Aldous Huxley entitled his "The Doors of Perception" from a line of Blake's: "If the doors of perception were cleansed everything would appear to man as it is, infinite." On mobile at the moment but Huxley was definitely directly influenced by Blake, and later went on to influence much of the psychedelic era.

In the realm of the written word, William Blake is sometimes thought to have used something like the automatic writing favored by the early Surrealists.

Blake talked about it more like mysticism - he said his poem "Jerusalem" was dictated to him and that he "dared not pretend to be other than the Secretary" to whoever was doing the dictating.

In the First Surrealist Manifesto, Andre Breton defines Surrealism itself as:

SURREALISM, n. m. Pure psychic automatism by means of which one intends to express, either verbally, or in writing, or in any other manner, the actual functioning of thought. Dictated by thought, in the absence of any control exercised by reason, free of any aesthetic or moral concern.

Automatic writing was one of the first tools Breton and his followers used to create Surrealist works.

Basically, it means writing down stuff without conscious thought and figuring out what it means after the fact.

Breton, again from the first manifesto, explains (or, well, "explains" is almost definitely the wrong word):

...the surrealist atmosphere created by automatic writing, which I desire to place within reach of everyone, is especially conducive to the production of the most beautiful images. One might even say that, in this vertiginous race, the images appear, as the mind’s sole means of steering. The mind, little by little, becomes convinced of the supreme reality of these images. At first confining itself to submitting to them, it soon perceives that they stimulate its powers of reason, increasing its knowledge accordingly.