Now this is an empirical discovery. No amount of a priori cogitation could possibly have revealed this fact to us. It is nevertheless, argues Kripke, a necessary fact. For statements of identity are guaranteed by logic to be true in every possible world. Everything is the thing it is and not some other thing .
Every object is necessarily self-identical.
The relationship between these three pairs of concepts is therefore much more complicated then had at first appeared. The question marks are hotly debated.