Introspection is "looking within". Why should science not be interested in that? It is an aspect of reality. It is not more or less real than galaxies or atoms. I know that it is a very perplexing one when one holds a physicalist metaphysical commitment, which is easy to confuse with some notion of "no-nonsense modern scientific standard", and so there is a temptation to pretend the undeniable is not there, or that it is "ill defined" in some way.