Abstract
In the absence of any plausible reductionist account of consciousness in nonmentalistic terms, the HOT theory says that the best explanation for what makes a mental state conscious is that it is accompanied by a thought (or awareness) that one is in that state. I discuss HOT theory with special attention to how Leibnizian theses can help support it and how it can shed light on Leibniz's theory of perception, apperception, and consciousness. It will become clear how treating Leibniz as a HOT theorist can solve some of the problems he faced and some of the puzzles posed by commentators, e.g. animal mentality and the role of reason and memory in self-consciousness.