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CAUSALITYC.G. II, ch. 30
It is therefore clear from what we have said that the necessity which arises from an efficient cause in some cases depends on the disposition of the agent alone; but in others, on the disposition of both agent and patient. Consequently, if this disposition, according to which the effect follows of necessity, be absolutely necessary both in the agent and in the patient, then there will be absolute necessity in the efficient cause, as with things that act necessarily and always.
On the other band, if this disposition be not absolutely necessary, but removable, then from the efficient cause no necessity will result, except on the supposition that both agent and patient possess the disposition necessary for acting. Thus we find no absolute necessity in those things that are sometimes impeded in their activity either through lack of power or the violent action of a contrary; such things, then, do not act always and necessarily, but in the majority of cases.
C.G. II, ch. 39, par. 3
Chance is found only in things that are possibly otherwise; and the source of this possibility is matter, and not the form, which indeed determines the matter, which is a reservoir of multiple possibilities.
C.G. III, ch.2, par.8
If an agent did not incline toward some definite effect, all results would be a matter of indifference for him. Now, he who looks upon a manifold number of things with indifference no more succeeds in doing one of them than another. Hence, from an agent contingently indifferent to alternatives no effect follows, unless he be determined to one effect by something. So, it would be impossible for him to act. Therefore, every agent tends toward some determinate effect, and this is called his end.
S.T. I, q.105, a.5
The operative powers which are seen to exist in things would be bestowed on things to no purpose, if things produced nothing through them. Indeed, all things created would seem, in a way, to be purposeless, if they lacked an operation proper to them; since the purpose of everything is its operation. For the less perfect is always for the sake of he more perfect. Consequently, just as the matter is for the sake of the form, so the form which is the first act is for the sake of its operation, which is the second act.
S.T. I, q.104, a.1
Every effect depends on its cause, so far as it is its cause. But we must observe that an agent may be the cause of the becoming of its effect, but not of its being. This may be seen both in artificial and in natural things. For the builder causes the house in its becoming, but he is not the direct cause of its being. For it is clear that the being of the house is a result of its form, which consists in the putting together and arrangement of the materials, and which results from the natural qualities of certain things. Thus the cook prepared the food by applying the natural activity of fire; and in the same way a builder constructs a house, by making use of cement, stones, and wood, which are able to be put together in a certain order and to conserve it. Therefore the being of the house depends on the nature of these materials, just as its becoming depends on the action of the builder.
The same principle applies to natural things For if an agent is not the cause of a form, as such, neither will it directly be the cause of the being which results from that form; but it will be the cause of the effect only in its becoming.
Now it is clear that of two things in the same species one cannot be essentially the cause of the other's (~) form as such, since it would then be the cause of its own form, since both forms have the same nature; but it can be the cause of (+) this form inasmuch as it is in matter-in other words, it may be the cause that this matter receives this form. And this is to be the cause of becoming, as when man begets man, and fire causes fire.
Thus whenever a natural effect is such that it has an aptitude to receive from its active cause an impression specifically the same as that in the active cause, then (+) the becoming of the effect depends on the agent, but not (~) its being.
S.T., I, q.46, a.2, ad 7
In efficient causes it is impossible to proceed to infinity per se. Thus, there cannot be an infinite number of causes that are per se required for a certain effect; for instance, that a stone be moved by a stick, the stick by the hand, and so on, to infinity. But it is not impossible to proceed to infinity accidentally as regards efficient causes; for instance, if all the causes thus infinitely multiplied should have the order of only one cause, while their multiplication is accidental; e.g., as an artificer acts by means of many hammers accidentally, because one after the other is broken. It is accidental, therefore, that one particular hammer should act after the action of another, and it is likewise accidental to this particular man as generator to be generated by another man; for he generates as a man, and not as the son of another man. For all men generating hold one grade in the order of efficient causes, viz., the grade of a particular generator. Hence it is not (~) impossible for man to be generated by man to infinity; but this would be (+) impossible if the generation of this man depended on this man, and on an elementary body, and on the sun, and so on to infinity.