Summa contra Gentiles3.86 Chapter LXXXVI. That corporeal effects in this lower world do not result of necessity from the action of the heavenly bodies

Not only are the heavenly bodies unable to necessitate man’s choice, but even corporeal effects do not proceed from them of necessity.

For the impressions of universal causes are received by their effects according to the mode of the recipient. Now the things of this lower world are fluctuating and changeable, both by reason of matter which is in potentiality to several forms, and on account of the contrariety of forms and powers. Therefore the impressions of heavenly bodies are not received of necessity by these lower bodies.

Again. A remote cause does not lead to a necessary result, unless the middle cause be also necessary: in a syllogism, for instance, if the major premiss be a necessary statement, and the minor a contingent statement, the conclusion that follows is not necessary. Now heavenly bodies are remote causes, and the proximate causes of the effects here below are the active and passive forces in the bodies of this lower world; and these are not necessary, but contingent, causes, for they may fail in a few instances. Therefore the heavenly bodies do not produce necessary effects in these lower bodies.

Besides. The heavenly bodies are always moved in the same way. Consequently, if heavenly bodies produced an effect on these lower bodies of necessity, there would be no variety in the things that happen to them. Now, they are not always the same, but only for the most part. Therefore they do not happen necessarily.

Moreover. Many contingencies do not make one necessary thing: since, just as each one of them by itself may fail in its effect, so too may all of them together. Now, it is evident that in these lower bodies each thing that happens through the influence of the heavenly bodies is a contingency.

Therefore the things that happen here below through the influence of heavenly bodies are not necessarily connected, since it is evident that each one of them may be hindered.

Further. The action of the heavenly bodies is in keeping with their nature: so that they require matter on which to act. Consequently their action does not remove what is required by matter. Now the matter on which heavenly bodies act is the bodies of the lower world: and since these are by nature corruptible, they can fail in action just as they can fail in being, so that their nature requires that they should not produce their effects of necessity. Therefore the effects of heavenly bodies on the bodies of the lower world do not result of necessity.

Perhaps someone will say that the effects of heavenly bodies must necessarily follow, and yet potentiality is not therefore removed from this lower world, because each effect is in potentiality before it comes into being, and is then said to be possible; but when it is in act, it passes from potentiality to necessity, the whole of which process is subject to the heavenly movements, and consequently a given effect is not prevented from having been possible at one time, although it is necessary that it result at some time:—in fact Albumasar strives thus to plead the cause of possibility, in the First Book of his Introductorium.

But the case for possibility cannot be defended in this way. For there is one kind of possible which follows from that which is necessary. Because that which must be necessarily, is possible; since what cannot possibly be, is impossible, and that which is impossible, necessarily is not. Consequently what must necessarily be, must necessarily not be: which is impossible. Hence it is impossible that the same thing should be necessarily, and yet that at the same time it should be impossible for it to be. Therefore possible being follows from necessary being.

But it is not this kind of possible that we need defend in contradiction to the statement that effects result of necessity: but the possible which is contrary to necessary, in which sense we say that a thing may possibly be or not be. Now a thing is said to be possible or contingent not merely because it is at one time potential and at another time actual, as the foregoing reply supposes: since thus, even in the heavenly movements there is possibility and contingency. For the sun and moon are not always actually in conjunction or opposition, but sometimes actually and sometimes potentially: and yet these are necessary things, since such matters are subject to demonstration. But the possible or contingent that is contrary to the necessary, is of such a nature that there is no necessity for it to be, when it is not. And the reason for this is that it does not result necessarily from its cause. Thus we say that it is a contingency that Socrates will sit, whereas it is a necessity that he will die, because the latter results from its cause necessarily, and not the former. Consequently if it follows necessarily from the movements of the heavenly bodies that their effects will result at some time, there will be nothing possible or contingent contrary to that which is necessary.

We must observe, however, that Avicenna having a mind to prove that the effects of heavenly bodies result of necessity, offers the following argument (Metaph. x.). If an effect of the heavenly bodies be hindered, this must be due to some cause either voluntary or natural. Now every cause, whether voluntary or natural, is reducible to some heavenly principle. Therefore even the impediment to the heavenly bodies’ effect results from some heavenly principles. Consequently, if we take the whole heavenly order at once, it is impossible for its effect ever to fail. Whence he concludes that the heavenly bodies must necessarily produce effects in the lower things both voluntary and natural.

This argument, as Aristotle observes (2 Phys. iv.) was employed by some of the ancients, who denied the existence of chance and luck, for the reason that every effect has its definite cause, and that given the cause the effect follows of necessity; so that, since everything happens necessarily, nothing can be referred to luck or chance.

He solves this argument (6 Metaph.), by denying the two propositions on which it is based. One is that given any cause whatsoever, the effect must follow of necessity. For this is not true of every cause, since even the per se, proper and sufficient cause of a certain effect may be hindered through the clashing of another cause, so that it fails to produce that effect.—The other proposition which he denies is that not everything that exists in any way whatever has a per se cause, hut only that which exists per se; and things which exist accidentally, have no cause at all: for instance that a man be musical is to be ascribed to a cause, but that he be musical as well as white, is not due to any cause. Because whatever things concur on account of a cause, are mutually dependent by reason of that cause, whereas accidental things are not mutually dependent. Consequently they are not the result of a per se active cause, but are merely an accidental result: thus it is accidental to the teacher of music that his pupil be a white man, since it is beside his intention, for his intention is to teach one who has an aptitude for music.

Accordingly given any particular effect, we shall say that it had a cause from which it did not necessarily result, because it might have been hindered by the accidental concurrence of another cause. And although we may trace that concurrent cause to some higher cause, we cannot ascribe to any cause the concurrence itself that proved to be a hindrance. Consequently we cannot say that the hindrance to this or that effect is to be traced to some heavenly principle. Therefore we must not allow that the effects of heavenly bodies happen of necessity in this lower world.

Hence Damascene says in the Second Book that heavenly bodies do not cause the generation of things that are made, nor the corruption of things that are destroyed: because, to wit, their effects do not follow of necessity.

Aristotle too, says (2 De Somn. et Vigil.), that many things betokened by corporeal things, even heavenly bodies, by water for instance or wind, do not happen. For if a stronger movement arise than that which presaged the future, the latter fails in its effect: even so we often renounce our first intent, on account of other and better thoughts.

Ptolemy also says (i Quadrip. ii.): Again, we must not think that the things which occur through the influence of higher beings are inevitable, like those which happen by divine decree and are altogether unavoidable, and such as do actually and necessarily occur. He says again in the Centiloquium: These principles which I have given you, are midway between the necessary and the possible.