Question Four of Chapter Nine of the Third Decade
Whether signs applied to natural things operate naturally to produce certain effects
It is asked whether figures, signs, or characters can be applied to natural things in order to produce certain effects.
It is answered negatively. The reason is that, since such characters, figures, or signs naturally effect nothing, because a figure is not the principle of any natural action, such figures or images have their effect from the operation of demons.
Hence it is always unlawful to make them, because in such things express or tacit pacts with demons are present. Therefore practices of this kind are condemned in cause 26, question 2, chapter Illos, and by Thomas in the place cited above, in the reply to the second argument.