Descartes used the word “animal” (Latin animal, French animal) for animals in general, including human beings, and the term “beast” (Latin bestia or brutum, French bête) for nonhuman animals. He developed a new approach to nonhuman animals as mindless machines, lacking sentience. In line with the similarity between human and animal bodies (henceforth, “animal” means nonhuman animals), he treated the human body as a machine as well, albeit ensouled. Descartes recognized that animals behave so as to preserve their bodies in relation to environmental circumstances. His conception of animals as mere machines required that he explain such behavior using material mechanisms alone, in accordance with his doctrine that matter or body (corporeal substance) has extension as its essence, modified only by the properties of size, shape, position, and motion.

Discussion of animal cognition increased throughout the seventeenth century. Before Descartes’ animal machine hypothesis, debate was focused on whether animals reason. Michel de Montaigne famously argued that human and animal cognition do not differ in kind. Pierre Charron held a similar position, sparking a controversy that drew in Marin Cureau de la Chambre, whom Descartes knew in the 1640s, and Pierre Chanet. The latter two authors adhered to the received opinion among Scholastic Aristotelians (see Scholasticism) that the cognitive capacities of humans and animals differ in kind. These discussions included both natural history, describing animal behavior, and natural philosophy, seeking to explain those behaviors (Harrison 2004). Descartes focused on the latter, using few examples of animal behavior in his theoretical discussions. His followers, such as Antoine Le Grand, tested the animal-machine doctrine against the natural history literature.

Within the Aristotelian philosophy that Descartes knew well, all natural kinds, whether mineral, vegetable, or animal, were constituted by a substantial form inhering in matter. This “matter” was not conceived as a stand-alone substance and so is not Descartes’ bare extended corporeal substance. It serves as substrate for the substantial form, which is an active principle that directs the development of each kind of thing toward its characteristic properties. An acorn contains the form of the oak and, in growing, organizes its matter into an oak tree with branches and leaves. Animals, nonhuman and human, have their various substantial forms, with capacities that guide the animals’ behaviors in relation to environmental circumstances.