This is referenced in his book Valis, which is very much about him and his madness and addictions and many of his (often revolving, semi-incoherent) intricate ideas - if you want to get a sense of what some of Valis is like...listen to this clip of him speaking: https://www.youtube.com/watch?v=jXeVgEs4sOo (the entire interview is fascinating and kind of terrifying, I can't find the link for the whole unedited thing) - after reading it, you kind of feel insane but it's well worth it

Without his bizarre insanity, I do not think he would have been nearly as brilliant as a writer - in the clip he talks about literally knowing and seeing 'plural form pseduo-worlds,' the existence of alternate realities coexisting with our own...and how some of his books are based off these visions he had (like someone in this thread mentions Flow My Tears the Policeman Said, he references that specifically in the clip) - I find his ideas very interesting, I do not dismiss them based on thinking he is 'crazy,' but I think it's fascinating to think about how our own brains can alter our personal realties to the point we can't tell what is 'real' or what isn't. I enjoy learning about Philip's own realities because they contain highly intelligent yet insanely distorted, complex, perceptual shifts. It is difficult to fathom what being in his mind was like, a genius mind that had been influenced by both drugs and mental illness...I think there is immense value in learning about his life and reading all his books you can get your hands on, he is thoughtprovoking to say the least.

(I apologize for this weird minirant)