Summary: Physics

Book I

We consider that we truly know something when we understand its causes, principles, and elements. This is why the science of nature aims to discover the causes, principles, and elements of natural beings.

We first perceive things as a whole, but learning is a process that leads us 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 identify 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 false to claim, as they do, that all things are one. Aristotle asks: what is this unity? This question remains unresolved.

Indeed, Aristotle demonstrated 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, signifying that there are two entities.

This One cannot be either a quality or a quantity, as 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 because the continuous is infinitely divisible. If it is indivisible, it will possess neither quantity nor quality. Therefore, it can 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 shared a common rejection of the idea that being is plurivocal—that the same thing can be both one and multiple. For 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 physicists: are things formed through condensation and rarefaction, or through alteration, as proposed by Anaxagoras, among other theories?

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 contradictions, but they differ on 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 (as that would render reality unknowable), nor merely two. Indeed, an opposite alone cannot produce its counterpart: a third principle, distinct from the two opposites, is necessary.

As a result of his investigation, Aristotle determined the number of principles: they are three. But which ones? That is the question he now sets out to address.

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