The story of the young Freud's search for a brilliant career, familiar from his autobiography and standard sources, and of other relevant aspects of his adolescence, is reviewed in light of hypotheses about his unconscious fantasies. His plan to become a lawyer underwent a gradual change into a wish to make a great contribution to knowledge, like Darwin. But how did hearing the essay “On Nature” read tip the balance toward medicine? The author argues that an answer requires treating that text much the way an analyst interprets a dream or a set of free associations, and proceeds to do so. “On Nature” also foreshadows numerous themes in Freud's mature writings and in his professional practice. The paper concludes with reflections on activity and passivity in Freud's personality and professional work, and with a discussion of ways in which Darwin's theories and those of his followers revived interest in Nature-philosophy, themes from which, rather than the furor sanandi, drew Freud finally to medicine.