Summa Theologica

what can be foreknown about future things through the inspection of celestial bodies. — And regarding those things which happen of necessity, it is manifest that they can be foreknown through the consideration of the stars, just as astrologers announce future eclipses. Regarding the foreknowledge of future events from the consideration of the stars, various people have said various things: for there were those who said that the stars signify rather than cause the things that are announced through their consideration. But this is said unreasonably; for every corporeal sign is either an effect of that of which it is a sign (just as smoke signifies fire, by which it is caused), or it is its cause, or it proceeds from the same cause; and thus, while it designates the cause, it consequently signifies the effect; just as the rainbow sometimes signifies fair weather, in that its cause is the cause of fair weather. However, it cannot be said that the dispositions and motions of celestial bodies are effects of future events: nor again can they be reduced to some superior common cause that is corporeal; they can, however, be reduced to one common cause, which is Divine Providence. But the motions and positions of celestial bodies are disposed by Divine Providence for one reason, and the events of contingent future things for another reason; because the former are disposed according to the principle of necessity, so that they occur always and in the same way; but the latter according to the principle of contingency, so that they occur variably. Whence it cannot be that foreknowledge of the future is taken from the inspection of the stars, except as effects are foreknown from their causes. — However, two types of effects are withdrawn from the causality of celestial bodies: first, indeed, all effects contingent by accident, whether in human affairs or in natural things: because, as is proven (Metaphysics, bk. VI), a being by accident has no cause, and especially not a natural one, such as is the power of celestial bodies; because what happens by accident is neither a being properly speaking, nor one; just as if, by a stone falling, an earthquake occurs, or that while a man is digging a grave, a treasure is found; for these and the like are not simply one, but simply many. Now the operation of nature always terminates in something one, just as it proceeds from one principle, which is the form of the natural thing. Secondly, the acts of free will, which is a faculty of the will and reason, are withdrawn from the causality of celestial bodies. For the intellect or reason is not a body, nor the act of a corporeal organ, and consequently neither is the will, which is in the reason, as is evident through the Philosopher (On the Soul, bk. III). No body, however, can imprint upon an incorporeal thing. Whence it is impossible that celestial bodies imprint directly upon the intellect and will: for this would be to posit that the intellect does not differ from sense: which Aristotle (On the Soul, bk. II) imposes upon those who said that "such is the will in men as the father of men and gods brings in for the day," namely, the sun or heaven. Whence celestial bodies cannot be the cause in themselves of the operations of free will; they can, however, incline towards this dispositively, in that they imprint upon the human body, and consequently upon the sensitive powers, which are acts of corporeal organs, which incline toward human acts. Because, however, the sensitive powers obey reason, as is evident through the Philosopher (On the Soul, bk. III, and Ethics, bk. I), no necessity is imposed by this free will; but against the inclination of celestial bodies, man can act through reason. If, therefore, anyone uses the consideration of the stars to foreknow casual or fortuitous future events, or even to know with certainty