It's from John Steinbeck's Journal of a Novel, the collected daily letters he wrote to his editor while drafting East of Eden:
Writing is a very silly business at best. There is a certain ridiculousness about putting down a picture of life. And to add to the joke -- one must withdraw for a time from life in order to set down that picture. And third one must distort one's own way of life in order in some sense to simulate the normal in other lives. Having gone through all of this nonsense, what emerges may well be the palest of reflections.