and "certain knowledge", will be a line drawn. To one side, we maintain that our belief has inadequate epistemic grounding, and to the other side we maintain that our belief qualifies as "knowledge". Close to the line, we will be uncertain which is which. Further from the line, we will have a great deal of confidence in which is which. But the choice of where a particular belief falls along that continuum relative to the location of that line, is largely a subjective evaluation.
Whether A judges that p qualifies as knowledge will depend on the confidence that A has about the ceteris paribus conditions surrounding B's assertion. Whether we judge that p qualifies as knowledge for A will depend on the confidence that we have about the ceteris paribus conditions surrounding B's assertion. Three conditions constrain that degree of confidence. The first is the (explanatory) coherence of p with the rest of A's (or our) belief set (at least the contextually relevant portion thereof). The second condition is the past performance of the particular source of information -- either B specifically, or the institution that guarantees B (if there is one). And in the absence of any specific prior information on that matter, we adopt the prima facie presumption of general reliability inherent in all linguistic exchanges. The third condition is A's (or our) perception of the relative risks involved. The more A has to lose, or the more that B has to gain from p being false, the more stringent need be the epistemic standards applied in validating the grounds for believing p.
The "suitability" condition on "justification" remains simply one of coherence. The "best explanation" for why B asserts that p is, ceteris paribus, that B believes it for duly responsible reasons (B knows that p). Once testimony is accepted, there are many sources which come into play to supply additional grounds relevant to the warrant for belief. What Hume affirmed is that we normally enter the setting of testimony with a large range of well-founded beliefs (derived from a similar source, "experience"), which provides a basis to test or assess any new testimony. We approach the information imparted by testimony with a vast background of knowledge about the reliability of communication in general, supplemented (usually) by beliefs that B has nothing to gain and something to lose through error or deception. We do not have to establish the separate trustworthiness of B; it is enough if no evidence to the contrary is already available.
(1) McDowell, John. Meaning, Communication, and Knowledge, Philosophical Subjects: Essays Presented to P.F.Strawson, Zak Van Straaten (ed.), Oxford University Press, New York, New York. 1980.
(2) Chakrabarti, Arindam. On Knowing by Being Told, Philosophy East and West, Vol. 42, No. 3. (Jul., 1992), pg 430
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