I think it is generally acknowledged that we see the world through the forms and filters of perception, and by expanding that perception we appear to expand the world. The challenge is to keep your mental ecology integrated as you do it. It seems to me that PKD was able to open up whole new areas of neural processing, but because of his drug use and history of loss he didn't manage to integrate the new input with the old, so he felt as if he were several different people, living in different worlds.
I also feel a strong link between PKD and William Blake. I don't know whether he felt that himself, he never mentioned Blake as one of his alter egos as far as I know.
ETA - my son says 'Mum, that's been available for years' oh well, it was new to me
With thanks to Andy D's andyduckerlinks.