I happened to come across a biographical article about him this week, and it struck me how he described how his imagination worked:
(...) What I mean to say, is that your mind should be sufficiently developed to recognise its own limitations. I am convinced that what's produced by what we call "fantasy" in fact comes from a much deeper reality than whatever we can observe in a physical sense. (...)
It's fascinating that so many people who put their imagination to use seem to arrive at similar conclusions.