In this paper, I explore scientific attempts to articulate a unified theory of the serotonin system and to explain the effects of psychedelic substances. I consider how certain accounts of psychedelic action have focused primarily on cognitive states, and I propose some phenomenological insights to supplement these models and inform work on psychedelics as therapeutic agents. Specifically, I argue that considering “psychological transformation” to be a core desideratum of psychedelic therapy should lead us to investigate the role of affect in (...) guiding significant psychological change. The phenomenological tradition offers resources for describing the relationship between knowledge and affect, and I argue that epistemically significant affective states such as fascination and revelation are more central to psychedelic experience than is commonly acknowledged. I propose that understanding the role of affect in psychedelic experiences is critical to developing a plausible account of the potential for using these substances therapeutically. (shrink)