The Doors took their name from a book by Aldous Huxley called “The Doors of Perception,” by he took his title from an older and much stranger work, the “Marriage of Heaven and Hell” by William Blake. Here's the phrase in its original context:
“For the cherub with his flaming sword is hereby commanded to leave his guard at the tree of life, and when he does, the whole creation will be consumed and appear infinite and holy whereas it now appears finite & corrupt... This will come to pass by an improvement of sensual enjoyment... But first the notion that man has a body distinct from his soul is to be expunged; this I shall do, by printing in the infernal method, by corrosives, which in Hell are salutary and medicinal, melting apparent surfaces away, and displaying the infinite which was hid... 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 thro' narrow chinks of his cavern.”
As bizarre and incomprehensible as this might seem, it can actually be readily understood in the context of the underground traditions of mysticism and internal alchemy. What Blake is saying is that the teachings of organized religion have placed a barrier between humanity and the holiness of the world, preventing us from seeing the world as it truly is. This is essentially a Gnostic concept. According to Blake, when people learn to reject the traditional opposition of body and spirit, and begin to enjoy sensual pleasure rather than rejecting it as sinful, the barriers set up by religious tradition will fall away. The “infinite,” as Blake calls it, is always present, but the barriers to perceiving the infinite have to be burnt away as if with a corrosive acid before we can see the truth.
In other words, Blake saw sexuality and the rejection of conventional restrictions on its free expression as a key he could use to “break on through to the other side.” The Doors could not have chosen a more appropriate name!