Objection 1. It would seem that heavenly bodies impose necessity on things subject to their action. For given a sufficient cause, the effect follows of necessity. But heavenly bodies are a sufficient cause of their effects. Since, therefore, heavenly bodies, with their movements and dispositions, are necessary beings; it seems that their effects follow of necessity.
Objection 2. Further, an agent's effect results of necessity in matter, when the power of the agent is such that it can subject the matter to itself entirely. But the entire matter of inferior bodies is subject to the power of heavenly bodies, since this is a higher power than theirs. Therefore the effect of the heavenly bodies is of necessity received in corporeal matter.
Objection 3. Further, if the effect of the heavenly body does not follow of necessity, this is due to some hindering cause. But any corporeal cause, that might possibly hinder the effect of a heavenly body, must of necessity be reducible to some heavenly principle: since the heavenly bodies are the causes of all that takes place here below. Therefore, since also that heavenly principle is necessary, it follows that the effect of the heavenly body is necessarily hindered. Consequently it would follow that all that takes place here below happens of necessity.
On the contrary, The Philosopher says (De Somn. et Vigil. [De Divin. per Somn. ii): "It is not incongruous that many of the signs observed in bodies, of occurrences in the heavens, such as rain and wind, should not be fulfilled." Therefore not all the effects of heavenly bodies take place of necessity.
I answer that, This question is partly solved by what was said above (Article 4); and in part presents some difficulty. For it was shown that although the action of heavenly bodies produces certain inclinations in corporeal nature, the will nevertheless does not of necessity follow these inclinations. Therefore there is nothing to prevent the effect of heavenly bodies being hindered by the action of the will, not only in man himself, but also in other things to which human action extends.
But in natural things there is no such principle, endowed with freedom to follow or not to follow the impressions produced by heavenly agents. Wherefore it seems that in such things at least, everything happens of necessity; according to the reasoning of some of the ancients who supposing that everything that is, has a cause; and that, given the cause, the effect follows of necessity; concluded that all things happen of necessity. This opinion is refuted by Aristotle (Metaph. vi, Did. v, 3) as to this double supposition.
For in the first place it is not true that, given any cause whatever, the effect must follow of necessity. For some causes are so ordered to their effects, as to produce them, not of necessity, but in the majority of cases, and in the minority to fail in producing them. But that such cases do fail in the minority of cases is due to some hindering cause; consequently the above-mentioned difficulty seems not to be avoided, since the cause in question is hindered of necessity.
Therefore we must say, in the second place, that everything that is a being "per se," has a cause; but what is accidentally, has not a cause, because it is not truly a being, since it is not truly one. For (that a thing is) "white" has a cause, likewise (that a man is) "musical" has not a cause, but (that a being is) "white-musical" has not a cause, because it is not truly a being, nor truly one. Now it is manifest that a cause which hinders the action of a cause so ordered to its effect as to produce it in the majority of cases, clashes sometimes with this cause by accident: and the clashing of these two causes, inasmuch as it is accidental, has no cause. Consequently what results from this clashing of causes is not to be reduced to a further pre-existing cause, from which it follows of necessity. For instance, that some terrestrial body take fire in the higher regions of the air and fall to the earth, is caused by some heavenly power: again, that there be on the surface of the earth some combustible matter, is reducible to some heavenly principle. But that the burning body should alight on this matter and set fire to it, is not caused by a heavenly body, but is accidental. Consequently not all the effects of heavenly bodies result of necessity.
Reply to Objection 1. The heavenly bodies are causes of effects that take place here below, through the means of particular inferior causes, which can fail in their effects in the minority of cases.
Reply to Objection 2. The power of a heavenly body is not infinite. Wherefore it requires a determinate disposition in matter, both as to local distance and as to other conditions, in order to produce its effect. Therefore as local distance hinders the effect of a heavenly body (for the sun has not the same effect in heat in Dacia as in Ethiopia); so the grossness of matter, its low or high temperature or other such disposition, can hinder the effect of a heavenly body.
Reply to Objection 3. Although the cause that hinders the effect of another cause can be reduced to a heavenly body as its cause; nevertheless the clashing of two causes, being accidental, is not reduced to the causality of a heavenly body, as stated above.
Source: Summa Theologica (New Advent)