Mechanism versus Aristotelianism

Cartesian mechanism was opposed to scholastic Aristotelian science, which was supported by both Roman Catholic and Protestant theologians. These thinkers held that, because all things are created by God with a given nature, there can be no evolutionary development of animals or of the universe as a whole. For Aristotle, all living things possess a spirit or “soul,” which is the form, or organizing principle, of the matter out of which the organism is composed, as well as the source of its powers of growth and development, nutrition, perception, and (in humans) cognition. The soul is the essence, or nature, of the organism and its final cause—i.e., its purpose, or goal. Thus, the development of an acorn into an oak tree is explained by the fact that the acorn possesses a form that directs it toward this end.

Descartes rejected both the teleological, animistic view and the related theory of alchemy that there are vital forces in things. Cartesians denied the existence of what they considered occult or magical forces, insisting instead that only God and humans have spirits, wills, purposes, and ends. They conceived both animate and inanimate bodies as having no goals but as simply being pushed around passively. For Cartesians, science therefore consisted of looking not for final causes but rather for the laws that govern the motions of bodies.

By insisting on human free will, Descartes placed the human soul or mind, like God, outside deterministic nature. Because the body is a part of nature, however, the mind’s evident ability to control the body’s movements is, on Cartesian assumptions, inexplicable and miraculous and thus inconsistent with mechanistic determinism. Ironically, in Descartes’s system this ability is itself an occult or magical force.

Mind, body, and humanity

Most Cartesians believed that the mind and body interact. When asked how this is possible, Régis gave the standard Cartesian reply: human beings experience the interaction, and God can and does make it take place, even if we cannot understand how. As for the question of how ideas represent objects, Rohault spoke for all Cartesians when he asserted that God can make ideas represent material bodies without resembling them; no further explanation is necessary. In both of these replies, the Cartesians can be seen to abdicate philosophy for mysticism and theology.

According to the Thomists (adherents of the Aristotelian philosophical and theological system developed by St. Thomas Aquinas), the soul or mind is the form of the body. Although for Aristotle the form of an object is inseparable from the matter of which it is made, the Thomists held that the human soul is a “substantial form” that is miraculously able to exist independently of matter and thus to survive the death of the body. Descartes, by contrast, contended that the notion of substantial form is contradictory, because it assumes the separate existence of something that by definition can exist only in unity with matter. For Cartesians, the mind or soul is a substance existing in itself, independently of matter; thus, they were able to explain immortality without having to rely on the dubious assumption that the soul-form is a kind of substance. This view, however, creates a serious problem concerning the ultimate nature of human beings. According to Cartesians, sensible ideas arise from the union of mind and body for the sole purpose of preserving the body by presenting harmful things as painful and beneficial things as pleasurable. Human beings learn by experience what to seek and to avoid, and the memory of these experiences is preserved in the brain. Once the body dies, however, both the need for sensible ideas and their memory traces in the brain are destroyed. All the soul knows of matter after death is the general idea of extension. Because all bodily associations and memories are eliminated, however, individual personality is lost; each human being survives death only as an impersonal soul, identical to all other bodiless souls. Like the notion that animals are mere machines, the Cartesian conclusion that the sensible manifestations of this life are neither continued nor remembered in the next was unpopular.

Science and religion

In addition to the dualism of mind and matter in Cartesian metaphysics, there is a more general dualism in Cartesianism as a whole between a rationalist metaphysics and epistemology, which entails the existence of innate ideas of mind, matter, and God and the possibility of obtaining certain knowledge through reason, and an empiricist (and mechanistic) physics, according to which scientific knowledge, which is never certain, is gradually accumulated through observation and experience of the material world. Descartes’s insistence on the possibility of certain knowledge of God’s existence has led some commentators to present him primarily as an apologist for Christianity. Others, however, have argued that he was really an atheist and a materialist who made arguments for God’s existence only to protect himself from persecution by the church.

Although Descartes publicly denied an interest in theology, in letters he offered mechanistic explanations of transubstantiation. According to the Thomistic account of this mystery, the forms of bread and wine are miraculously sustained as substantial forms while their matter is replaced by Christ’s flesh and blood. Rohault appealed to the Cartesian view that sensible ideas are caused by configurations of the parts of material bodies to argue that, if bread and wine were replaced by flesh and blood whose parts had exactly the same configurations, the flesh and blood would look, feel, and taste like bread and wine. Although Rohault’s account still requires the miraculous replacement of bread and wine by flesh and blood, it does not rely on the self-contradictory notion of substantial form.

A deterministic Cartesian ethics was developed by the Flemish Calvinist philosopher Arnold Geulincx (1624–69). In his view, although one can do only what God has willed, one is free to accept what one must do willingly or unwillingly. Virtue consists in the humble, diligent, and obedient acceptance of the justice of God’s decrees in the light of reason, whereas sin and evil result from an egotistic (and futile) stand against God. This Stoic ethics, with its affinity to Calvinist and Jansenist predestinarianism, is as deterministic as Cartesian physics. It does, however, contradict Descartes’s claim that the human will is free not just to accept or reject the rightness of predetermined bodily actions but also to choose and cause particular actions.