In the chapters of his 'Physics' commented on here, Aristotle disagrees with Pre-Socratic philosophers about the basic principles that explain natural changes. But he finds some agreement among them that at least two contrary properties must be involved, for example hot and cold. His own view is that there are two contrary principles at a more abstract level: the form possessed at the end of a change and the privation of that form at the beginning. But there is also a third principle needed to supply continuity - the matter to which first privation and later form belong. Despite the apparent disagreements, Simplicius, the Neoplatonist commentator, wants to emphasise the harmony of all pagan Greek thinkers, as opposed to Christians, on such a basic matter as first principles. He therefore presents not only the Pre-Socratics and Aristotle, but also himself and earlier commentators of different schools as all in basic agreement.