علی امیریفرGrusch has actually produced no publicly facing empirical or objective evidence for his claims. He’s just told us (and Congress) that he has evidence, including the testimony of people who work inside the program, that the hypothesis is true. So his testimony (primarily about their testimony) is basically the only thing we have which could be considered evidence for his claim. But not only is testimony not nearly as dependable as people often assume; as David Hume taught us long ago, testimony is an especially unreliable guide to the truth when it comes to the occurrence of extraordinary events. Hume was talking about miracles, but his point would stand for claims about aliens as well.
Hume said that, when it comes to miracles, testimony cannot justify belief. Why? Miracles are a violation of natural law, but that the laws always hold is something he has seen, repeatedly, every day, with his own eyes. The evidence he has for that is repeated and direct. Miracles, on the other hand, are rare and testimony does not produce as much justification as seeing something with your own eyes. So the unique unrepeated evidence of someone telling him the laws were once broken (even if they are a generally reliable witness) will never outweigh the direct repeated evidence he has that the laws always hold. It’s much more likely that the witness is either lying or simply mistaken.
Likewise, the evidence I have that aliens have not visited Earth is repeated and direct. Not only have I walked around my entire life and seen no good evidence of aliens visiting Earth, I have seen claimed evidence that they have (including evidence people have claimed is “absolute proof” and “the best evidence you will ever see of aliens”) debunked, over and over and over and over again. The unique indirect evidence of someone’s testimony that they have seen such evidence will not be able to outweigh the direct repeated evidence I already have. It is much more likely that they are lying or mistaken.