PHILOSOPHY OF NATURE
Aristotle’s classification of the sciences, as we have seen, divides them firstly into the theoretical, which aim at knowledge for its own sake, the practical, which aim at knowledge as a guide to conduct, and the productive, which aim at knowledge to be used in making something useful or beautiful. The theoretical sciences are subdivided into ‘theology’ (or metaphysics), physics, and mathematics. Physics deals with things that have a separate existence but are not unchangeable (i.e. with ‘natural bodies’, which have in them a source of movement and rest), mathematics with things that are unchangeable but have no separate existence (i.e. with numbers and spatial figures which have only an adjectival existence, as qualifying substances); theology with things that both have separate existence and are unchangeable (i.e. with the substances which exist free from any connexion with matter); it owes its name to the fact that the chief of these pure substances is God.1 ‘Physics’ as thus defined is expounded by Aristotle in a long series of works. That these are thought of as forming a unity is indicated by the opening of the Meteorologica; Aristotle there claims to have dealt (1) with the first causes of nature (i.e. the constituent elements which in Physics I., II. he shows to be involved in all change), and with natural movement in general (Physics III.-VIII.); (2) with the order and movement of the stars (De Caelo, I., II.), the number and nature of the bodily elements and their transformation into each other (De Caelo, III., IV.); (3) with coming to be and passing away, in general (De Generatione et Corruptione). He proposes to deal (4) with ‘the things that happen in accordance with nature, but a nature less ordered than that of the first (or celestial) element, in the region that borders most closely on the movement of the stars’2 (Meteorologica); and (5) with animals and plants both in general and according to their kinds (the biological works). The movement, it will be seen, is from general to particular.3 The Physics deals in fact with natural body in general, with the common nature of all those bodies which have in themselves a source of movement and of rest. This includes not only living bodies but the elements and their inorganic compounds; these also have an innate tendency to movement—either in a circle, or from or towards the centre of the universe. Even manufactured things have a natural movement, inasmuch as their materials are natural bodies; but their movement as manufactured things is something imposed on them by the hand of the craftsman who makes them and of him who uses them.4 The Physics announces itself as dealing with ‘the science of nature,’ but offers at the start no account of what is meant by ‘nature.’ There lay behind it a whole series of works ‘on nature,’ for this had been a favourite title with the pre-Socratics; and in the light of these earlier works Aristotle could count on his meaning being sufficiently plain. He would be understood as intending to deal both with the ultimate stuff of which material bodies are made and with the nature and causes of the changes discernible in them. The importance of discovering causes is emphasised at the outset. The facts of experience are represented as a confused mass which must be analysed until we see its ultimate implications, the ‘origins,’ ‘causes,’ or ‘elements’ which are ‘clear by nature’ though to us initially obscure.5 Different views may be taken of these originative causes. But there is one view, Aristotle points out, which amounts to the abolition of natural philosophy—the view that reality is single, undivided, and unchangeable. We must take it as established by experience that change exists, and we must make this our basis. But Eleaticism has played so large a part in Greek thought that Aristotle can-not brush it aside by a mere appeal to experience; he proceeds to point out various confusions on which it rests.6
SUBSTRATUM, FORM, PRIVATION
The views of the ‘natural philosophers’ (as opposed to the Eleatics, who in principle denied the existence of nature) are of two main kinds, Some hold that there is one kind of underlying body from which all other things are generated by condensation and rarefaction. Others hold that there are fundamental qualitative differences between things but that all things have been sifted out of a single mass in which all the ‘contrarieties’ were present. The latter view is subjected to criticism.7 What Aristotle finds common to all previous schools is that they recognise contraries as first principles. Rare and dense, solid and void, being and not being, up and down, before and behind, straight and curved -such opposites play important parts in all the earlier theories. This follows from the nature of first principles. (1) They must not be generated one from another, nor from other things, and (2) all other things must be generated from them. The primary contraries, whatever they are, evidently satisfy these conditions. But the doctrine may be confirmed by a more elaborate argument. Everything in the world requires the presence of a particular character in that out of which it is to emerge; that is, if we rule out accidental connexions. The white can come to be out of the musical only because the not-white happens to be musical; strictly it comes into being out of the not-white, i.e. from what is black, or intermediate between black and white. And inter-mediates are formed by a mixture of contraries, so that at bottom what change to any state presupposes is the contrary of that state.8
There are thus at least two first principles. There cannot be an infinite number. For (1) if there were, being would be unknowable; (2) substance is one genus, and one genus has only one fundamental contrariety; (3) it is possible to derive reality from a finite number of principles, and a simple explanation, where it is possible, is better than a more complex one; (4) some contraries are obviously derivative, but first principles must be eternal, non-derivative. But we cannot cut our principles down to two, as economy might suggest. For (1) density does not act on rarity nor vice versa; love does not bring together strife nor does strife separate love; there must be a third thing which the one brings together and the other divides. (2) There seems to be nothing whose substance is through and through one of two contraries. Contraries are essentially adjectival; they presuppose a substance in which they inhere. (3) Substance is never contrary to substance. To treat contraries as the first principles, then, is to derive substance from non-substances; but there can be nothing more primary than substance. We must, then, presuppose a tertium quid, recurring in this to the view of the early thinkers who supposed a single material substratum of all things. But we must not identify this ultimate substratum with any one of the obvious elementary bodies; fire, air, earth, and water include contrarieties in their nature—e.g, fire moves up, earth down. It would be more reasonable to identify the substratum with something intermediate between the four ‘elements.’
A single substratum, and contraries differing by excess and defect of some quality,—these are the principles which a simple study of change reveals, and these are in fact the principles at which earlier thought has arrived. Nothing is gained, and some-thing is lost, by recognising more than three principles. Of passive principles one is clearly enough; but if we allow more than one pair of contrary active principles, each pair will require a separate passive principle to work on. Besides, substance, being a single genus, can only have principles distinguished by order of priority, not generically different fundamental principles. We are safe then in saying that there are neither less than two nor more than three first principles.9 We speak of two different sorts of thing as coming to be; we say ‘the man becomes musical’ and we say ‘the unmusical becomes musical.’ In the former case that which becomes persists, in the latter it passes away. But whether we say ‘a- becomes b-’ or ‘not b becomes b,’ what always happens is that a-not-b becomes ab. The product contains two elements (a substratum and a form), but a third element is presupposed by the change (the privation of the form). The substratum, before the change, was numerically one, but included two distinguishable elements—that which was to persist through the change and that which was to be replaced by its opposite. Thus we get three presuppositions of change -matter, form, privation.10 Earlier thinkers had been baffled by the problem of becoming; that which is apparently could not come to be out of that which is, nor yet out of that which is not. Aristotle solves the difficulty by pointing out (1) that nothing comes into being simply from not-being. A thing comes into being from its privation, which is indeed slmpliciter not-being, but it comes into being from it not simpliciter but incidentally; it could not come into being from bare privation, but only from privation in a substratum. And again nothing comes into being simpliciter from being. It comes into being from that which incidentally is, but not from it as being, but as not being the particular thing that comes to be. (2) The difficulty is removed by the distinction of grades of being—potentiality and actuality; a thing comes from that which is it potentially but not actually.11
The matter and form of physical things, it must be noted, are elements distinguishable by thought but inseparable in reality. Matter never exists bare but always informed. It exists wit...