One of the routes to being epistemically justification that p is true is to be told by a reliable reporter that p. This is justification by testimony. On the other hand, I might be told something by you and not accept it as testimony, but instead treat the fact that you told it to me as evidence, doing a Bayesian update of my credences based on the fact that you said the thing. If I treat your saying that p in this way, my credence that p may go up and may go down (e.g., if I think you're lying so that your asserting that p makes it more likely that p is false). Moreover, there is a difference between accepting your testimony and taking your assertion of p to be evidence for the truth of p. I want to focus on this difference, and say a few things about the difference between lying and deception.
The first point I want to make is that I can only take your assertion that p is true to be testimony if either you made the assertion to me, or I stand at the end of a chain where you told it to A1, A1 told it to A2, and so on, until we get to someone who told it to me. Suppose that instead I stand in no such chain. Instead, I overhear your saying that p to B. Then I cannot properly accept p as testimony, since you were not speaking to me. That you asserted p to B is evidence for p, if I think it is likely you were telling the truth to B, but it is not testimony to me. Testimony is at least a ternary relation: A testifies that p to B (where B might be an individual or a group). If I am not testified to, I cannot properly believe on testimony.
Here's one reason. It is perfectly permissible to speak in ways that your interlocutor understands but which others will misunderstand. You do have a responsibility to ensure your interlocutor will not misunderstand. If you have a foreign vegan guest whom you know to think that the English word "egg" denotes a kind of plant (eggplant, say), you are neglecting not just your hostly responsibilities but your responsibilities as a speaker in saying: "This dish is made of egg." In fact, you are lying—in speaking, you are inviting your guest to take your words as testimony to the dish being made of a kind of a plant. You are giving false witness.
When we speak we have a responsibility not to be misunderstood by an interlocutor, and the interlocutor in turn gain the right—barring defeaters—to take what she thinks you've said as true. This right is tied to the responsibility. But when I overhear your asserting p to B, you have no responsibility to ensure my understanding you—your only responsibility is to ensure that B understands. Thus you do not confer on me the right to take what I think you've said as true.
Suppose this is right. Assume that you know that George is at Mark's house, but want to mislead me. You are talking with Frank and notice that I am listening in (maybe I am behind the arras, and you hear a rustle). You tell Frank: "George is at Jennifer's house", but you do so with a wink that ensures Frank doesn't take your words as literal truth. I don't see the wink, of course, so I come to have evidence that George is at Jennifer's house. But you haven't lied to me. You haven't lied to me because you weren't speaking to me, though you expected me to hear. Your properly speakerly responsibilities were to Frank, and these you fulfilled.
Note, too, that it may be that you are not even intending that I believe George is at Jennifer's house. It could be that you are merely intending that I take myself to have evidence for George's being at Jennifer's house. (This point is based on an idea of Mark Murphy's.) And in intending this, you are intending that I believe something true, viz., that I have evidence for George's being at Jennifer's house. My believing this is likely all you need for your purposes, since whether I actually believe on the evidence or not, presumably the presence of the evidence will get me to look for George at Jennifer's house, if I want to find him.
I once read in an early 20th century moral theology textbook (Smith, I think) that someone who is tortured and says something false is no more lying than an actor on stage, because she is not really engaging in the practice of assertion, but is uttering words more like a madman (I am very loosely paraphrasing the main idea from memory). Here is one way of saying something like this in the above setting. If I am torturing you, you shouldn't expect me to take your words as testimony. Instead, you would expect me to take your words as your way of saying whatever it takes to get the pain to stop. Thus if you say something that is false, you are not offering me testimony, but simply bringing it about that I have evidence of the Bayesian, not testimonial, sort.
In summary, here are some claims that I suspect are true, though I have not given much of an argument for many, or perhaps any, of them:
- Lying is not just deceitful or misleading speech. It is speech that provides (or maybe: is intended to provide) false testimony to the person being lied to. When you are not providing testimony to me, e.g., because you are not talking to me, you are not lying to me.
- It is a speaker's responsibility not to be misunderstood by the interlocutors.
- There is a difference between being known to be a listener and being spoken to, and only someone spoken to can be lied to.
- Credences should be differently updated on testimony than on evidence.
- There is an epistemic virtue, which I will call "the doxastic aspect of faith", which is the virtue of appropriately updating on testimony.
- Presumably, demons believe many claims made by Jesus, because they have evidence that Jesus is God and that God does not lie. However, this need not be the same as even the doxastic aspect of faith, because it may be that the demons are updating on Jesus's words considered as evidence, and not as testimony directed to them. Likewise, it would be possible for a human being to come to believe that what Jesus said on some topic is true without having even the doxastic aspect of faith in Jesus.