Imperishable οὐσίαι are exempt from all the kinds of change to which perishable οὐσίαι are subject, with the single exception of change of place: while the heavenly bodies are eternal and do not suffer qualitative change, they move in perfectly regular circular orbits. If, as Aristotle maintains, these heavenly bodies together with all sublunary things make up physical reality, φύσις must be defined accordingly, ie. not in terms of the changeability which characterizes sensible οὐσίαι only, but strictly as change of place. The key principle of Aristotelian physics is movement (κίνησις) in this specific sense. In Book II of the Physics, we read that φύσις is 'a principle or cause of being moved and of being at rest in that to which it belongs primarily' (192b,22-23). Similarly in Book V (1015a,13-15) of the Metaphysics, 'φύσις in the primary and strict sense is the substance of things which have in themselves, as such, a source of movement'.
Aristotle explains his definition by contrasting physical things with the products of art (τέχνη), eg. a bed or a coat (Phys.192b,17). These latter things do not, qua the particular artefacts that they are, possess an inherent principle of movement. Since such artefacts are themselves made from physical substances, they have an impulse to movement which derives from their constituting materials, but this is not an inherent impulse. The bed does not, qua bed, possess any principle of movement, but in so far as it is made of wood, it will partake of the inherent tendency of earth (one of the four elements) to move downwards. Fire, by contrast, has an inherent tendency to move upwards. All complex physical things have an inherent impulse to movement, with the particular direction of the motion being in each case dependent on the combination of elements in the complex thing. The simple heavenly bodies move eternally and unchangeably in their proper place, while the movement of complex physical bodies resolves itself, due to the characteristic movements of the four elements each towards its respective proper place, into an eternal cycle of formation and dissolution. Physical things have the source of