Terence Kemp McKenna (November 16, 1946 - April 3, 2000) was an American ethnobotanist, mystic, psychonaut, lecturer, author, and an advocate for the responsible use of naturally occurring psychedelic plants. He spoke and wrote about a variety of subjects, including psychedelic drugs, plant-based entheogens, shamanism, metaphysics, alchemy, language, philosophy, culture, technology, environmentalism, and the theoretical origins of human consciousness. He was called the "Timothy Leary of the '90s", "one of the leading authorities on the ontological foundations of shamanism", and the "intellectual voice of rave culture". McKenna formulated a concept about the nature of time based on fractal patterns he claimed to have discovered in the I Ching, which he called novelty theory, proposing this predicted the end of time in the year 2012.

In the early 1980s, McKenna began to speak publicly on the topic of psychedelic drugs, becoming one of the pioneers of the psychedelic movement. His main focus was on the plant-based psychedelics such as psilocybin mushrooms (which were the catalyst for his career), ayahuasca, cannabis, and the plant derivative DMT. He conducted lecture tours and workshops promoting natural psychedelics as a way to explore universal mysteries, stimulate the imagination, and re-establish a harmonious relationship with nature. Though associated with the New Age and Human Potential Movements, McKenna himself had little patience for New Age sensibilities. He repeatedly stressed the importance and primacy of the "felt presence of direct experience", as opposed to dogma.  In addition to psychedelic drugs, McKenna spoke on a wide array of subjects including; shamanism; metaphysics; alchemy; language; culture; self-empowerment; environmentalism, techno-paganism; artificial intelligence; evolution; extraterrestrials; science and scientism; the Web; virtual reality (which he saw as a way to artistically communicate the experience of psychedelics); and aesthetic theory, specifically about art/visual experience as information representing the significance of hallucinatory visions experienced under the influence of psychedelics.  McKenna soon became a fixture of popular counterculture with Timothy Leary once introducing him as "one of the five or six most important people on the planet" and with comedian Bill Hicks' referencing him in his stand-up act and building an entire routine around his ideas. McKenna also became a popular personality in the psychedelic rave/dance scene of the early 1990s, with frequent spoken word performances at raves and contributions to psychedelic and goa trance albums by The Shamen, Spacetime Continuum, Alien Project, Capsula, Entheogenic, Zuvuya, Shpongle, and Shakti Twins. In 1994 he appeared as a speaker at the Starwood Festival, documented in the book Tripping by Charles Hayes.  McKenna published several books in the early-to-mid-1990s including: The Archaic Revival; Food of the Gods; and True Hallucinations. Hundreds of hours of McKenna's public lectures were recorded either professionally or bootlegged and have been produced on cassette tape, CD and MP3. Segments of his talks have gone on to be sampled by many musicians and DJ's.  McKenna was a colleague and close friend of chaos mathematician Ralph Abraham, and author and biologist Rupert Sheldrake. He conducted several public and many private debates with them from 1982 until his death. These debates were known as trialogues and some of the discussions were later published in the books: Trialogues at the Edge of the West and The Evolutionary Mind.Answer this question using a quote from the following article:

What was the subject of these books?
spoke on a wide array of subjects including; shamanism; metaphysics; alchemy; language;