Defining ‘psychedelic’

The term 'psychedelic' was coined in 1956 from Greek roots meaning 'soul-manifesting' or 'soul-illuminating,' intended to name a drug category defined by its ability to induce a characteristic subjective state. This study examined the main subjective effects of psilocybin, ketamine, and MDMA. Over two hundred participants rated items about their experiences with all three drugs. Factor analyses revealed three or four independent dimensions of subjective experience. A machine learning classifier successfully predicted which drug a person had taken from the effects reported, confirming that the three drugs produce categorically distinct experiences: psilocybin induces visions and psychological insight, ketamine induces dissociation, and MDMA induces pro-social feelings such as love. Psilocybin is thus an exemplar psychedelic drug, definable by its induction of a psychedelic state characterized by visions and insight.

Study at a glance

Abstract

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.