As many know, David Hume set out the problem that we cannot infer and “ought” from an “is”. That is, we cannot infer what morally ought to be the case from what is in fact the case. Does Aquinas make such an inference in his natural law ethics?
In fact, Aquinas doesn’t make such an inference and arguably doesn’t make any inference at all. Aquinas argues that it is an axiom or metaphysical first principle that we ought to “do good and avoid evil”. In fact this principle or belief is foundational and not derived or inferred from other beliefs. Therefore, it would be wrong-headed to suppose that Aquinas is inferring an “ought” from an “is”, when he isn’t making an inference to begin with. Similarly, it would be misguided to ask Aquinas to justify his principle. Why? Because it’s a first principle or basic belief, and as such, doesn’t demand further justification.
Basic beliefs are self-justifying. That is, they don’t derive their justification from other beliefs.