^Suppose we grant that testimony is an extremely important source of knowledge. We’re then faced with the question of how the beliefs we form on the basis of testimony achieve the level of knowledge. Someone tells you something, or you read something, and you believe it. Is that enough to count as knowledge if what you believe turns out to be true? How does that work?
^One explanation was given by David Hume:
b
[A]s the evidence, derived from witnesses and human testimony, is founded on past experience, so it varies with the experience, and is regarded either as a proof or a probability, according as the conjunction between any particular kind of report and any kind of object has been found to be constant or variable.^In other words, it is not enough simply to be told something or to read something in order to have good evidence. Instead, if you believe something on the basis of someone’s testimony, you have to support your belief on the basis of an argument.
^This may seem surprising, given that the last time we encountered Hume he was arguing that inductive reasoning never provides us with actual reasons for our beliefs.
However, when it comes to testimony, Hume suggests that the evidence we derive from testimony is simply a form of inductive inference.
^Suppose you receive testimony from Tim. Hume thinks that you ought to reason according to something like the following basic structure:
1 Tim, who has certain attributes that are relevant to you taking him seriously in this kind of situation, gives you some piece of information.
2 Generally, when speakers who are similar to Tim—and in similar types of situations—give you some piece of information, their reports are reliable.
3 Thus, (probably) the information Tim has given you is likely true.
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S ay you’re a detective and you’re trying to gather information about the location of a suspect at the time a crime occurred. You ask the suspect’s secretary, who doesn’t particularly like the suspect and who has no knowledge of when the crime occurred, and she gives you information about the suspect’s activities, from which you gather that the suspect was nowhere near the location of the crime at the time the crime occurred.
In this case, the relevant attributes of the testifier include her lack of motivation to lie to give an alibi to the suspect as well as her lack of knowledge about the time for which the suspect would need an alibi. In a situation like this one, you can reasonably assume that the secretary is telling the truth and conclude that the suspect probably has a strong alibi for the time in question.
When you consider examples like this, Hume’s view about testimony sounds pretty strong.
^Because Hume requires that you have positive reasons, in the form of an argument, for accepting that testimony as evidence for knowledge, we can call his view inferentialist non-presumptivism. It’s inferentialist because you need to support your acceptance of testimony by means of an inductive inference. And it’s
truth; you have to have positive reasons for accepting someone’s testimony if you’re going to rely on that testimony as evidence for knowledge.
^Even though Hume’s view is one of the most popular positions historically about the nature of evidence that
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^Pretty soon after Hume proposed the theory, other philosophers noted problems with it. One of the strongest critics of Hume’s inferentialism was philosopher Thomas Reid, who raised a number of criticisms of Hume’s theory of testimony—of which we will focus on two.
] Reid criticizes Hume for getting the phenomenology of believing on the basis of testimony wrong. Normally, when someone tells you something and you don’t have a particular reason not to believe the person, then you just do believe them, without rehearsing any sort of argument in support of your belief.
] Reid asks us to consider how children learn from the people around them and suggests that children aren’t capable of using inductive inference to support their reliance on other people’s testimony. Not only do they lack the reasoning skills, but they also lack the experience needed to have a basis for an inductive argument.
If Hume were correct, Reid says, you would expect children to be the most mistrustful of testimony, while the most knowledgeable,
most experienced people would be the most trusting of testimony. As Reid notes, however, this is not the case. Children are quite willing to accept the testimony of the people around them.
^Reid defends a rival theory about the way that testimony provides support for knowledge. Unfortunately, it is not entirely clear what theory Reid advances to replace Hume’s. We’ll consider two options.
^Both rival theories to Hume share one big similarity: They assert that, at least sometimes, you can know something on the basis of someone’s testimony even if you aren’t aware of any positive reasons in support of your belief of that person’s testimony.
In other words, they reject Hume’s requirement that you can’t know something on the basis of testimony unless you rehearse positive reasons for accepting that testimony.
^Where the two challengers to Hume differ, however, is in their explanation for why they reject Hume’s positive reasons requirement.
EXTERNALISM
^One motivation for rejecting Hume’s positive reasons requirement comes from externalism. An externalist theory of knowledge on the basis
of testimony would allow that if you are in circumstances in which the testimony you’re receiving is a reliable source of information, you can
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acquire knowledge on the basis of that testimony as long as the reason you believe it is that your acceptance of the testimony involves a reliable method.
^Most externalist theories of testimonial knowledge think that the phrase “your acceptance of the testimony involves a reliable method”
involves at least two components.
] You can’t have any positive reason to question the circumstances or the speakers. If you’re in a used car showroom and you know that the salesperson earns most of his or her money from commissions, then you shouldn’t automatically trust the salesperson when he or she says the car you’re interested in has never been in any major accidents.
] You also have some set of unconscious abilities that monitor the trustworthiness or honesty of your sources of information. In face-to-face communication, this would include signals the speaker gives off that you might not even be
consciously aware of but that are supposed to alert you if your brain is triggered by anything suspicious.
Let’s call this the unconscious monitoring requirement.
O ur culture is full of evidence that many of us at least intuitively accept the idea of an unconscious monitoring requirement. For example, you might read an interview with a policeman who caught a serial killer during a routine traffic stop and perhaps the policeman says, “Something just seemed off about the guy when I stopped him.”
I
n our discussions of perception and memory, we’ve seen that externalists allow for the possibility that we might not know when our perceptual abilities are reliable but that we can still know information we acquire using those abilities.For the sort of externalist theories we’ve been considering, it is enough that the methods we use to acquire information are in fact reliable. Those theories don’t require that we also have good evidence that they are reliable.
Theories of Knowledge LECTURE 17 Knowledge derived from Testimony
^An advantage of the externalist take on testimony is that it can treat testimony as completely on par with perception or memory. As with those sources of knowledge, we are not always aware when we are reliable or what the mechanism is that ensures we are reliable. What is important in all cases, for the externalist, is the fact that we are reliable.
^Hume’s non-presumptivist inferentialist theory differs from externalism, then, in one very important respect: It requires you to have a positive argument in support of your acceptance of someone’s
testimony. In contrast, the externalist theory only requires that someone’s testimony actually is a reliable source of information if you are going to rely on that testimony.
^However, like Hume, the externalist also thinks you have to be sensitive to relevant features of the testimony that serve as indicators of its reliability.
It’s just that the externalist thinks that you can be sensitive to these features unconsciously and that you don’t have to use them to formulate an explicit argument in support of your acceptance of a given piece of testimony.