Defining 'psychedelic'.

Humphry Osmond coined the term 'psychedelic' in 1956, conjoining 'psyche' for 'soul' and 'delic' for 'to manifest' or 'illuminate'. Soul-illumination is a compound noun that describes a psychological state or process. Osmond intended for it to be a taxonomic noun-naming, not just a state-but a category of drug that can induce this psychological effect as its principal action. Consistent with the etymology of psychedelic, the present work respects the fundamental importance of phenomenology. Accordingly, we examine the main subjective effect of three different psychoactive drugs, psilocybin, ketamine, and MDMA (3,4-methylenedioxymethamphetamine) (variable label, Drug). Over 200 participants rated Delphi-generated subjective rating scale items based on their personal experiences with all 3 drugs. Factor analyses revealed three or four sufficiently independent dimensions of subjective experience (variable label, Effects). A machine-learning classifier successfully predicted Drug from Effects, validating the hypothesis that psilocybin, ketamine and MDMA have categorically distinct subjective effect profiles, differentiable by (1) visions and psychological insight (psilocybin), (2) dissociation (ketamine) and (3) pro-social and loving feelings (MDMA). We conclude that psilocybin is an exemplar psychedelic-a category of drug definable by the induction of a psychedelic state. The quintessential psychedelic phenomenon is a subjective state characterized by visions and psychological insight.