> If Crazy Carl says that aliens abducted his prize turnip, that is in itself evidence that aliens did the deed. It is wildly unpersuasive evidence, but nonetheless evidence.

Even as someone who has made a similar point recently, I think this is going too far. Carl claiming aliens is evidence of something weird going on with Carl, but there's still no reason to take his claim at face value. More likely Carl has a screw loose (good to know!) or is pulling a con.