in moderating it. Despite health warnings from colleague Wilhelm Fliess, he remained a smoker, eventually suffering a buccal cancer. Freud suggested to Fliess in 1897 that addictions, including that to tobacco, were substitutes for masturbation, "the one great habit." 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. Though Freud was reluctant to associate his psychoanalytic insights with prior philosophical theories, attention has been drawn to analogies between his work and that of both Schopenhauer and Nietzsche. In 1908, Freud said that he occasionally read Nietzsche, and was strongly fascinated by his writings, but did not study him, because he found Nietzsche's "intuitive insights" resembled too much his own work at the time, and also because he was overwhelmed by the "wealth of ideas" he encountered when he read Nietzsche. Freud sometimes would deny the influence of Nietzsche's ideas. One historian quotes Peter L. Rudnytsky, who says that based on Freud's correspondence with his adolescent friend Eduard Silberstein, Freud read Nietzsche's The Birth of Tragedy and probably the first two of the Untimely Meditations when he was seventeen. In 1900, the year of Nietzsche's death, Freud bought his collected works; he told his friend, Fliess, that he hoped to find in Nietzsche's works "the words for much that remains mute in me." Later, he said he had not yet opened them. Freud came to treat Nietzsche's writings "as texts to be resisted far more than to be studied." His interest in philosophy declined after he had decided on a career in neurology. Freud read William Shakespeare in English throughout his life, and it has been suggested that his understanding of human psychology may have been partially derived from Shakespeare's plays.