I have one point about this claim you make: "Similarly, even if NCB is otherwise trusting of CB, he shouldn't accept CB's reports about color!"
One of the conditions of the case is that NCB does not know that CB is colorblind. If he did, then of course NCB should not trust CB's judgments about color. Similarly, if Jim knew that Susan couldn't distinguish between Judy and Trudy, then he shouldn't (completely) trust her assertions about Judy. However, both CB and Susan are generally reliable testifiers, so there is a strong prima facie presumption to trust what they assert.
I think you're right, but I don't know whether your argument will be persuasive; the people with Graham's intuition might very well also have the corresponding intuition in the color case. Not being such a person, I can't say for sure.
I agree with your objection, but I think there might be a way of modifying it slightly to make Graham's counterexample work:
Suppose that, for some reason, Jim knows that Trudy could not have broken the vase. Say, for instance, that he knows she has a severe phobia or allergic reaction to vases and never would pick one up in the first place... but Susan, not knowing Trudy at all, doesn't know this. It seems to me that it might make sense in that situation to say that Susan has far too little information to have real knowledge that Judy broke the vase (even though she believes so based on what she has seen enough to report that to Jim), whereas Jim does have enough information to say that he "knows" Judy broke the vase.
Of course, in that case, Jim doesn't "know that p on the basis of S's testimony", since he knows it inferentially - on the basis of S's testimony he knows that either Judy or Trudy broke the vase (maybe that's testimony plus inference too), and then on the basis of this and his other knowledge, he infers that Judy broke the vase.
But yeah, I agree with the other people that I modus ponens-ed where Graham modus tollens-ed. I think your example makes it somewhat clearer, because instead of one of these unlikely defeaters (an unknown twin) we have a perfectly ordinary defeater (color).
In the original article, Jim does know that Trudy was in the other room. But, as easwaran usefully points out, for it to be testimonial knowledge per se, it cannot be the result of an inference from that knowledge. Graham thinks that it can function as a restorer for Jim without being an inference and with all the epistemic merit of the belief deriving from Jim's trust in the testifier. I think he's wrong about that, so I tried to redescribe the case without that component. What is clear is that Graham wants Jim's knowledge to be based on the testimony and not on any inference.
Ah, I see your and easwaran's point. I guess I'm pretty skeptical about being able to separate pure testimonial knowledge from inferential knowledge. I think any time someone listens to a testimony, they automatically interpret it in the context of the knowledge they already possess. (Some inferences are made just in deciphering the meaning of the statements made, for instance.) So it's hard to say what knowledge is gained on the basis of the testimony versus inference. But other than that, I agree. To the extent that it makes sense to talk about purely testimonial knowledge, it looks like the original principle holds up.
Might the problem with Graham's counterexample be that it is too coarse-grained when it comes to "trustworthiness?" Ie, Susan might be generally trustworthy and Jim might trust her in general, but with respect to distinguishing between Judy and Trudy Susan is not trustworthy (because she cannot tell them apart). So because she is not trustworthy/reliable in this respect, Jim cannot come to know anything about Judy/Trudy from testimony by Susan? So even if Jim doesn't know that Susan can't tell Judy/Trudy apart Susan is still unreliable/untrustworthy (if Judy had not broken the vase and Trudy had instead, Susan would still claim that Judy had broken the vase) and having faith/trust in an unreliable person is not a way to gain knowledge? Similarly for CB : even though NCB doesn't know that CB is colorblind (presumably CB doesn't know this either), CB is still in fact unreliable with respect to the color of red things and green things and so NCB's trust in CB's judgments is misplaced and not a way for NCB to come to know something
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I have one point about this claim you make: "Similarly, even if NCB is otherwise trusting of CB, he shouldn't accept CB's reports about color!"
One of the conditions of the case is that NCB does not know that CB is colorblind. If he did, then of course NCB should not trust CB's judgments about color. Similarly, if Jim knew that Susan couldn't distinguish between Judy and Trudy, then he shouldn't (completely) trust her assertions about Judy. However, both CB and Susan are generally reliable testifiers, so there is a strong prima facie presumption to trust what they assert.
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Suppose that, for some reason, Jim knows that Trudy could not have broken the vase. Say, for instance, that he knows she has a severe phobia or allergic reaction to vases and never would pick one up in the first place... but Susan, not knowing Trudy at all, doesn't know this. It seems to me that it might make sense in that situation to say that Susan has far too little information to have real knowledge that Judy broke the vase (even though she believes so based on what she has seen enough to report that to Jim), whereas Jim does have enough information to say that he "knows" Judy broke the vase.
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But yeah, I agree with the other people that I modus ponens-ed where Graham modus tollens-ed. I think your example makes it somewhat clearer, because instead of one of these unlikely defeaters (an unknown twin) we have a perfectly ordinary defeater (color).
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In the original article, Jim does know that Trudy was in the other room. But, as easwaran usefully points out, for it to be testimonial knowledge per se, it cannot be the result of an inference from that knowledge. Graham thinks that it can function as a restorer for Jim without being an inference and with all the epistemic merit of the belief deriving from Jim's trust in the testifier. I think he's wrong about that, so I tried to redescribe the case without that component. What is clear is that Graham wants Jim's knowledge to be based on the testimony and not on any inference.
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