But because every cause, in so far as it is a cause, is naturally prior to what is caused, it should be noted that "prior" is said in two ways, as Aristotle says in Bk.XVI of On Animals (c.f.On the Generation of Animals II, 6, 742a21).Through this diversity, something can be called both prior and posterior and cause and caused with respect to the same thing.
For something is called prior to another in generation and time or prior in substance and completeness.Therefore, since the operation of nature is from the imperfect to the perfect and from the incomplete to the complete, the imperfect is prior to the perfect according to generation and time, but the perfect is prior to the imperfect according to substance: for example, it can be said that the man is prior to the boy in substance and completeness, but the boy is prior to the man in generation and time.But, although in things capable of coming-to-be, the imperfect is prior to the perfect and potency is prior to act--considering that in any subject that what is prior is imperfect rather than perfect and in potency rather than in act--nevertheless, absolutely speaking, it is necessary that what is in act and perfect be prior; because what reduces potency to act is in act and what perfects the imperfect is perfect.The matter is prior to the form in generation and time, for that to which something comes is prior to that which comes to it.But the form is prior to the matter in substance and complete being because the matter has complete being only through the form.Similarly the efficient cause is prior to the end in generation and time, since the movement towards the end is brought about by the efficient cause; but the end is prior to the efficient cause, in so far as it is efficient cause, in substance and completeness, since the action of the efficient cause brings about completeness only through the end.Accordingly, these two causes, the material and the efficient are prior by way of generation;but the form and the end are prior by way of perfection.
And it should be noted that there are two kinds of necessity, namely, absolute necessity and conditional necessity.That necessity is absolute which proceeds from prior causes in the order of generation, and these are the material and the efficient causes; for example the necessity of death which comes about from matter, namely from the disposition of contrary components--and it is called absolute because there is no impediment to it.This necessity is also called the necessity of matter.On the other hand, conditional necessity proceeds from causes which are posterior in generation, namely, from the form and the end; for example, we say that it is necessary that there be conception if a man is to be generated.And this necessity is also called conditional, because it is not absolutely necessary that this woman conceive but only under this condition, namely, if a man is to be generated.And this necessity is called the necessity of the end.
And it should be noted that three of the causes, namely, the form, the end and the efficient cause, can coincide in one thing as is evident in the coming-to-be of fire.
For fire brings fire to be, therefore fire is the efficient cause in so far as it brings to be; and again, fire is the form in so far as it makes to be in act what before was in potency; and again it is the end in so far as it is the intention of the agent and in so far as the operation of the agent is terminated in it.But there are two kinds of ends, namely, the end of the generation and the end of the thing generated, such as is evident in the generation of a knife: for the form of a knife is the end of the generation, but cutting, which is the operation of the knife, is the end of the thing generated, that is, the knife.However, sometimes the end of the generation coincides with the other two above-mentioned causes (the form and the efficient cause), namely, when the generation is of what is similar in species; for example, when a man generates a man or an olive tree generates an olive tree which (coincidence of form, efficient cause, and end) cannot be understood of the end of the thing generated.
Nevertheless it should be noted that the end is identical with the form numerically, because it is the same something numerically which is the form generated and is the end of generation.But the end of the generation is not identical with the efficient cause in the same number but in the same species.For it is impossible that the maker and the thing made be numerically the same, but they can be specifically the same; for example, when a man generates a man, the man generating and the man generated are different by number but of the same species.However, the matter cannot coincide with the other causes because the matter by the fact that it is a being in potency, has the notion of imperfection; but the other causes, since they are in act, involve the notion of perfection; moreover, the perfect and the imperfect do not coincide in the same thing.
Accordingly, having seen that there are four causes, namely, the efficient, the material, the formal, and the final it should be known that these same causes are divided in many ways.There can be prior cause and posterior cause, as when we say that both the art and the doctor are the cause of health, but the art is the prior cause and the doctor the posterior cause.And the same division holds for the formal cause and the other causes.And notice carefully that we should always lead back a question to the first cause.
