It’s not so much of a live question these days of astrological forces. Nevertheless, it used to be, and these arguments are of interest because of it.
1 Not only is it impossible for the celestial bodies to impose necessity on human choice; in fact, not even corporeal effects in things here below necessarily result from them.
2 For the impressions of universal causes are received in their effects according to the mode of the recipients. Now, these lower things are fluctuating and do not always maintain the same condition: because of matter which is in potency to many forms and because of the contrariety of forms and powers. Therefore, the impressions of celestial bodies are not received in these lower things by way of necessity.
3 Again, an effect does not result from a remote cause unless there be also a necessary intermediate cause; just as in syllogisms, from a necessary major and a contingent minor, a necessary conclusion does not follow. But celestial bodies are remote causes, whereas the proximate causes of lower effects are the active and passive powers in these lower things, which are not necessary causes, but contingent, for they may fail in a few instances. So, effects in these lower bodies do not follow of necessity from the motions of the celestial bodies.
4 Besides, the motion of the celestial bodies always is in the same mode. So, if the effect of the celestial bodies on these lower ones came about from necessity, the events in lower bodies would always happen in the same way. Yet they do not always occur in the same way, but in most cases. So, they do not come about by necessity.
Notes Cause is conditional is a shorter way of saying it.
5 Moreover, it is not possible for one necessary thing to come to be out of many contingent things, because, just as any contingent thing of itself can fall short of its effect, so, too, all of them may together. Now, it is obvious that the individual effects that are accomplished in these lower things, as a result of the impression of celestial bodies, are contingent. Therefore, the combination of these events that occur in lower things as a result of the impression of celestial bodies is not a necessary one, for it is plain that any one of them may be prevented from happening.
6 Moreover, the celestial bodies are agents in the order of nature; they need matter on which to act. So, the need for matter is not removed as a result of the action of celestial bodies. Now, the matter on which the celestial bodies act consists of the lower bodies which, being corruptible in their nature, may be just as able to fail in their operations as they are able to fail in their being. Thus, their nature has this characteristic: they do not produce their effects by necessity. Therefore, the effects of the celestial bodies do not come about by necessity, even in the lower bodies.
7 But someone will say, perhaps, that the effects of the celestial bodies must be accomplished. Yet, possibility is not removed from the lower bodies by this fact, because each effect is in potency before it comes about. So, it is then called possible, but when it now becomes actual, it passes from possibility to necessity. All of this comes under the control of the celestial bodies; and so, the fact that the effect is at one time possible is not removed in this way, even though it is necessary that this effect be produced at another time. Indeed, this is the way that Albumasar, in his book, Introduction to Astronomy, tries to defend the possible.
8 But one cannot defend this meaning of the possible. For there is a sort of possibility that depends on what is necessary. Indeed, what is necessary in regard to actual being must be possible in regard to being; and what is not possible in relation to being is impossible in regard to being; and what is impossible in regard to being is necessarily nonbeing. Therefore, what is necessary in relation to being is necessary in relation to non-being. But this is impossible. So, it is impossible for something to be necessary in relation to being, yet not possible in regard to this being. Therefore, possible being follows from necessary being.
Notes Physicists take note.
9 As a matter of fact, we do not have to defend this meaning of possible against the statement that effects are caused by necessity, but, rather, the possible that is opposed to the necessary, in the sense that the possible is called that which can be, and also not be.
Now, a thing is not called possible, or contingent, in this way from the sole fact that it is at one time in potency and at another time in act, as the preceding answer takes it. In fact, in that preceding sense there is possibility and contingency even in celestial motions, for there is not always an actual conjunction or opposition of the sun or moon. Rather, it is sometimes actually so, sometimes potentially so; yet these events are necessary, for demonstrations of such events may be given.
But the possible, or contingent, that is opposed to the necessary has this characteristic: it is not necessary for it to happen when it is not. This is indeed so, because it does not follow of necessity from its cause. Thus, we say that Socrates will sit is a contingent fact, but that he will die is necessary, because the second of these facts follows necessarily from its cause, whereas the first does not. So, if it follows necessarily from the celestial motions that their effects will occur at some time in the future, then the possible and contingent that is opposed to the necessary is thereby excluded.
10 Moreover, we should note that, in order to prove that the effects of the celestial bodies come about by necessity, Avicenna uses an argument like this in his Metaphysics [X, 1]. If any effect of the celestial bodies is blocked, this must be due to some voluntary or natural cause. But every voluntary or natural cause is reducible to some celestial source. Therefore, even the blocking of the effects of the celestial bodies results from some celestial sources. So, if the entire order of celestial things be taken together, it is impossible for its effect ever to fail to come about. Hence he concludes that the celestial bodies produce necessarily the effects which must occur in these lower bodies, both the voluntary and the natural ones.
11 But this way of arguing, as Aristotle says in Physics [II, 4], was used by some of the ancients who denied chance and fortune on the basis of the view that there is a definite cause for every effect. If the cause be granted, then the effect must be granted. Thus, since everything occurs by necessity, there is nothing fortuitous or by chance.
12 He answers this argument, in Metaphysics VI [2-3], by denying two propositions which the argument uses. One of these is: “if any cause be granted, it is necessary to grant its effect.”
Indeed, this is not necessary in the case of all causes, for a certain cause, though it may be the direct, proper and sufficient cause of a given effect, may be hindered by the interference of another cause so that the effect does not result.
The second proposition that he denies is: “not everything that exists in any way at all has a direct cause, but only those things that exist of themselves; on the other hand, things that exist accidentally have no cause.”
For instance, there is a cause within a man for the fact that he is musical, but there is no cause for the fact that he is at once white and musical. As a matter of fact, whenever plural things occur together because of some cause they are related to each other as a result of that cause, but whenever they occur by accident they are not so related to each other. So, they do not occur as a result of a cause acting directly; their occurrence is only accidental. For instance, it is an accident to the teacher of music that he teaches a white man; indeed, it is quite apart from his intention; rather, he intends to teach someone who is capable of learning the subject.
13 And thus, given a certain effect, we will say that it had a cause from which it did not necessarily follow, since it could have been hindered by some other accidentally conflicting cause. And even though it be possible to trace this conflicting cause back to a higher cause, it is not possible to trace this conflict, which is a hindrance, back to any cause. Thus, it cannot be said that the hindrance of this or that effect proceeds from a celestial source. Hence, we should not say that the effects of celestial bodies come about in these lower bodies as a result of necessity.
14 Hence, Damascene says, in Book II [De fide orthodoxa], that “the celestial bodies are not the cause of any process of generating things that come into being, or of the process of corrupting things that are corrupted”; that is to say, these effects do not come about of necessity from them.
15 Aristotle also says, in On Sleep II, that “of those signs which occur in bodies, and even of the celestial signs, such as movements of water and wind, many of their results do not come about. For, if another movement occurs, stronger than the one which is a sign of the future, then the event does not happen; just as many of our well laid plans, which were suitable to be accomplished, come to no result, because of the interference of higher powers.”
16 Ptolemy, too, in his Fourfold Work, says: “Again, we should not think that higher events proceed inevitably, like things that happen under divine control and which can in no way be avoided, nor as things which come about truly and of necessity.” He also says in the Centiloquium: “These prognostications that I give you are midway between the necessary and the possible.”