Introspective knowledge by acquaintance is knowledge we have by being directly aware of our phenomenally conscious states. In this paper, I argue that introspective knowledge by acquaintance is a sui generis kind of knowledge: it is irreducible to any sort of propositional knowledge and is wholly constituted by a relationship of introspective acquaintance. My main argument is that this is the best explanation of some epistemic facts about phenomenal consciousness and introspection. In particular, it best explains the epistemic asymmetry between (...) a subject who has never had a certain phenomenal state and one who has. I also consider two theoretical objections to my claim: an objection from disunity and an objection from mysteriousness. I show that these objections can be answered and that introspective knowledge by acquaintance being sui generis remains a live option on the table. (shrink)