“If the doors of perception were cleansed, everything would appear to man as it is: infinite.”

Source: Liber Null & Psychonaut (1987), p. 28
Context: Man considers himself center of will and a center of perception. Will and perception are not separate but only appear so to the mind. The unity which appears to the mind to exert twin functions of will and perception is called Kia by magicians. Sometimes it is called the spirit, or soul, or life force, instead.

Source: The Martyrdom of Man (1872), Chapter IV, "Intellect", p. 417.

Aldous Huxley, using the term "the doors of perception" which originated with William Blake in The Marriage of Heaven and Hell. It is sometimes credited to Morrison because he cited it in interviews as the inspiration for the name The Doors and without always crediting Huxley as the source.
Misattributed

Source: Life of Galileo (1939)

Source: The Perks of Being a Wallflower

Source: Learning How to Learn: Psychology and Spirituality in the Sufi Way

Now, I don't actually think that's the first thing he wrote: he probably wrote 'my brain is melting' ten thousand times, but it was the book that the critics latched on to.
Is It Bill Bailey? (TV, 1998)

The Levinas reader by Levinas, Emmanuel p. 134

Source: 1990s and beyond, The Book of Probes : Marshall McLuhan (2011), p. 67