Summary: Physics

Book I

We can be said to truly know something only when we understand its causes, principles, and elements. This is why natural science aims to discover the causes, principles, and elements of natural beings.

We first perceive things as a whole, but learning is a process through which we come to discover their parts. This is why we must proceed from what is general to what is particular ¹, from what is clear to us to what is clear by nature.

The principle is the origin of a thing. Aristotle seeks to establish the characteristics of the principle that underlies all things. This principle is either one or many. If it is one, it must be either immobile (Parmenides or Melissos) or in motion (physicists). If they are multiple, are they finite or infinite? Are they of one kind or of opposite kinds?

In opposition to Parmenides and Melissos, Aristotle asserts that motion exists, and that all things, or at least some things, are in motion. Moreover, it is wrong to claim, as they do, that all things are one. Aristotle asks: what is this unity? This question remains unresolved.

Indeed, Aristotle showed in the Categories that being is plurivocal: it manifests as being in potency, in act, in itself, by accident, and in other modes.

Moreover, a principle is always the principle of something: the very term 'principle' implies a duality — there are always two entities involved.

This One can be neither a quality nor a quantity, since these predicates always refer to a subject, thereby implying duality.

Either the One is continuous or it is indivisible. If it is continuous, the One is multiple — for the continuous is infinitely divisible. If it is indivisible, it possesses neither quantity nor quality — and can therefore be neither infinite, as Melissos claims, nor finite, as Parmenides maintains.

Finally, what exactly does it mean to say that all things are One? If "being One" means "being identical", then they employ the language of Heraclitus, good will be the same as evil ². And their discourse will not be about the unity of beings, but about their non-existence ³.

In fact, the pre-Socratics all rejected the idea that being is plurivocal — that the same thing can be both one and multiple — since such a view would be contradictory, introducing contradiction into the very heart of being.

By contrast, for Aristotle, beings are multiple either by definition (the definition of "white" differs from that of "quick") or by division (a whole is composed of several parts). He therefore rejects the monism of Parmenides.

If everything is not One, how then are different things formed? Aristotle examines the doctrines of various natural philosophers: are things formed through condensation and rarefaction, through alteration as proposed by Anaxagoras, or through other processes?

After criticising the notion of a single principle (monism), Aristotle examines the doctrine of several pre-Socratics, who held that there are two opposing principles and that the plurality of beings arises from their opposition.

All these thinkers agree that the principles are the contraries, but they differ as to which contraries are fundamental.

For example, according to Democritus, they are being and void. Others propose pairs such as heat and cold, or even and odd, among other opposites.

Indeed, Aristotle holds that principles can be neither one (as shown by his refutation of monism), nor infinite (for that would render reality unknowable), nor merely two — for an opposite alone cannot produce its counterpart: a third principle, distinct from the two opposites, is required.

His investigation thus leads Aristotle to determine the number of principles: there are three. But which ones? That is the question he now sets out to answer.

¹ Physics, Book I, 184a
² ibid., 185b
³ ibid.