Summary: Physics

Book I

We consider that we know a thing when we know its causes, principles and elements. This is why the science of nature has the vocation of finding the causes, principles and elements of natural beings.

We first know things as a whole, but learning is a process that leads us to discover their parts: that is why we must go from things in general to particular things ¹, from things clear to us to things clear by nature.

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

Against Parmenides and Melissos, Aristotle asserts that motion exists, and that all things or at least some things are moved. On the other hand, it is false to maintain, as they do, that all things are one. What is this unity, asks Aristotle? This remains undetermined.

In fact, Aristotle showed in the Categories that being is plurivocal: we find being in potency, in act, being by itself, by accident, etc.

Moreover the principle is always principle of something: the very term principle implies a duality (that there are two things).

This One cannot be either quality or quantity because these two predicates always refer to the subject; again, this implies 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 have neither quantity nor quality. It will therefore be neither infinite as Melissos says, nor finite as Parmenides maintains.

Finally, what is the exact meaning of the idea that all things are One? If "being One" means "being synonymous", then they employ the language of Heraclitus, good will be the same as evil ². And their discourse will not be about the fact that the beings are One, but that they are nothing ³.

In fact, the pre-Socratics were united by the same rejection of the idea that being is plurivocal, namely that the same thing is both one and multiple. For then that would be a contradiction, and it would introduce contradiction into the heart of being.

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

How, since everything is not One, are different things formed? Aristotle examines the doctrines of the various physicists: should we believe that they are formed by condensation and rarefaction, by alteration (Anaxagoras), etc.?

After criticising the idea of a One principle (monism), Aristotle examines the doctrine held by several pre-Socratics according to which there are two contrary principles, and that it is from their opposition that the plurality of beings is born.

All these thinkers agree that the principles are the contradictions but differ on the choice of the contraries.

For example, for Democritus it is being and void. For others, heat and cold, even and odd, and so on.

In fact, for Aristotle, principles can be neither one (as evidenced by his refutation of monism), nor infinite in number (because reality would be unknowable), nor two. Indeed, alone, an opposite cannot produce its opposite: a third principle different from the two opposites is needed.

Aristotle determined, as a result of this investigation, the number of principles: they are three. Which ones? That is the question he is now going to tackle.

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