Freud revised his theory of dreams significantly. First, he discovered the role of the id in dreaming. He hypothesized that the function of dreaming is to gratify an infantile sexual wish, in accordance with the pleasure principle, and thereby to preserve sleep. However, he learned later that anxiety generated by recent events plays a greater role in stimulating dream formation than he had thought and that some dreams actually generate anxiety. These clinical observations caused him, in large part, to revise his theories of dreaming, of anxiety, and of the mental apparatus.