Summa Theologica

And if indeed someone attempts to foreknow future events from the consideration of the position and motion of the stars, this would pertain to astrologers, who are also called genethliaci (birthday calculators), on account of their consideration of birth days. But if through the motions or cries of birds, or of any animals whatsoever, or through the sneezes of men or the twitching of limbs, this pertains generally to augury; which is said from the chirping (garritus) of birds, just as auspice (auspicium) is from the inspection of birds; the first of which pertains to the ears, the second to the eyes; for in birds these things are especially accustomed to be considered. But if such a consideration is made regarding the words of men spoken without intention, which one twists toward the future that he wishes to foreknow, this is called an omen; and as Valerius Maximus says (lib. 1, ch. 5, in the beginning), "the observation of everything is bound up with some contact of religion, since it is believed to consist not in fortuitous motion, but in divine providence; which caused that, when the Romans were deliberating whether they should migrate to another place, by chance at that time a certain centurion exclaimed: Standard-bearer, set up the standard, here we shall stay best; which word they took, when heard, as an omen, and abandoned the plan of moving." If, however, certain dispositions of figures occurring to the sight in some bodies are considered, it will be another species of divination: for divination taken from the lines of the hand is called chiromancy, as if hand-divination (for χείρ in Greek is called hand): but divination from certain signs appearing in the shoulder-blade of some animal is called spatulamancy. — To the second genus of divination, however, which is without express invocation of demons, pertains the divination which is done from the consideration of those things that come from certain actions performed seriously by men to inquire into something hidden; whether through the drawing of points, which pertains to the art of geomancy; or through the consideration of the figures that proceed from molten lead thrown into water; or from certain slips, written or unwritten, stored in secret, while one considers which one he receives; or even from uneven sticks set out, as to which one receives the larger or smaller; or the throwing of dice, as to who throws more points; or even when one considers what happens to someone opening a book; all of which have the name of lots. Thus, therefore, it is clear that there is a triple genus of divination: the first of which is through manifest invocation of demons, which pertains to necromancers; the second is through the mere consideration of the disposition or motion of another thing, which pertains to augurs; the third is when we do something so that something hidden may be manifested to us, which pertains to lots. Under each of these, many things are contained, as is clear from what has been said (in the body of the article).

To the first, therefore, it must be said that in all the aforesaid there is the same general reason for sinning, but not the same specific one; for it is much graver to invoke demons than to do certain things by which it is worthy that demons should insert themselves.

To the second, it must be said that the knowledge of future or hidden things is the ultimate end from which the general reason of divination is taken. But the diverse species are distinguished according to their proper objects or matters, namely, in as much as the knowledge of hidden things is considered in diverse things.

To the third, it must be said that the things which diviners attend to are not considered by them as signs with which they express what they already know, as happens