Mechanical Philosophy

Gassendi and Descartes published the first systematic and the most influential accounts of the mechanical philosophy. Their treatises spelled out the fundamental terms of the mechanical philosophy and functioned as programmatic statements, describing what such a philosophy would look like in practice. Although both men agreed that all physical phenomena should be explained in terms of matter and motion, they differed about the details.

Gassendi believed that God had created indivisible atoms and endowed them with motion. The atoms, colliding in empty space (the void), are the constituents of the physical world. In his massive Syntagma philosophicum (published posthumously in 1658; Philosophical treatise), Gassendi set out to explain all the qualities of matter and all the phenomena in the world in terms of atoms and the void. He argued for the existence of the void—a controversial claim at the time—on both conceptual and empirical grounds, appealing to recent barometric experiments of Evangelista Torricelli (1608–1647) and Blaise Pascal (1623–1662). The primary qualities of Gassendi's atoms were size, shape, and mass. He attempted to explain all the qualities of bodies—light, color, sound, taste, smell, heaviness, and lightness—in atomic terms. Among the qualities he included were the so-called occult qualities, which seemed to involve action at a distance and had generally resisted explanation in mechanical terms. After laying the foundations for his philosophy, Gassendi gave an account of the entire creation: the heavens, the inanimate world, the animate world, and the human soul.

Writing in the manner of a Renaissance humanist, Gassendi saw himself as the restorer of the philosophy of Epicurus. Deeply concerned with Epicurus's heterodox ideas, Gassendi, a Catholic priest, sought to modify ancient atomism so that it would be acceptable to seventeenth-century Christians. Accordingly, he insisted on God's creation of a finite number of atoms, on God's continuing providential relationship to the creation, on free will (both human and divine), and on the existence of an immaterial, immortal human soul that, he claimed, God infuses into each individual at the moment of conception.

Gassendi was not a materialist. He argued for the existence of an incorporeal, immortal soul and also believed in the existence of incorporeal angels and demons. In addition to the immaterial, immortal soul, Gassendi claimed that there exists a material, sensible soul, composed of very fine and swiftly moving particles. This material soul (which animals also possess) is responsible for vitality, perception, and the less abstract aspects of understanding. The material soul is transmitted from one generation to the next in the process of biological reproduction. Gassendi's ideas were brought to England by Walter Charleton (1620–1707) and popularized in France by François Bernier (1620–1688).

Although Descartes also articulated a full-fledged mechanical philosophy in his Principia philosophiae (1644; Principles of philosophy), his ideas were quite different from Epicurean atomism. Impressed by the rigor of mathematical reasoning and Galileo's mathematization of physics, Descartes wanted to develop a mathematical approach to the mechanical philosophy. In contrast to Gassendi's atomism, Descartes was a plenist, claiming that matter fills all space. He claimed tht matter is infinitely divisible, thus denying the existence of both atoms and the void. He believed that matter possesses only one primary quality, geometrical extension. This belief provided foundations for his attempted mathematization of nature. Descartes drew a sharp distinction between matter and mind, considering thinking to be the essential characteristic of the mind. Like Gassendi's doctrine of the immortal soul, Descartes's concept of mind established the boundaries of mechanization in the world.

Descartes attempted to deduce the laws of motion—the conservation of motion and the principle of inertia—from first principles. From the laws of motion, he attempted to derive mathematical laws of impact. Although these laws were inadequate, even in seventeenth-century terms, their prominent place in his system reflects their importance in a mechanical philosophy, according to which contact and impact are the only causes in the physical world. Having established the physics that he considered fundamental to his system, Descartes proceeded to give mechanical explanations of all the phenomena in the world, including cosmology, light, the qualities of material things, and even the human body.

Like Gassendi, Descartes intended his philosophy to replace Aristotelianism. He hoped that the Jesuit colleges would adopt the Principia philosophiae as a physics textbook in place of the Aristotelian texts still in use. His hopes were dashed posthumously, however, when the Roman Catholic Church condemned his book in 1662 and then placed it on the Index of prohibited books one year later, in response to his attempt to give a mechanical explanation of Real Presence (the doctrine holding that Christ is actually present in the Eucharist).

William Harvey (1578–1657), a medical practitioner and teacher, inspired Descartes's mechanical philosophy by his experimental proof of the circulation of the blood, published in Exercitationes Anatomica de Motu Cordis et Sanduinis in Animalibus (1628). Harvey's natural philosophy was actually more Aristotelian than mechanical, a point manifest in his work on embryology, in which he adopted an Aristotelian explanation of generation—epigenesis, according to which the embryo is formed from the fluids contributed by both parents in the process of reproduction. Nevertheless, impressed by Harvey's use of mechanical analogies to describe the flow of blood, Descartes attempted to develop a complete physiology based on his own mechanical principles. Gassendi was not convinced by Harvey's evidence and rejected the circulation of the blood.

Another mechanical philosopher, Thomas Hobbes (1588–1679), was the specter haunting more orthodox natural philosophers. Hobbes's philosophy seemed—to the seventeenth-century reader—to be materialistic, deterministic, and possibly even atheistic. In The Elements of Philosophy (1642–1658), Hobbes propounded a complete philosophy—of matter, of man, and of the state—according to mechanistic principles. Although the details of his mechanical philosophy were not very influential among natural philosophers, his mechanical account of the human soul and his thoroughly deterministic account of the natural world alarmed the more orthodox thinkers of his day. His claims underscored their fears that the mechanical philosophy would lead to materialism, deism, and even atheism.