>>12130762See munchausen trilemma.
https://en.wikipedia.org/wiki/M%C3%BCnchhausen_trilemmaOr more simply put, if you ask why and try to keep asking why you'll arrive at one of the following:
-Axiomatic (simply is, irreducible)
-Circular (logic forms closed loops which refer to themselves)
-Infinite (endless chain of whys)
Define implicitly defines itself if you take it as a function of language. Otherwise you're left unable to move forward because all that you do relies on the mind and the senses, and even if you define what it is to define, you cannot validate that which is used to do so.
