Recent scientific research suggests that altered states of consciousness induced by classic psychedelic drugs can cause durable psychological benefits in both healthy and patient populations. The phenomenon of ‘psychedelic transformation’ has many philosophically provocative aspects, not least of which is the claim commonly made by psychedelic subjects that their transformation is centrally due to some kind of learning or knowledge gain. Can psychedelic experiences really be a source of knowledge? From the vantage point of philosophical materialism or naturalism, a negative (...) answer is tempting because psychedelic subjects often claim drug-facilitated knowledge of non-natural, transcendent realities. This fact, combined with common conceptions of these drugs as ‘hallucinogenic’ or ‘psychotomimetic’, invites the conclusion that claims of epistemic benefit from psychedelic experience are uniformly false. However, several recent proposals have been made in the literature about naturalistically acceptable epistemic benefits that might arise from psychedelic use. In this chapter I review these proposals, classifying them in accordance with standard epistemological categories, and discuss the arguments for and against them. I also offer some suggestions for future research. (shrink)