De principiis naturae

Having seen, then, that there are four genera of causes, it should be known that it is not impossible for the same thing to have several causes, as a statue whose causes are bronze and the craftsman — but the craftsman as efficient cause, bronze as matter. Nor is it impossible for the same thing to be the cause of contraries, as the helmsman is the cause of the safety of the ship and of its sinking, but of the latter by absence, of the former by presence.

It should also be known that it is possible for one and the same thing to be cause and caused with respect to the same thing, but in different ways: as walking is the cause of health as efficient, while health is the cause of walking as an end, for walking is sometimes for the sake of health; and likewise the body is the matter of the soul, while the soul is the form of the body. For the efficient is called a cause with respect to the end, since the end is not in act except through the operation of the agent; but the end is called the cause of the efficient, since the agent operates only through intending the end. Hence the efficient is the cause of that which is the end — that health may be — yet it does not make the end to be an end; and so it is not the cause of the causality of the end, that is, it does not make the end to be final: as the physician makes health to be in act, yet does not make health to be an end.

The end, however, is not the cause of that which is the efficient, but is the cause that the efficient be efficient; for health does not make the physician to be a physician — and I mean the health that is actually the end of the worker — but it makes the physician to be efficient. Hence the end is the cause of the causality of the efficient, because it makes the efficient to be efficient: and likewise it makes matter to be matter and form to be form, since matter does not receive form except on account of the end, and form does not perfect matter except on account of the end. Hence it is said that the end is the cause of causes, because it is the cause of causality in all causes.

For matter is said to be the cause of form insofar as form exists only in matter; and likewise form is said to be the cause of matter, insofar as matter has being in act only through form: for matter and form are said relatively to each other, as is said in II Physics. For the efficient is the cause of the end insofar as it is ordered to what follows: as parts are to the whole and the simple to the composite.

But because every cause, insofar as it is a cause, is naturally prior to what is caused, it should be known that prior is said in two ways, as Aristotle says in XVI On Animals: through their diversity something can be called prior and posterior with respect to the same thing, both cause and caused. For something is said to be prior to another in generation and time, and again in substance and completeness. Since, therefore, the operation of nature proceeds from the imperfect to the complete, the imperfect is prior to the perfect in generation and time, but the perfect is prior in completeness: as it can be said that the man is before the boy in substance and completeness, but the boy is before the man in generation and time. But although in generable things the imperfect is prior to the perfect and potency prior to act, considering in one and the same thing that first it is imperfect and in potency and then perfect and in act, still speaking simply act and the perfect must be prior, because what reduces potency to act is in act, and what perfects the imperfect is perfect. Matter is indeed prior to form in generation and time, for what a thing comes to is prior to what comes to it; form, however, is prior to matter in perfection, because matter has complete being only through form. Likewise the efficient is prior to the end in generation and time, since motion toward the end comes about from the efficient; but the end is prior to the efficient insofar as it is efficient in substance and completeness, since the action of the efficient is not completed except through the end. Therefore these two causes, namely matter and the efficient, are prior by the way of generation, while form and end are prior by the way of perfection. And it should be noted that necessity is twofold, namely absolute necessity and conditional necessity. Absolute necessity is that which proceeds from the causes prior in the way of generation, which are matter and the efficient, as the necessity of death which comes from matter and from the disposition of the contraries composing [the thing]; and this is called absolute because it has no impediment; this is also called the necessity of matter. Conditional necessity, however, proceeds from the causes posterior in generation, namely from form and end, as when we say that conception must exist if a man is to be generated; and this is conditional, because this woman's conceiving is not necessary simply, but under a condition: if a man is to be generated. And this is called the necessity of the end. And it should be known that three causes can coincide in one, namely form, end, and efficient, as is clear in the generation of fire: for fire generates fire, so fire is the efficient cause insofar as it generates; and again fire is form insofar as it makes to be in act what was previously in potency; and again it is the end insofar as it is the terminus set by the agent and insofar as the operations of the agent are terminated in it.

But the end is twofold, namely the end of generation and the end of the thing generated: as in the generation of a knife: for the form of the knife is the end of the generation, but cutting, which is the operation of the knife, is the end of the knife itself, namely of the thing generated. The end of generation sometimes coincides, from two of the causes mentioned, namely when generation comes about from a like in species, as man generates man and the olive generates the olive; which cannot be understood of the end of the thing generated.

It should be known, however, that the end coincides with the form in the same in number, because the end of generation and the form of the thing generated are the same in number: as the soul is the end and the form of the man generated. But with the efficient it does not coincide in the same in number, but in the same in species; for it is impossible for the maker and the made to be the same in number, but they can be the same in species: as when man generates man, the generating man and the generated man are diverse in number, but the same in species. Matter, however, does not coincide with the end in the same in number nor in the same in species, because matter is imperfect while the end is perfect: but the other causes, since they are in act, have the character of the perfect; and the perfect and the imperfect do not coincide in the same.