Objection 1. It would seem that it is not unlawful to practice the observances of the magic art. A thing is said to be unlawful in two ways. First, by reason of the genus of the deed, as murder and theft: secondly, through being directed to an evil end, as when a person gives an alms for the sake of vainglory. Now the observances of the magic art are not evil as to the genus of the deed, for they consist in certain fasts and prayers to God; moreover, they are directed to a good end, namely, the acquisition of science. Therefore it is not unlawful to practice these observances.
Objection 2. Further, it is written (Daniel 1:17) that "to the children" who abstained, "God gave knowledge, and understanding in every book, and wisdom." Now the observances of the magic art consist in certain fasts and abstinences. Therefore it seems that this art achieves its results through God: and consequently it is not unlawful to practice it.
Objection 3. Further, seemingly, as stated above (Article 1), the reason why it is wrong to inquire of the demons concerning the future is because they have no knowledge of it, this knowledge being proper to God. Yet the demons know scientific truths: because sciences are about things necessary and invariable, and such things are subject to human knowledge, and much more to the knowledge of demons, who are of keener intellect, as Augustine says [Gen. ad lit. ii, 17; De Divin. Daemon. 3,4. Therefore it seems to be no sin to practice the magic art, even though it achieve its result through the demons.
On the contrary, It is written (Deuteronomy 18:10-11): "Neither let there be found among you... anyone... that seeketh the truth from the dead": which search relies on the demons' help. Now through the observances of the magic art, knowledge of the truth is sought "by means of certain signs agreed upon by compact with the demons" [Augustine, De Doctr. Christ. ii, 20; see above (92, 2). Therefore it is unlawful to practice the notary art.
I answer that, The magic art is both unlawful and futile. It is unlawful, because the means it employs for acquiring knowledge have not in themselves the power to cause science, consisting as they do in gazing certain shapes, and muttering certain strange words, and so forth. Wherefore this art does not make use of these things as causes, but as signs; not however as signs instituted by God, as are the sacramental signs. It follows, therefore, that they are empty signs, and consequently a kind of "agreement or covenant made with the demons for the purpose of consultation and of compact by tokens" [Augustine, De Doctr. Christ. ii, 20; see above (92, 2). Wherefore the magic art is to be absolutely repudiated and avoided by Christian, even as other arts of vain and noxious superstition, as Augustine declares (De Doctr. Christ. ii, 23). This art is also useless for the acquisition of science. For since it is not intended by means of this art to acquire science in a manner connatural to man, namely, by discovery and instruction, the consequence is that this effect is expected either from God or from the demons. Now it is certain that some have received wisdom and science infused into them by God, as related of Solomon (1 Kings 3 and 2 Chronicles 1). Moreover, our Lord said to His disciples (Luke 21:15): "I will give you a mouth and wisdom, which all your adversaries shall not be able to resist and gainsay." However, this gift is not granted to all, or in connection with any particular observance, but according to the will of the Holy Ghost, as stated in 1 Corinthians 12:8, "To one indeed by the Spirit is given the word of wisdom, to another the word of knowledge, according to the same Spirit," and afterwards it is said (1 Corinthians 12:11): "All these things one and the same Spirit worketh, dividing to everyone according as He will." On the other hand it does not belong to the demons to enlighten the intellect, as stated in I, 109, 3. Now the acquisition of knowledge and wisdom is effected by the enlightening of the intellect, wherefore never did anyone acquire knowledge by means of the demons. Hence Augustine says (De Civ. Dei x, 9): "Porphyry confesses that the intellectual soul is in no way cleansed by theurgic inventions," i.e. the operations "of the demons, so as to be fitted to see its God, and discern what is true," such as are all scientific conclusions. The demons may, however, be able by speaking to men to express in words certain teachings of the sciences, but this is not what is sought by means of magic.
Reply to Objection 1. It is a good thing to acquire knowledge, but it is not good to acquire it by undue means, and it is to this end that the magic art tends.
Reply to Objection 2. The abstinence of these children was not in accordance with a vain observance of the notary art, but according to the authority of the divine law, for they refused to be defiled by the meat of Gentiles. Hence as a reward for their obedience they received knowledge from God, according to Psalm 118:100, "I have had understanding above the ancients, because I have sought Thy commandments."
Reply to Objection 3. To seek knowledge of the future from the demons is a sin not only because they are ignorant of the future, but also on account of the fellowship entered into with them, which also applies to the case in point.
Source: Summa Theologica (New Advent)