Summa Theologica

2. Furthermore, a human act derives its species from its end, as was held above (1-2, qu. 2, art. 3, and qu. 18, art. 6). But every divination is ordered toward one end, namely, the foretelling of future things. Therefore, every divination is of one species.

3. Furthermore, signs do not diversify the species of sin; for whether someone disparages by words, or by writing, or by gesture, it is the same species of sin. But divinations do not seem to differ except according to the different signs from which the foreknowledge of future things is taken. Therefore, there are not diverse species of divination.

But on the contrary, Isidore (Etymol. lib. VIII, ch. 9) enumerates diverse species of divination.

CONCLUSION. — It is certain that there are three genera of divination, under which various species of divination exist: one of which is by manifest invocation of demons, which pertains to necromancers; the second is taken from the disposition and motion of another thing, which pertains to augurs; the third pertains to lots.

I answer that, as has been said (previous art.), every divination uses some counsel or help of demons for the foreknowledge of a future event; which indeed is either expressly implored, or else, beyond the intention of man, a demon secretly inserts himself to foretell certain future things that are unknown to men, but known to him through the modes spoken of in the first (qu. 57, art. 3). Now demons, when expressly invoked, are wont to foretell future things in many ways. Sometimes, indeed, by certain prestige-filled apparitions, they insert themselves into the sight and hearing of men to foretell future things; and this species is called prestige (præstigium), from the fact that the eyes of men are dazzled. Sometimes, however, through dreams; and this is called divination of dreams. Sometimes, indeed, through the apparition or speech of some dead persons, and this species is called necromancy, because, as Isidore says (Etymol. lib. VIII, ch. 9, before the middle), νεκρὸν in Greek is called a dead body, and μαντεία is called divination, because "by certain incantations, with blood applied, the resurrected dead seem to divine and to answer the questions put to them." Sometimes, however, they foretell future things through living men, as is evident in those who are possessed; and this is divination by pythons; and, as Isidore says (loc. cit.), "pythons are named from the Pythian Apollo, who was said to be the author of divining." Sometimes, however, they foretell future things through certain figures or signs that appear in inanimate things; and if these appear in some earthly body, for instance in wood, or iron, or polished stone, it is called geomancy; but if in water, hydromancy; if in the air, aeromancy; if in fire, pyromancy; but if in the entrails of animals sacrificed on the altars of demons, it is called haruspicy. Divination, however, which is done without the express invocation of demons, is divided into two genera. The first of which is when we consider something in the dispositions of certain things to foreknow future events.