As we have seen Physics I identifies matter and form as ‘the causes and principles’ from which naturally generated substances first come into being (e.g. 190b10–23; cf. Metaph. VII 8). On this model, matter plays the role of the subject from which the change proceeds while form is the positive state acquired as a result of the change. When Aristotle returns to the principles of natural generation in GC II 9, he goes to great lengths to show that these two principles alone are not sufficient to account for the change. Instead, we must posit a third principle over and above those two.