In given situation I would be more curious that if the dragon is invisible, incorporeal, immune to all tests ... then what exactly made Carl believe there is a dragon?

Yeah, I know, there are many possible explanations. Perhaps the dragon can become visible and corporeal when the dragon wants to; and it happened when only Carl was near, because unlike me, he is a nice and humble person or whatever, so the dragon likes him. Alternatively, Carl derived the dragon's existence from the philosophical first principles, and if I disagree with him, here are hundred books with thousand pages each, and I am welcome to show exactly where the authors made a mistake. :(