Comment by exo-pla-net

It's not an assumption. Self awareness has been scientifically tested in many species, most prominently by using the mirror test: testing if an animal can recognize itself in a mirror by observing if its behavior changes after surreptitiously placing onto its body a mark, which the animal can only see through the mirror. One can argue about the merits of the mirror test, but it's inaccurate to say that scientists are merely making assumptions. They're doing their best to rigorously test self awareness.