The author describes the influence on Freud's thought of certain of his predecessors and contemporaries. Freud stated that Darwin's theories and Goethe's essay on nature were factors in his decision to become a medical student. The six years he spent at the Brücke Institute 'probably provided him with the basic elements of his theories that culminated later in his discovery of psychoanalysis'. Two currents of thought that influenced Freud strongly at the Brücke Institute were the 'evolutionistic orientation' of Darwin and the physiology of Helmholtz. Brentano's courses on Aristotle exerted another important influence on Freud. 'Whenever students of Freud find it hard to follow one part or the other of his theories, it would probably be of help to go back to some of Aristotle's doctrines. The libido theory and the supremacy of genitality could perhaps be more easily understood if one recalls Aristotle's view that the higher levels of organization contain the lower levels and something more.'