In a 1974 letter to the literary critic Peter Fitting (who a year later would publish the essay ‘”Ubik”: The Deconstruction of Bourgeois SF’) Dick talks about tachyons which he read about in an article by Arthur Koestler in the July ’74 issue of Harper’s magazine. Since (the hypothetical) tachyons travel faster than the speed of light they would travel in a reverse time direction and open up the possibility of precognition.
He recounts how he read about vitamin megadosing to improve brain efficiency while doing research for his novel A Scanner Darkly, so he loaded up on water-soluble vitamins one night and for eight hours hallucinated a series of colored graphics that resembled abstract art. He believed later this was due to the reduction of the inhibitory neurotransmitter GABA in his brain.
Following all this he begins to receive messages from the sky. At first he thinks he’s picking up on some ESP experiment from the USSR but then concludes these messages are not man-made but rather a tachyon bombardment that lets him see the future. All of this is a sort of scientific explanation for what the Israelites, for instance, experienced when they imagined they were contacted by God.
In a postscript he decides these cosmic broadcasts weren’t necessarily directed at him but they may have influenced him to write Ubik. Before sending the letter he returns to follow up on this theory that perhaps all of his books were written by him as a result of this tachyon bombardment. He could foresee the future and that’s why more and more people are telling him they feel they are living in a world that caught up to his stories.
He ends the letter teasing an idea for a sequel to The Man in the High Castle which he never completed. You can read the first two chapters of this unfinished book in the collection of essays and other ephemera The Shifting Realities of Philip K. Dick edited by Lawrence Sutin.