Question Three of Chapter Nine of the Third Decade
Whether it is lawful to use natural things in order to acquire bodily health
It is asked whether it is lawful to use natural things in order to acquire bodily health.
It is answered that, if natural things are applied to effects for which they are believed to possess a natural power, it is neither superstitious nor unlawful.
But if certain other things are added—for example, some signs, characters, names, or any other observances which manifestly possess no natural efficacy—it will be superstitious and unlawful.
These things are from Thomas in the place cited above, article 2.