Nutrition provides Aristotle a way of distinguishing form and matter in living things. It also provides the way of binding form and matter. On the one hand, living activity takes part in continuously different matter, in any individual. On the other, nutrition, specified by any one indivisible kind of living thing, must take part in matter of the requisite kind. Between them these two facts explain the sense in which living is in a subject without being an attribute. For the soul is the cause of the living thing: it brings it about that there is a living body. Nutrition is the way that the soul achieves this. This explains how soul can then serve as the substance of living things in the Metaphysics.