Who said

Reality is what's still there after you stop believing in it.

Here's a claim of origination.

Reality is that which, when you stop believing in it, doesn't go away.

Philip K. Dick, How to Build a Universe That Doesn't Fall Apart Two Days Later, 1978. Apparently a 1978 (undelivered) speech published as the introduction to a collection of Short Stories, "I Hope I Shall Arrive Soon," Doubleday, 1985 and St Martins Mass Market Paper, Reprint edition, 1987. Amazon.com.

Considering how perceptive I find this sentence, I was surprised and disappointed upon reading the essay from which it was taken. It seemed much too concerned with coincidences having religious overtones and implications. The essay lacked the simplicity, insight, directness, and clarity of the sentence.