Defining ‘psychedelic’

Humphry Osmond coined the term ‘psychedelic’ in 1956, conjoining ‘psyche’ for ‘soul’ and ‘delic’ from ‘dêlos’ for ‘to manifest’ or ‘illuminate.’ Soul-illumination is an adjective that describes a psychological state or process. However, Osmond’s intention was to use the adjective to name— not just a state— but a category of drug that can induce the subjective effect as its principal action; thus, when used in this way, psychedelic becomes a ‘nominalized adjective;’ describing a ‘thing’ (i.e., a drug) that can induce the described state. Consistent with the etymology of psychedelic, the present work is guided by phenomenology, recognizing its fundamental ontology. Accordingly, we examine the main subjective effect of three different psychoactive drugs, psilocybin, ketamine, and MDMA (variable label, Drug). Over two-hundred participants rated Delphi-derived subjective rating scale items based on their personal experiences with all three drugs. Factor analyses revealed 3 or 4 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 are categorically distinct as determined by their differential ability to induce the following Effects: 1) visions and psychological insight (psilocybin), 2) dissociation (ketamine) and 3) pro-social feelings, epitomized by feelings of love (MDMA). We conclude that psilocybin is an exemplar psychedelic drug— a category of drug definable by the induction of a psychedelic state— the quintessential psychedelic phenomenon. This state is characterized by visions and psychological insight.