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 .