Writing in the Journal of Humanistic Psychology [1], psychologist John Heider acknowledges that a major catalyst in the development of transpersonal psychology [1] was “the widespread use and abuse of psychedelic or mind-manifesting substances such as marijuana, LSD and mescaline…. The psychedelic drugs gave incontrovertible proof that altered states of consciousness had reality and that paths toward transcendent experience existed.”
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[1] https://www.thebereancall.org/taxonomy/term/56/psychology