Change, motion, and explanation › Place, the void, and the moving container

Book IV asks what a body's place is. Aristotle distinguishes it from the body's own matter or form and from a separately existing spatial interval. His answer identifies place with the first unmoving boundary of what contains the body (Physics IV.4). Both "boundary" and "unmoving" matter.

A vessel provides a first illustration of containment. Aristotle explicitly distinguishes a vessel, which can be carried, from place as an immovable container. His river example makes the difficulty visible: the immediately surrounding water can flow past a boat, while a broader containing arrangement supplies the fixed reference. Removing the unmoving qualification turns his account into a different one.

The account has substantial physical commitments. Aristotle rejects a separately existing void and argues against the idea that locomotion requires empty gaps. Bodies can make room for one another. He also considers how different media affect speed and argues that motion in a void would create problems for the ratios he uses to compare movement (Physics IV.6–9).

We can assess the reasoning without insulating it from physical criticism. His assumptions about motion and resisting media do not become correct merely because they appear in a philosophical inquiry. A philosophical interest in place does not require defending all the physical conclusions he draws from it. The task is to distinguish which assumptions an argument uses and whether its conclusion follows from adequate assumptions.