Question Five of Chapter Nine of the Third Decade
Whether it is lawful to observe certain signs in order to foreknow future good or bad fortunes
It is asked whether it is lawful to observe certain signs in order to foreknow some future good or bad fortunes, such as illnesses, states of health, or things of this kind, as physicians do.
It is answered that, if anyone considers a presage of future things from a cause which appears expressly through signs, it will not be unlawful.
For example, a servant may fear a beating when he sees his master’s anger. Similarly, if such a thing could occur, someone might fear harm to another’s child from a fascinating eye. In a similar manner, the causes of illnesses are known from certain signs.
Therefore physicians lawfully observe such signs, from which they can lawfully judge concerning health or illness, death or life.
These things are from Thomas, Second Part of the Second Part, question 96, article 3.