On Astrological Judgments, by Thomas Aquinas

Because you asked if it was permissible to use astrological judgments, wishing to satisfy your request, I have taken care to write about what the sacred teachers transmit regarding this matter.

Concerning astrological judgments, it’s important, first and foremost, to know that the power of celestial bodies extends to influencing the inferior bodies. Augustine says in Book V of “The City of God”, “It is not entirely unreasonable to say that some celestial influences can reach only to the differences in our bodies.” Therefore, if someone uses astrological judgments to predict physical effects, such as weather conditions, health or illness, or the abundance and scarcity of crops, and other similar events which depend on physical and natural causes, it seems to be no sin. For all men, in relation to these effects, make some observation of celestial bodies: like farmers who sow and reap at certain times observed according to the sun’s movement; sailors who avoid voyages during the full moon, or during the moon’s eclipse; doctors observe critical days in illnesses, which are determined according to the course of the sun and the moon. Hence, it is not inappropriate to use astrological judgments for physical effects, according to some other more hidden observations of stars.

However, one must firmly hold that man’s will is not subject to the necessity of the stars; otherwise, free will would perish. Without it, neither good deeds would be attributed to man for merit nor bad deeds for fault. Therefore, every Christian must firmly believe that those things which depend on man’s will, like all human actions, are not subjected to the necessity of the stars. And therefore, Jeremiah 10 says, “Do not be afraid of the signs of the heavens, which the nations fear.”

But the devil, in order to lead everyone into error, involves himself in the works of those who are intent on astrological judgments; and therefore Augustine says in Book II of “Genesis according to the Letter”, “We must admit, when true things are said by astrologers, they are said by a most hidden impulse which unknowing human minds suffer; and since this is done to deceive men, it is the work of unclean and seductive spirits, to whom it is permitted to know certain true things about temporal matters.” And therefore Augustine says in Book II of “On Christian Doctrine” that such observations of the stars should be attributed to certain pacts made with demons. But a Christian must altogether avoid making a pact or association with demons, according to the words of the Apostle in 1 Corinthians 10, “I do not want you to be partners with demons.” And therefore, it must be firmly believed that it is a serious sin to use astrological judgments concerning those things which depend on man’s will.