Timothy Leary was one of the speakers at the Human Be-In concert in San Francisco in 1967. He was a prominent figure in the Haight at that time, so much so that some people wore buttons saying, "Timothy Leary is God".
The Second Doctor once had a deep discussion with him, during which Jessica Willamy overheard the Doctor saying, "It's just a matter of tuning in," which caused Leary to say, "I think I might use that, if you don't mind". The Doctor later confided to Ben that Leary was "quite a visionary". (PROSE: Wonderland)
Behind the scenes Edit
The implication of the overheard conversation in Wonderland is that the Doctor inspired Leary's famous mantra, "Turn on, tune in, drop out".