Timothy Francis Leary was an American psychologist and writer, known for his advocacy of psychedelic drugs. During a time when drugs like LSD and psilocybin were legal, Leary conducted experiments at Harvard University under the Harvard Psilocybin Project, resulting in the Concord Prison Experiment and the Marsh Chapel Experiment. Both studies produced useful data, but Leary and his associate Richard Alpert were fired from the university.

Leary believed LSD showed therapeutic potential for use in psychiatry. 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 (SM²LE), and he developed the eight-circuit model of consciousness in his book Exo-Psychology (1977).

During the 1960s and 1970s, Leary was arrested regularly and was held captive in 29 different prisons throughout the world. President Richard Nixon once described Leary as "the most dangerous man in America."