Leary believed that LSD showed potential for therapeutic use in psychiatry. He used LSD himself and developed a philosophy of mind expansion and personal truth through LSD.After leaving Harvard, he continued to publicly promote the use of psychedelic drugs and became a well-known figure of the counterculture of the 1960s. He popularized catchphrases that promoted his philosophy, such as "turn on, tune in, drop out", "set and setting", and "think for yourself and question authority". He also wrote and spoke frequently about transhumanist concepts involving space migration, intelligence increase, and life extension and developed the eight-circuit model of consciousness in his book Exo-Psychology (1977). He gave lectures, occasionally billing himself as a "performing philosopher".
During the 1960s and 1970s, he was arrested often enough to see the inside of 36 prisons worldwide.President Richard Nixon once described Leary as "the most dangerous man in America".