Since some things are such as to come-to-be and pass-away, and since coming-to-be in fact occurs in the region about the centre, we must explain the number and the nature of the originative sources of all coming-to-be alike: for a grasp of the true theory of any universal facilitates the understanding of its specific forms.
The originative sources, then, of the things which come-to-be are equal in number to, and identical in kind with, those in the sphere of the eternal and primary things. For there is one in the sense of matter, and a second in the sense of form: and, in addition, the third originative source must be present as well. For the two first are not sufficient to bring things into being, any more than they are adequate to account for the primary things.
Now cause, in the sense of material origin, for the things which are such as to come-to-be is that which can be-and-not-be: and this is identical with that which can come-to-be-and-pass-away, since the latter, while it is at one time, at another time is not. (For whereas some things are of necessity, viz. the eternal things, others of necessity are not. And of these two sets of things, since they cannot diverge from the necessity of their nature, it is impossible for the first not to he and impossible for the second to he. Other things, however, can both be and not he.) Hence coming-to-be and passing-away must occur within the field of that which can be-and not-be. This, therefore, is cause in the sense of material origin for the things which are such as to come-to-be; while cause, in the sense of their end, is their figure or form-and that is the formula expressing the essential nature of each of them.
But the third originative source must be present as well--the cause vaguely dreamed of by all our predecessors, definitely stated by none of them. On the contrary (a) some amongst them thought the nature of the Forms was adequate to account for coming-to-be. Thus Socrates in the Phaedo first blames everybody else for having given no explanation; and then lays it down; that some things are Forms, others Participants in the Forms, and that while a thing is said to be in virtue of the Form, it is said to come-to-be qua sharing in, to pass-away qua losing, the Form. Hence he thinks that assuming the truth of these theses, the Forms must be causes both of coming-to-be and of passing-away. On the other hand (b) there were others who thought the matter was adequate by itself to account for coming-to-be, since the movement originates from the matter.
Neither of these theories, however, is sound. For (a) if the Forms are causes, why is their generating activity intermittent instead of perpetual and continuous-since there always are Participants as well as Forms? Besides, in some instances we see that the cause is other than the Form. For it is the doctor who implants health and the man of science who implants science, although Health itself and Science itself are as well as the Participants: and the same principle applies to everything else that is produced in accordance with an art. On the other hand (b) to say that matter generates owing to its movement would be, no doubt, more scientific than to make such statements as are made by the thinkers we have been criticizing. For what alters and transfigures plays a greater part in bringing, things into being; and we are everywhere accustomed, in the products of nature and of art alike, to look upon that which can initiate movement as the producing cause. Nevertheless this second theory is not right either.
For, to begin with, it is characteristic of matter to suffer action, i.e. to be moved: but to move, i.e. to act, belongs to a different power. This is obvious both in the things that come-to-be by art and in those that come to-be by nature. Water does not of itself produce out of itself an animal: and it is the art, not the wood, that makes a bed. Nor is this their only error. They make a second mistake in omitting the more controlling cause: for they eliminate the essential nature, i.e. the form. And what is more, since they remove the formal cause, they invest the forces they assign to the simple bodies-the forces which enable these bodies to bring things into being-with too instrumental a character. For since (as they say) it is the nature of the hot to dissociate, of the cold to bring together, and of each remaining contrary either to act or to suffer action, it is out of such materials and by their agency (so they maintain) that everything else comes-to-be and passes-away. Yet (a) it is evident that even Fire is itself moved, i.e. suffers action. Moreover (b) their procedure is virtually the same as if one were to treat the saw (and the various instruments of carpentry) as the cause of the things that come-to-be: for the wood must be divided if a man saws, must become smooth if he planes, and so on with the remaining tools. Hence, however true it may be that Fire is active, i.e. sets things moving, there is a further point they fail to observe--viz. that Fire is inferior to the tools or instruments in the manner in which it sets things moving.