Question Eight of Chapter Nine of the Third Decade
Whether, besides the Creed and the Lord’s Prayer, it is lawful to recite charms or use other observances when gathering herbs
It is asked whether it is lawful, when gathering herbs or for any illness whatsoever, to recite certain charms or place certain writings upon human beings or animals besides the Creed and the Lord’s Prayer.
It is answered that it is not lawful to place anything upon human beings and animals besides the Creed and the Lord’s Prayer. If, however, anything superstitious is added, it is a grave sin.
Hence it is said in the Decretals, cause 26, question 5, chapter Non liceat Christianis: “Nor, when gathering herbs which are medicinal, is it lawful to attend to any observances or incantations, unless only with the Creed or the Lord’s Prayer, so that the creator of all things—namely, God alone—may be honored.”
Second, this same point is proved through what is said in the title On Sortilege, chapter Ex tuarum tenore. It says there that a certain priest went with someone to a private place, not with the intention of invoking a demon, but so that by inspection of an astrolabe he might recover property stolen from a certain church. Although he proposed to do this from good zeal and simplicity, his sin was nevertheless most grave.
These things are from Thomas, Second Part of the Second Part, question 96, article 4, and Alexander, Second Part of the Summa, question 52, article 6.