Between the age of Homer (mid- eighth century) and the age of Socrates (late fifth century), the Greeks began systematic rational study of the natural order and the moral order. Aristotle distinguishes those who talk about gods and offer poetic or mythological accounts from those who offer rational accounts that can be seriously studied:

The school of Hesiod and all the theologians considered only what was persuasive to themselves, and thought little of us . . . But it is not worth seriously examining the sophistries of mythology, whereas we must interrogate those who present a rational demonstration.

He calls the second group 'students of nature' or 'naturalists' (phusiologoi), as opposed to Hesiod and his followers, because they abandon mythology to ask a new question, about the nature (phusis) of things. Aristotle's comments on the 'mythologists' are unsympathetic, indeed unfair; but he has good reason to believe that a new movement began with Thales (c. 625-c.545), 'the originator of this sort of philosophy'. He rightly thinks it is worth his while to conduct a rational discussion with these thinkers.

Many have followed Aristotle in taking the naturalists to be the first philosophers and scientists. To see if this judgement is right, we ought to see what is distinctive of these thinkers.

Aristotle claims that the naturalists identify the nature with the 'matter' or 'basic subject' of things:

Most of the first philosophers thought the only origins of everything were material. For, they say, there is some ‘subject’ that all beings come from, the first thing they come to be from and the last thing they perish into, the substance remaining throughout and changing in its attributes; and this is the elementary ‘basis’ and the origin of beings. And for this reason they think nothing either comes to be or perishes, since they assume that in every change this nature [i.e. the subject] persists. For just as we say Socrates does not come to be unqualifiedly whenever he becomes good or musical, and does not perish ‘unqualifiedly’ whenever he loses these states, since the subject, Socrates himself, remains, so also, ‘they say’, nothing else ‘either comes to be or perishes unqualifiedly’, since there must be some nature, either one or more than one, that persists while the other things come to be from it.

We can also recognize a continuing subject in more extensive changes: if we make a square lump of bronze that was one centimetre wide into a round coin that is two centimetres in diameter, the very same lump of bronze is the continuing subject, its dimensions and shape alone having changed. The continuing subject may not be continuously observable as easily as the bronze is: if we cook oats and water to make porridge, the result may not look much like oats or water, but still they are what it is, and they are continuing subjects that have undergone change.

If we find the continuing subject of change, we seem to find the nature of things. If we take an ordinary subject, we can ask what its nature is, or what the subject really is; and a reasonable answer will tell us what its basic, underlying subject is. It is not Socrates' nature to be tanned or pale, since he could lose these properties and still exist; but we might say it is his nature to be a man, and the man is the basic subject. Similarly, we can say that porridge is oats and water that have been cooked together; this is a better answer than 'Porridge is a sticky mess' or 'Porridge is light grey', because it allows us to say how the porridge came into being. It also allows us, in principle, to explain the properties and behaviour of the porridge; if we know what oats and water are like and how they interact, we can predict what will happen when porridge is eaten by someone with a specific type of body. In Aristotle's view, the naturalists want to explain the world as a whole in the same way. They want to find the nature of things by finding their basic matter.

In Homer the nature and constitution of things does not play the primary role in explaining what happens to them. He often explains events by some external divine agency affecting the sea to produce a storm, or affecting human bodies to produce a plague. In so far as they appeal to the nature and constitution of things, the naturalists assume that this Homeric view is mistaken. In their view, things seem random, or to require divine intervention, only because we do not know enough about the constituent stuffs and processes. . .