Objection 1. It would seem that we should not distinguish several species of divination. Where the formality of sin is the same, there are not seemingly several species of sin. Now there is one formality of sin in all divinations, since they consist in entering into compact with the demons in order to know the future. Therefore there are not several species of divination.
Objection 2. Further, a human act takes it species from its end, as stated above (I-II, 01, 3; I-II, 18, 6). But all divination is directed to one end, namely, the foretelling of the future. Therefore all divinations are of one species.
Objection 3. Further, signs do not vary the species of a sin, for whether one detracts by word writing or gestures, it is the same species of sin. Now divinations seem to differ merely according to the various signs whence the foreknowledge of the future is derived. Therefore there are not several species of divination.
On the contrary, Isidore enumerates various species of divination (Etym. viii, 9).
I answer that, As stated above (Article 2), all divinations seek to acquire foreknowledge of future events, by means of some counsel and help of a demon, who is either expressly called upon to give his help, or else thrusts himself in secretly, in order to foretell certain future things unknown to men, but known to him in such manners as have been explained in I, 57, 3. When demons are expressly invoked, they are wont to foretell the future in many ways. Sometimes they offer themselves to human sight and hearing by mock apparitions in order to foretell the future: and this species is called "prestigiation" because man's eyes are blindfolded [praestringuntur]. Sometimes they make use of dreams, and this is called "divination by dreams": sometimes they employ apparitions or utterances of the dead, and this species is called "necromancy," for as Isidore observes (Etym. viii) in Greek, nekron "means dead and manteia divination, because after certain incantations and the sprinkling of blood, the dead seem to come to life, to divine and to answer questions." Sometimes they foretell the future through living men, as in the case of those who are possessed: this is divination by "pythons," of whom Isidore says that "pythons are so called from Pythius Apollo, who was said to be the inventor of divination." Sometimes they foretell the future by means of shapes or signs which appear in inanimate beings. If these signs appear in some earthly body such as wood, iron or polished stone, it is called "geomancy," if in water "hydromancy," if in the air "aeromancy," if in fire "pyromancy," if in the entrails of animals sacrificed on the altars of demons, "aruspicy."
The divination which is practiced without express invocation of the demons is of two kinds. The first is when, with a view to obtain knowledge of the future, we take observations in the disposition of certain things. If one endeavor to know the future by observing the position and movements of the stars, this belongs to "astrologers," who are also called "genethliacs," because they take note of the days on which people are born. If one observe the movements and cries of birds or of any animals, or the sneezing of men, or the sudden movements of limbs, this belongs in general to "augury," which is so called from the chattering of birds [avium garritu], just as "auspice" is derived from watching birds [avium inspectione]. These are chiefly wont to be observed in birds, the former by the ear, the latter by the eye. If, however, these observations have for their object men's words uttered unintentionally, which someone twist so as to apply to the future that he wishes to foreknow, then it is called an "omen": and as Valerius Maximus [De Dict. Fact. Memor. i, 5 remarks, "the observing of omens has a touch of religion mingled with it, for it is believed to be founded not on a chance movement, but on divine providence. It was thus that when the Romans were deliberating whether they would change their position, a centurion happened to exclaim at the time: 'Standard-bearer, fix the banner, we had best stand here': and on hearing these words they took them as an omen, and abandoned their intention of advancing further." If, however, the observation regards the dispositions, that occur to the eye, of figures in certain bodies, there will be another species of divination: for the divination that is taken from observing the lines of the hand is called "chiromancy," i.e. divination of the hand (because cheir is the Greek for hand): while the divination which is taken from signs appearing in the shoulder-blades of an animal is called "spatulamancy."
To this second species of divination, which is without express invocation of the demons, belongs that which is practiced by observing certain things done seriously by men in the research of the occult, whether by drawing lots, which is called "geomancy"; or by observing the shapes resulting from molten lead poured into water; or by observing which of several sheets of paper, with or without writing upon them, a person may happen to draw; or by holding out several unequal sticks and noting who takes the greater or the lesser. or by throwing dice, and observing who throws the highest score; or by observing what catches the eye when one opens a book, all of which are named "sortilege."
Accordingly it is clear that there are three kinds of divination. The first is when the demons are invoked openly, this comes under the head of "necromancy"; the second is merely an observation of the disposition or movement of some other being, and this belongs to "augury"; while the third consists in doing something in order to discover the occult; and this belongs to "sortilege." Under each of these many others are contained, as explained above.
Reply to Objection 1. In all the aforesaid there is the same general, but not the same special, character of sin: for it is much more grievous to invoke the demons than to do things that deserve the demons' interference.
Reply to Objection 2. Knowledge of the future or of the occult is the ultimate end whence divination takes its general formality. But the various species are distinguished by their proper objects or matters, according as the knowledge of the occult is sought in various things.
Reply to Objection 3. The things observed by diviners are considered by them, not as signs expressing what they already know, as happens in detraction, but as principles of knowledge. Now it is evident that diversity of principles diversifies the species, even in demonstrative sciences.
Source: Summa Theologica (New Advent)