Hume's argument for testimony
1) If someone tells you they have seen a miracle, there are two other options: either they really have seen a miracle, or they are saying something that isn't true.
2) A miracle is a suspension of the laws of nature. We have never observed this. So we should consider it extremely unlikely that it would happen.
3) We have often observed people say things that aren't true. So we should consider this happening to be quite likely.
C1) Therefore, it is more likely that someone is saying something that isn't true than that a miracle really occurred.
C2) Therefore, when someone tells you they have seen a miracle, we should not believe that a miracle had occurred.