If the doors of perception were cleansed every thing would appear to man as it is, infinite. For man has closed himself up, till he sees all things through narrow chinks of his cavern.
William Blake, The Marriage of Heaven and Hell (1790–1793)
Aldous Huxley’s book The Doors of Perception takes its title from this quote: http://en.wikipedia.org/wiki/The_Doors_of_Perception
The full text is available at Erowid: http://www.erowid.org/psychoactives/writings/huxley_doors.shtml