Philip K. Dick, interviewed by Joe Vitale, "The World of Philip K. Dick: An Interview with America's Most Brilliant Science-Fiction Writer" in The Aquarian (October 11, 1978).
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There’s nothing good about drugs. Drugs kill you and they break down your head. They eat your head. In “White Rabbit,” Grace Slick says, “feed your head.” But I say, “What are you really feeding it?” You’re feeding it itself. Drugs cause the mind to feed on itself.