Nature as an inner principle of change

If we are to understand what it is for man by nature to desire to understand, we must understand what it is for something to exist by nature (phusei). Aristotle begins Physics II by saying that existent things can be divided into those which exist by nature and those which exist from other causes. The Greek word which is translated as ‘cause’ does not mean cause in the modern sense: namely, an antecedent event sufficient to produce an effect. Rather, it means the basis or ground of something. Aristotle later says that we do not understand something until we know why it is what it is: and the cause gives us ‘the why.’ We shall discuss Aristotle's conception of cause later. For the moment, the important point is that Aristotle thinks that to say that something exists by nature is to cite its cause.

Aristotle thinks he can unproblematically identify the things that exist by nature. The paradigms are living organisms – animals and plants – but he also includes their parts and the ‘simple bodies’ – earth, air, fire, and water. The task, for Aristotle, is to find the characteristic feature which distinguishes natural items from everything else. ‘Each of them,’ he says, ‘has within itself a principle of change and rest.’ The ability to grow is obvious in plants and animals, and animals can move about their environment, but even the simple elements have tendencies to move in fixed directions.