Testimony and Knowledge:
Contrasting and Evaluating the Humean and Reidian Models
According to David Hume, “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 eye-witnesses and spectators.”[i] The majority of a person’s knowledge is based on second-hand evidence (I read in a book that
PART I: Hume’s Theory of Testimony
Although Hume was creating his theory of testimony in Section 10 of his Enquiry Concerning Human Understanding to decide if it is reasonable to believe in religious miracles, we can apply his theory to all testimonial evidence. Where Hume wrote, “That no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavors to establish…”[ii] we can interpret this as saying that no testimony is sufficient on its own to show the truth of any proposition unless it is more likely that the speaker is telling the truth than it is that the proposition is false. Here Hume asks us to weigh the credibility of the witness against the probability of the proposition’s being true.
Hume suggests that the credibility of a witness with respect to the proposition can be supported or denied by our observational evidence of this witness’s track record. Where an honest, competent witness will serve as a credible witness, “A man delirious, or noted for falsehood and villainy, has no manner of authority with us.”[iii] If no track record is available for this individual, we may also look at past experiences of witnesses similar to this person. For example, if Channel 5 News hired a new meteorologist, we would tend to take her weather reports with the same authority (or lack of authority) as the previous one as she is associated with all trained experts on weather prediction.
At the other end of the scale, we must weigh the probability of the proposition’s being true. We consider our past experiences or world view with respect to the proposition.[iv] The proposition’s plausibility is proportional with how it seems to jive with our world view. For example, the proposition that water freezes at 32 degrees is very plausible for a native Minnesotan, but almost unthinkable for Hume’s Indian prince who had never observed weather below the freezing point.[v]
Now that we have both ends of the scale—the credibility of the witness on one end and the plausibility of the proposition on the other—let us see it in action using Hume’s example of miracles. Imagine that one of Christ’s prophets tells you that he saw Jesus rise from the dead. Since this person claims to be a first-hand witness, and being associated with Christianity’s commandment that thou shalt not lie, you could consider this prophet to be credible. Consider, however, that the proposition that a person rose from the dead is against your world view of nature—that which dies remains to be dead. Since the scale is already tipped too far toward the improbability of the proposition, you have trouble believing in this proposition no matter how credible the prophet seems to be.
To sum up Hume’s theory of testimony, given (1) that a person A testifies a proposition p to B, we balance credibility against implausibility and believe p based on (1) only if c > i.
Part II: Reid’s Theory of Testimony
............Where Hume demands that we have evidence of a witness’s credibility before believing in a testified proposition, Thomas Reid reverses the burden of proof by suggesting that it is reasonable for someone to believe in a testified proposition unless the believer has good reason to think that the witness is not credible. In other words, we are reasonable to trust everyone until we have reason not to. This view is rooted in Reid’s belief that human beings are naturally predisposed (by principles “implanted in us by the Supreme Being”) to tell the truth, to seek guidance from others, and to trust each other.[vi] Since we are naturally credulous, deception is an exception to the rule. So it follows, according to Reid, that testimony should be believed unless it is proven to be the false exception.
It should also be noted that the concept of the believer’s world view can also be applied to Reid’s model. For an example, let us go back to the newly hired meteorologist on Channel 5 news. The Reidian would believe her first forecast since there is no reason not to (remember that the Humean believes her because of evidence acquired by observing meteorologists as whole). Now imagine that the meteorologist says that tomorrow it will rain cats and dogs… literally. Because this claim violently contradicts his world view, even the Reidian could justify not believing the forecast as the claim itself gives reason to consider the witness incredulous.
To sum up Reid’s theory of testimony, given (1) that a person A testifies a proposition p to B, B is justified in believing p unless B has reason to believe that A is not credulous with respect to p, or that p is implausible given B’s world view.
Part III: Critical Assessment of Both Theories
One of the major criticisms of Hume’s testimony theory is that it may lead us to shut our eyes to important, truthful information when we do not have the means to evaluate the witness’s credibility. If you are a stranger in a strange land or a child, your world view and experience with others is so minimal that you cannot judge if a witness is credible—nor can you weigh the plausibility of the claim itself. It would seem that a Humean in this case could never be justified in believing anybody while a Reidian would be justified until he has reasons not to. While this may be true, let us observe the possible consequences of each theory. Imagine a Humean and Reidian traveling as strangers in a strange land. If both are approached by a scoundrel who has some magic beans to sell, the Humean would demand evidence that the beans are indeed magic, that the seller has credentials, testimonials from other buyers, etc. The Reidian would assert that the seller has a natural propensity to be credulous. Since she is a stranger in a land where magic beans could exist, and she has no reason to doubt the credibility of the seller—she buys the beans.
What about a simple need of testimony? Imagine our travelers need directions to the strange land’s museum of philosophy (strange land, indeed!). While the Reidian can immediately accept the local’s directions, would the Humean miss out on the trip to the museum since he has no one to trust? For these simple issues it is important to note that while Hume demanded the look-before-you-leap approach to accepting testimony, he also noted that belief is not a bipolar, off/on issue: “A wise man, therefore, proportions his belief to the evidence… All probability, then, supposes an opposition of experiments and observations; where the one side is found to overbalance the other, and to produce a degree of evidence proportioned to the superiority.”[vii] This means that we have degrees of belief that are proportional to the evidence that we already have. While getting directions, the fact that the local is a local could serve as enough evidence that he can provide directions to the museum. (He may also observe physical “tells” regarding the local’s confidence while giving the directions). The Humean may not trust this stranger with 100% certainty, but he could estimate that he has an 80% chance of being a helpful citizen and that outweighs the 20% chance that he’s a liar or an incompetent. Thus, the Humean will have the same shot at getting to the museum as the Reidian.
In the case of a child who does not have the cognitive means to weigh these probabilities and degrees of belief, Hume’s critics are correct when they suggest that a child is never justified to believe testimony under the Humean model. I would argue, however, that this is not a serious knock against Hume. If a Reidian criticized my theory of testimony by saying it did not accurately model the mind of a child, I would say, “Good!” as I observe that children readily believe in Easter Bunnies, Santa, and almost anything else they hear. It is important, though, to note that children are not doomed to always lack this justification. An older child may start to be justified in believing testimony after creating a world view and a list of trustworthy witnesses based on a few years of trial and error experience.
In “Egoism in Epistemology,”[viii] Richard Foley asks us to consider two types of testimony. The first is the type we’ve been dealing with so far: (1): A testifies that p to B. The second is a sort of self-testimony: (2): B testifies that p to B (I see the rock so I tell myself the rock exists). Taking Reid’s lead that witnesses have a fundamental authority—that the witness is to be believed simply in virtue of (1) itself[ix]—Foley argues that it should be just as (prima facie) reasonable for B to believe in p in the first case as it is when B is self-testifying. Logically, his argument could look something like this:
A) It is prima facie reasonable for B to believe p given (2)
B) If (A), it is prima facie reasonable for B to believe p given (1)
C) Therefore, it is prima facie reasonable for B to believe p given (1)[x]
A Humean would argue against premise B’s claim that equates a person’s self-testimony to testimony from a second party (citing derivative authority in (1)—that B has no reason to believe p without evidence that A is credible[xi]) while the Reidian cites fundamental authority and sees no difference between premises A and B. One merely has to point to the possibility that in (1), the witness could be incompetent, insincere, or have bad intentions to disallow premise B and refute Foley’s argument. Foley would call those who cannot accept this argument epistemic egoists—or those who trust themselves more than they trust others—but based on Hume’s claim that there is no human weakness worse than credulity, I think Hume would not take the term ‘epistemic egoist’ to be derogatory.
.............Lastly, it is important to show that social conventions may serve as evidence to support a witness’s credibility when no other evidence about this witness’s track record is available. This is important as it seems that the greatest criticism against the Humean theory of testimony is that without this evidence, the Humean seems to be agnostic about all testimony. Notice that our courts establish severe punishments to those who are caught lying while under oath. This social convention allows jurors to accept the testimonies of the witnesses without any previous knowledge of their track records or competence. We also have the social convention of frowning upon liars and cheats (and make it illegal when property or health is at stake), so we may use this convention as evidence that a stranger will most likely tell the truth when asked for simple testimony. This differs from the Reidian approach as Reid believes this urge to tell the truth is an innate property of human behavior installed by God,[xii] while Hume believes that it is constructed socially for a community’s greater good.[xiii]
Part IV: Conclusion in Support Of Hume
...............Hume’s critics have claimed that his theory is too exclusive and hinders us from reasonably believing in important information when we cannot access the witness’s credibility. I have shown that this credibility can be supported from other sources such as social conventions or implied expertise (e.g.: getting directions from a local). Also, using Hume’s balance theory, we have a sound model for balancing the probability of a testimony’s truth against the witness’s credibility. If the local tells us that the museum of philosophy is in an Atlantis-like enclosure underneath the ocean, we slide our scale towards the improbable and ask a different local. If the local tells us that the museum is two blocks north, we are reasonable to believe him—and justified to walk those two blocks. Reid’s theory lacks Hume’s street smarts. There are too many thieves, liars, and incompetents in our world to accept his believe-others-as-you’d-believe-yourself model. This is why we are ‘sworn in’ in court; this is why psychologists display their credentials; and this is why we need virus protection for internet browsing. Hume’s model can serve as an efficient firewall as we sift through this information age.
[i] David Hume, An Enquiry Concerning Human Understanding, Tom L. Beauchamp, ed. (Oxford University Press, 1999), P.170
[ii] Hume, P.174
[iii] Hume, P. 171
[iv] Hume, P. 170-1
[v] Hume, P. 172
[vi] Michael Root, “Hume On The Virtues of Testimony,” (American Philosophical Quarterly, Jan 2001, Vol. 38, No.1) P 26-7
[vii] Hume, 170
[viii] Richard Foley, “Egoism in Epistemology,” From Socializing Epistemology, Frederick F. Schmitt, ed., (Rowman & Littlefield, 1994)
[ix] C.A.J Coady, A Philosophical Study, (Oxford University Press, 1992) p.23
[x] While this argument is in not Foley’s own words, it is taken from Root’s lecture notes based on Foley’s article sited in viii.
[xi] Coady, p. 22
[xii] Root, p. 26-7,
[xiii] Root, p. 21-23
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