The Mechanical Philosophy: How the World Became a Machine
From Descartes and Hobbes to Boyle and Newton: the doctrine that the universe is inert matter in motion under mathematical laws, with no purposes, no forms, and no Aristotelian teleology. The deep conceptual shift of the seventeenth century.
The Mechanical Philosophy
The mechanical philosophy — the doctrine that the natural world is a system of matter in motion, governed by laws of impact and contact action, with no purposes, no forms, and no Aristotelian teleology — was the deepest conceptual shift of the Scientific Revolution. By the late seventeenth century, the mechanical philosophy had displaced the Aristotelian-Scholastic natural philosophy that had dominated the European universities for four centuries, and had become the working assumption of every new natural philosophy in the major European countries. The transition was not, in any sense, sudden: it took most of the seventeenth century, it was conducted by many hands, and it was resisted at every stage. But the result, by the time of Newton’s Principia in 1687, was a new conception of nature — what historians sometimes call the mechanization of the world-picture — that has been the working framework of the physical sciences ever since.
This page looks at what the mechanical philosophy actually claimed, how it developed, and what its implications were for the broader project of natural philosophy.
The Core Claims
The mechanical philosophy had, in its seventeenth-century form, three core claims. The first was the universality of matter: the doctrine that everything in the natural world, including what had previously been thought to be immaterial, is made of matter, in the sense of substance that has spatial extension, mass, and the capacity to move. The Aristotelian-Scholastic tradition had distinguished sharply between matter and form, and had attributed to living things, in particular, a non-material soul that organised the body and directed its growth and behaviour. The mechanical philosophy rejected this distinction, and held that the phenomena of life could, in principle, be explained as the products of the motion of matter.
The second was the reduction of qualities to figure and motion. The Aristotelian tradition had distinguished between the primary qualities of bodies (the ones that belong to the bodies themselves: figure, size, solidity) and the secondary qualities (the ones that are produced by the interaction of the bodies with the senses of the perceiver: colour, sound, taste, smell, warmth). The mechanical philosophy held that all the apparent qualities of bodies, including the secondary ones, are, in the last analysis, effects of the figure and motion of the corpuscles that compose the bodies. A red apple, on this view, is red because the surface of the apple reflects the long-wavelength end of the visible spectrum; the redness is not a property of the apple itself, but a property of the interaction between the apple and the perceiver.
The third was the elimination of teleology — the doctrine that the natural world contains no purposes, no ends, no Aristotelian final causes. Aristotelian science had explained the growth of an acorn into an oak, or the flight of a stone toward the centre of the earth, in terms of the telos or end of the process. The mechanical philosophy rejected this kind of explanation, and held that all natural phenomena are, in the last analysis, the product of the efficient causes of motion and impact. A stone falls, on the mechanical view, not because it is seeking its natural place at the centre of the earth, but because the matter of the stone is being acted on by the matter around it. The acorn grows into an oak, on the mechanical view, not because the form of the oak is already present in the acorn, but because the corpuscles of the acorn are organised in a particular way that produces, by the ordinary laws of matter in motion, the growth of the oak.
The Major Figures
The mechanical philosophy was, in its seventeenth-century form, the work of a substantial number of natural philosophers, and the variations among the different versions are considerable. The most important figures are the following.
René Descartes (1596–1650) is the most influential single proponent. The Cartesian natural philosophy, set out in the Principia Philosophiae of 1644, is the most comprehensive version of the mechanical philosophy in the seventeenth century. Descartes held that the world is composed of three kinds of matter: ordinary matter, which makes up the visible bodies; subtle matter, which fills the spaces between the visible bodies; and a still more subtle matter, which makes up the sun and the stars. All natural phenomena are produced by the motion and impact of these three kinds of matter. The Cartesian natural philosophy also included a substantial cosmology — the vortex theory of the solar system, in which the planets were carried around the Sun by huge whirlpools of subtle matter — and a substantial physiology — the account of the human body as a machine, set out in the posthumous Treatise on Man of 1662.
Thomas Hobbes (1588–1679) is the second major proponent, and the most thoroughgoing. The Hobbesian natural philosophy, set out in De Corpore of 1655, is the most aggressively materialist of the seventeenth-century versions. Hobbes held that the only real things in the world are matter in motion, and that all other things — including, most consequentially, the human mind — are, in the last analysis, products of the motion of matter. The Hobbesian natural philosophy was, on the whole, more extreme than the Cartesian, and it had a more limited influence on the actual development of the new science.
Robert Boyle (1627–1691) is the most important British proponent. The mechanical chemistry of Boyle, set out in The Sceptical Chymist of 1661 and in a long series of experimental papers, treated the phenomena of chemistry as the products of the size, shape, and motion of corpuscles. Boyle also conducted the long series of experiments on the vacuum and the air pump that established the mechanical philosophy of gases. The Boyle–Hooke experiments on the air pump, in particular, established the principal evidence for the mechanical philosophy in the English-speaking world.
Isaac Newton (1642–1727) is the most important late proponent, and the most paradoxical. Newton accepted the broad program of the mechanical philosophy — the doctrine that the natural world is a system of matter in motion under laws that can be described mathematically — but he also accepted the existence of universal gravitation, which seemed difficult to explain by contact action. The Newton of the Principia describes the solar system as a system of matter in motion under the law of gravitation, but he does not offer a mechanical account of the cause of gravitation. The Hypotheses non fingo passage in the General Scholium to the Principia — “I do not feign hypotheses” — is the most famous statement of the philosophical difficulty. Newton’s later work in the Opticks — the famous Queries — explored the possibility of an aether that would transmit gravitational force, but without arriving at a settled view.
The Critique: Newton, Action at a Distance, and the Leibniz–Clarke Debate
The mechanical philosophy was, by the late seventeenth century, the working assumption of the new science, but it was also the target of a sustained critique, most famously in the Leibniz–Clarke correspondence of 1715–1716. The principal objection was that the mechanical philosophy, committed to contact action, could not explain the action at a distance that Newton’s law of universal gravitation seemed to require. The force that the Sun exerts on the Earth, on Newton’s law, is a force that acts across empty space, and the mechanical philosophy had no good account of how a force could be transmitted across empty space.
The Leibnizian objection, sharpened in the Leibniz–Clarke correspondence and in Leibniz’s various polemical works, was that Newton’s gravitation was a return to the occult qualities of the scholastic philosophy. Clarke, replying on Newton’s behalf, argued that the empirical success of the law of universal gravitation was a sufficient justification of the law, regardless of the absence of a mechanical account of the cause. The debate was less a contest of science than a contest of metaphysical commitments, and the two sides talked past one another to a considerable degree. The eventual triumph of Newton’s physics in the eighteenth century, especially in France after the work of Voltaire and Émilie du Châtelet, was a triumph of the explanatory power of the theory rather than of the metaphysical commitments of either side.
The full account of the Leibniz–Clarke debate is given in the Wikipedia entry on the Leibniz–Clarke correspondence (a useful summary, though readers should consult the primary text, edited by Roger Ariew in Individuation, the Leibniz–Clarke Correspondence, and Scientific Innovation, Hackett, 2000), and in the Stanford Encyclopedia of Philosophy entry on the debate. The longer account of the broader history is in E. J. Dijksterhuis, The Mechanization of the World Picture (Oxford, 1961; originally Dutch, 1950), and in Alexandre Koyré, From the Closed World to the Infinite Universe (Johns Hopkins, 1957), and in the more recent I. Bernard Cohen, The Newtonian Revolution (Cambridge, 1980).
The Longer-Term Legacy
The mechanical philosophy has, eventually, been the working framework of the physical sciences. The program of the mechanical philosophy — the reduction of nature to matter in motion under mathematical laws — is the program that the physical sciences have, on the whole, followed. The success of the program in mechanics and astronomy, in the eighteenth and nineteenth centuries, was extended in the twentieth century to the atomic and subatomic realm by the quantum-mechanical description of matter, and to the cosmic realm by Einstein’s general theory of relativity. The mechanical philosophy has, in this sense, been the most successful research program in the history of science, and the conceptual shift of the seventeenth century has been the most consequential single shift in the history of Western thought.
The mechanical philosophy has also, eventually, been the source of the deepest conceptual difficulties of modern science. The action-at-a-distance problem, which Newton left unsolved, was eventually dissolved by the field concept in the nineteenth century, and again by general relativity in the twentieth. The reduction-of-qualities problem, which Descartes and Boyle left unsolved, was eventually addressed by the philosophy of perception, but the question of how the secondary qualities of objects are produced by the interaction of matter with the senses of the perceiver is still, in the twenty-first century, a major question in the philosophy of mind. The program of the mechanical philosophy is, in this sense, still in progress.
For the broader history of the philosophy of science of the period, see Philosophy of Science in the Scientific Revolution, Rationalism vs Empiricism, and What Is the Scientific Method?. For the practical application of the new mechanism to the new physics, see The Laws of Motion and Universal Gravitation.