Objection 1. It would seem that observances directed to the alteration of bodies, as for the purpose of acquiring health, or the like, are lawful. It is lawful to make use of the natural forces of bodies in order to produce their proper effects. Now in the physical order things have certain occult forces, the reason of which man is unable to assign; for instance that the magnet attracts iron, and many like instances, all of which Augustine enumerates (De Civ. Dei xxi, 5,7). Therefore it would seem lawful to employ such like forces for the alteration of bodies.
Objection 2. Further, artificial bodies are subject to the heavenly bodies, just as natural bodies are. Now natural bodies acquire certain occult forces resulting from their species through the influence of the heavenly bodies. Therefore artificial bodies, e.g. images, also acquire from the heavenly bodies a certain occult force for the production of certain effects. Therefore it is not unlawful to make use of them and of such like things.
Objection 3. Further, the demons too are able to alter bodies in many ways, as Augustine states (De Trin. iii, 8,9). But their power is from God. Therefore it is lawful to make use of their power for the purpose of producing these alterations.
On the contrary, Augustine says (De Doctr. Christ. ii, 20) that "to superstition belong the experiments of magic arts, amulets and nostrums condemned by the medical faculty, consisting either of incantations or of certain cyphers which they call characters, or of any kind of thing worn or fastened on."
I answer that, In things done for the purpose of producing some bodily effect we must consider whether they seem able to produce that effect naturally: for if so it will not be unlawful to do so, since it is lawful to employ natural causes in order to produce their proper effects. But, if they seem unable to produce those effects naturally, it follows that they are employed for the purpose of producing those effects, not as causes but only as signs, so that they come under the head of "compact by tokens entered into with the demons" [Augustine, De Doctr. Christ.; see above (92, 2). Wherefore Augustine says (De Civ. Dei xxi, 6): "The demons are allured by means of creatures, which were made, not by them, but by God. They are enticed by various objects differing according to the various things in which they delight, not as animals by meat, but as spirits by signs, such as are to each one's liking, by means of various kinds of stones, herbs, trees, animals, songs and rites."
Reply to Objection 1. There is nothing superstitious or unlawful in employing natural things simply for the purpose of causing certain effects such as they are thought to have the natural power of producing. But if in addition there be employed certain characters, words, or any other vain observances which clearly have no efficacy by nature, it will be superstitious and unlawful.
Reply to Objection 2. The natural forces of natural bodies result from their substantial forms which they acquire through the influence of heavenly bodies; wherefore through this same influence they acquire certain active forces. On the other hand the forms of artificial bodies result from the conception of the craftsman; and since they are nothing else but composition, order and shape, as stated in Phys. i, 5, they cannot have a natural active force. Consequently, no force accrues to them from the influence of heavenly bodies, in so far as they are artificial, but only in respect of their natural matter. Hence it is false, what Porphyry held, according to Augustine (De Civ. Dei x, 11), that "by herbs, stones, animals, certain particular sounds, words, shapes and devices, or again by certain movements of the stars observed in the course of the heavens it is possible for men to fashion on earth forces capable of carrying into effect the various dispositions of the stars," as though the results of the magic arts were to be ascribed to the power of the heavenly bodies. On fact as Augustine adds (De Civ. Dei x, 11), "all these things are to be ascribed to the demons, who delude the souls that are subject to them."
Wherefore those images called astronomical also derive their efficacy from the actions of the demons: a sign of this is that it is requisite to inscribe certain characters on them which do not conduce to any effect naturally, since shape is not a principle of natural action. Yet astronomical images differ from necromantic images in this, that the latter include certain explicit invocations and trickery, wherefore they come under the head of explicit agreements made with the demons: whereas in the other images there are tacit agreements by means of tokens in certain shapes or characters.
Reply to Objection 3. It belongs to the domain of the divine majesty, to Whom the demons are subject, that God should employ them to whatever purpose He will. But man has not been entrusted with power over the demons, to employ them to whatsoever purpose he will; on the contrary, it is appointed that he should wage war against the demons. Hence in no way is it lawful for man to make use of the demons' help by compacts either tacit or express.
Source: Summa Theologica (New Advent)