(In discussing the causes of coming-to-be) we must first investigate the matter, i.e. the so-called elements. We must ask whether they really are elements or not, i.e. whether each of them is eternal or whether there is a sense in which they come-to-be: and, if they do come-to-be, whether all of them come-to-be in the same manner reciprocally out of one another, or whether one amongst them is something primary. Hence we must begin by explaining certain preliminary matters, about which the statements now current are vague.
For all (the pluralist philosophers)--those who generate the elements as well as those who generate the bodies that are compounded of the elements--make use of dissociation and association, and of action and passion. Now association is combination; but the precise meaning of the process we call combining has not been explained. Again, (all the monists make use of alteration: but) without an agent and a patient there cannot be altering any more than there can be dissociating and associating. For not only those who postulate a plurality of elements employ their reciprocal action and passion to generate the compounds: those who derive things from a single element are equally compelled to introduce acting. And in this respect Diogenes is right when he argues that unless all things were derived from one, reciprocal action and passion could not have occurred. The hot thing, e.g. would not be cooled and the cold thing in turn be warmed: for heat and cold do not change reciprocally into one another, but what changes (it is clear) is the substratum. Hence, whenever there is action and passion between two things, that which underlies them must be a single something. No doubt, it is not true to say that all things are of this character: but it is true of all things between which there is reciprocal action and passion.
But if we must investigate action-passion and combination, we must also investigate contact. For action and passion (in the proper sense of the terms) can only occur between things which are such as to touch one another; nor can things enter into combination at all unless they have come into a certain kind of contact. Hence we must give a definite account of these three things--of contact, combination, and acting.
Let us start as follows. All things which admit of combination must be capable of reciprocal contact: and the same is true of any two things, of which one acts and the other suffers action in the proper sense of the terms. For this reason we must treat of contact first. every term which possesses a variety of meaning includes those various meanings either owing to a mere coincidence of language, or owing to a real order of derivation in the different things to which it is applied: but, though this may be taken to hold of contact as of all such terms, it is nevertheless true that contact in the proper sense applies only to things which have position. And position belongs only to those things which also have a Place: for in so far as we attribute contact to the mathematical things, we must also attribute place to them, whether they exist in separation or in some other fashion. Assuming, therefore, that to touch is--as we have defined it in a previous work--to have the extremes together, only those things will touch one another which, being separate magnitudes and possessing position, have their extremes together. And since position belongs only to those things which also have a place, while the primary differentiation of place is the above and the below (and the similar pairs of opposites), all things which touch one another will have weight or lightness either both these qualities or one or the other of them. But bodies which are heavy or light are such as to act and suffer action. Hence it is clear that those things are by nature such as to touch one another, which (being separate magnitudes) have their extremes together and are able to move, and be moved by, one another.
The manner in which the mover moves the moved not always the same: on the contrary, whereas one kind of mover can only impart motion by being itself moved, another kind can do so though remaining itself unmoved. Clearly therefore we must recognize a corresponding variety in speaking of the acting thing too: for the mover is said to act (in a sense) and the acting thing to impart motion. Nevertheless there is a difference and we must draw a distinction. For not every mover can act, if (a) the term agent is to be used in contrast to patient and (b) patient is to be applied only to those things whose motion is a qualitative affection--i.e. a quality, like white or hot, in respect to which they are moved only in the sense that they are altered: on the contrary, to impart motion is a wider term than to act. Still, so much, at any rate, is clear: the things which are such as to impart motion, if that description be interpreted in one sense, will touch the things which are such as to be moved by them-while they will not touch them, if the description be interpreted in a different sense. But the disjunctive definition of touching must include and distinguish (a) contact in general as the relation between two things which, having position, are such that one is able to impart motion and the other to be moved, and (b) reciprocal contact as the relation between two things, one able to impart motion and the other able to be moved in such a way that action and passion are predicable of them.
As a rule, no doubt, if A touches B, B touches A. For indeed practically all the movers within our ordinary experience impart motion by being moved: in their case, what touches inevitably must, and also evidently does, touch something which reciprocally touches it. Yet, if A moves B, it is possible-as we sometimes express it-for A merely to touch B, and that which touches need not touch a something which touches it. Nevertheless it is commonly supposed that touching must be reciprocal. The reason of this belief is that movers which belong to the same kind as the moved impart motion by being moved. Hence if anything imparts motion without itself being moved, it may touch the moved and yet itself be touched by nothing-for we say sometimes that the man who grieves us touches us, but not that we touch him.
The account just given may serve to distinguish and define the contact which occurs in the things of Nature.