Objection 1. It would seem that divination by the stars is not unlawful. It is lawful to foretell effects by observing their causes: thus a physician foretells death from the disposition of the disease. Now the heavenly bodies are the cause of what takes place in the world, according to Dionysius (Div. Nom. iv). Therefore divination by the stars is not unlawful.
Objection 2. Further, human science originates from experiments, according to the Philosopher (Metaph. i, 1). Now it has been discovered through many experiments that the observation of the stars is a means whereby some future events may be known beforehand. Therefore it would seem not unlawful to make use of this kind of divination.
Objection 3. Further, divination is declared to be unlawful in so far as it is based on a compact made with the demons. But divination by the stars contains nothing of the kind, but merely an observation of God's creatures. Therefore it would seem that this species of divination is not unlawful.
On the contrary, Augustine says (Confess. iv, 3): "Those astrologers whom they call mathematicians, I consulted without scruple; because they seemed to use no sacrifice, nor to pray to any spirit for their divinations which art, however, Christian and true piety rejects and condemns."
I answer that, As stated above (1 and 2), the operation of the demon thrusts itself into those divinations which are based on false and vain opinions, in order that man's mind may become entangled in vanity and falsehood. Now one makes use of a vain and false opinion if, by observing the stars, one desires to foreknow the future that cannot be forecast by their means. Wherefore we must consider what things can be foreknown by observing the stars: and it is evident that those things which happen of necessity can be foreknown by this mean,: even so astrologers forecast a future eclipse.
However, with regard to the foreknowledge of future events acquired by observing the stars there have been various opinions. For some have stated that the stars signify rather than cause the things foretold by means of their observation. But this is an unreasonable statement: since every corporeal sign is either the effect of that for which it stands (thus smoke signifies fire whereby it is caused), or it proceeds from the same cause, so that by signifying the cause, in consequence it signifies the effect (thus a rainbow is sometimes a sign of fair weather, in so far as its cause is the cause of fair weather). Now it cannot be said that the dispositions and movements of the heavenly bodies are the effect of future events; nor again can they be ascribed to some common higher cause of a corporeal nature, although they are referable to a common higher cause, which is divine providence. on the contrary the appointment of the movements and positions of the heavenly bodies by divine providence is on a different principle from the appointment of the occurrence of future contingencies, because the former are appointed on a principle of necessity, so that they always occur in the same way, whereas the latter are appointed on a principle of contingency, so that the manner of their occurrence is variable. Consequently it is impossible to acquire foreknowledge of the future from an observation of the stars, except in so far as effects can be foreknown from their causes.
Now two kinds of effects escape the causality of heavenly bodies. On the first place all effects that occur accidentally, whether in human affairs or in the natural order, since, as it is proved in Metaph. vi [Ed. Did. v, 3, an accidental being has no cause, least of all a natural cause, such as is the power of a heavenly body, because what occurs accidentally, neither is a "being" properly speaking, nor is "one"--for instance, that an earthquake occur when a stone falls, or that a treasure be discovered when a man digs a grave--for these and like occurrences are not one thing, but are simply several things. Whereas the operation of nature has always some one thing for its term, just as it proceeds from some one principle, which is the form of a natural thing.
In the second place, acts of the free-will, which is the faculty of will and reason, escape the causality of heavenly bodies. For the intellect or reason is not a body, nor the act of a bodily organ, and consequently neither is the will, since it is in the reason, as the Philosopher shows (De Anima iii, 4,9). Now no body can make an impression on an incorporeal body. Wherefore it is impossible for heavenly bodies to make a direct impression on the intellect and will: for this would be to deny the difference between intellect and sense, with which position Aristotle reproaches (De Anima iii, 3) those who held that "such is the will of man, as is the day which the father of men and of gods," i.e. the sun or the heavens, "brings on" [Odyssey xviii, 135].
Hence the heavenly bodies cannot be the direct cause of the free-will's operations. Nevertheless they can be a dispositive cause of an inclination to those operations, in so far as they make an impression on the human body, and consequently on the sensitive powers which are acts of bodily organs having an inclination for human acts. Since, however, the sensitive powers obey reason, as the Philosopher shows (De Anima iii, 11; Ethic. i, 13), this does not impose any necessity on the free-will, and man is able, by his reason, to act counter to the inclination of the heavenly bodies.
Accordingly if anyone take observation of the stars in order to foreknow casual or fortuitous future events, or to know with certitude future human actions, his conduct is based on a false and vain opinion; and so the operation of the demon introduces itself therein, wherefore it will be a superstitious and unlawful divination. On the other hand if one were to apply the observation of the stars in order to foreknow those future things that are caused by heavenly bodies, for instance, drought or rain and so forth, it will be neither an unlawful nor a superstitious divination.
Wherefore the Reply to the First Objection is evident.
Reply to Objection 2. That astrologers not unfrequently forecast the truth by observing the stars may be explained in two ways. First, because a great number of men follow their bodily passions, so that their actions are for the most part disposed in accordance with the inclination of the heavenly bodies: while there are few, namely, the wise alone, who moderate these inclinations by their reason. The result is that astrologers in many cases foretell the truth, especially in public occurrences which depend on the multitude. Secondly, because of the interference of the demons. Hence Augustine says (Gen. ad lit. ii, 17): "When astrologers tell the truth, it must be allowed that this is due to an instinct that, unknown to man, lies hidden in his mind. And since this happens through the action of unclean and lying spirits who desire to deceive man for they are permitted to know certain things about temporal affairs." Wherefore he concludes: "Thus a good Christian should beware of astrologers, and of all impious diviners, especially of those who tell the truth, lest his soul become the dupe of the demons and by making a compact of partnership with them enmesh itself in their fellowship."
This suffices for the Reply to the Third Objection.
Source: Summa Theologica (New Advent)