In one of my favorite anecdotes about Foucault, someone asks him why he writes books. He responds by saying something like "When I begin to write a book, I do not know how it will come out, what it will say in the end. If I already did, I wouldn't need to write it. "
The late philosopher Morris R. Cohen of CCNY was asked by a student in the metaphysics course, Professor Cohen, how do I know that I exist? The keen old prof replied, And who is asking?