In the late eighteenth and early nineteenth centuries, philosophers including Kant and Hegel draw a sharp distinction between the human and the animal. The human is self-conscious, the animal is not; the human has moral worth, the animal does not. By the mid to late nineteenth century, these claims are widely rejected. As scientific and philosophical work on the cognitive and motivational capacities of animals increases in sophistication, many philosophers become suspicious of the idea that there is any divide between (...) human beings and other animals. This paper traces the transitions in eighteenth- and nineteenth-century thought about animals. My focal point is the notion of drive or instinct (Trieb, Instinkt). Although in sporadic usage during earlier times, the drive concept explodes in the eighteenth and nineteenth centuries. It begins playing central roles in three distinct areas: embryology, ethology, and metaphysics. In embryology, drive describes a force, inaccessible in itself but whose results are visible and susceptible to scientific and philosophical study, governing organic development. In ethology, drives are the sources of seemingly deliberate, highly articulated, yet non-conscious activities, which are directed at ends of which the animal is ignorant. In metaphysics, drive describes the human essence. I focus on the way in which the emergence of the drive concept in each of these three domains undermines the idea that there is any sharp distinction between the human and the animal. I conclude by considering how, in light the collapse of the human/animal divide, ethical theories are reshaped. (shrink)