Freud had greatly admired his philosophy tutor, Brentano, who was known for his theories of perception and introspection. Brentano discussed the possible existence of the unconscious mind in his "Psychology from an Empirical Standpoint" (1874). Although Brentano denied its existence, his discussion of the unconscious probably helped introduce Freud to the concept. Freud owned and made use of Charles Darwin's major evolutionary writings, and was also influenced by Eduard von Hartmann's "The Philosophy of the Unconscious" (1869). Other texts of importance to Freud were by Fechner and Herbart with the latter's "Psychology as Science" arguably considered to be of underrated significance in this respect. Freud also drew on the work of Theodor Lipps who was one of the main contemporary theorists of the concepts of the unconscious and empathy.