Motion

Motion is a kind of change, so it is worth beginning with change generally. Change is puzzling because it requires something both to remain the same (so one thing changes) and to become different (so change has occurred), a contradiction according to some pre-Socratic philosophers. Aristotle’s analysis of change dissolved the paradox: an underlying subject must remain the same thing, but which properties inhere in it change. But then what causes change? According to Aristotle things possess forms, or ideal states, and change in order to achieve them: the form of an acorn is an oak so the acorn grows to become an oak. In the seventeenth century, Aristotle’s account was superseded by a mechanistic understanding of change. According to Descartes the only fundamental properties of bodies are geometrical – shape, size and motion – and all other properties are to be explained in terms of them: all physical change is merely change in the geometrical arrangements of bodies caused by their collisions with one another. The success of Newton’s theory of planetary motion showed that Descartes’ picture was incomplete, and that a limited number of forces, such as gravity, are also needed to account for change.

Turning to motion specifically, we first ask what causes it. Is it, as the Aristotelians came to believe, ‘impetus’: moving bodies possess an internal force that overcomes their natural resistance to motion? Following Galileo, modern science takes constant, straight-line motion to be natural, requiring no cause, and inertia (or momentum) as a measure of how much external force is required to bring a body to rest. Second, there is the question of whether motion is always relative; or do the laws of physics support a sense of motion in which ‘x moves’ – without mention of a reference body – is determinately true or false? In fact almost all interpretations of motion in almost all systems of mechanics (Aristotle’s, Descartes’, Newton’s, relativity) imply that there is such a sense.