Does Aquinas falls into the naturalistic fallacy in his moral approach?
“[P]hilosophers […] have objected that Aquinas’s metaphysical approach to ethics is a non-starter, on the grounds that it ignores the “fact/value distinction.” For as Hume famously argued, conclusions about what ought to be done (which are statements about “value”) cannot be inferred from premises concerning what is the case (statements of “fact”). To assume otherwise, it is claimed, is to commit the “naturalistic fallacy.”
From the traditional Thomistic point of view, however, there simply is no “fact/value distinction” in the first place. More precisely, there is no such thing as a purely “factual” description of reality utterly divorced from “value,” for “value” is built into the structure of the “facts” from the get-go. A gap between “fact” and “value” could exist only given a mechanistic-cum-nominalistic understanding of nature of the sort commonly taken for granted by modern philosophers, on which the world is devoid of any objective essences or natural ends. No such gap, and thus no “fallacy” of inferring normative conclusions from “purely factual” premises, can exist given an Aristotelian–Thomistic essentialist and teleological conception of the world. “Value” is a highly misleading term in any case, and subtly begs the question against critics of the “fact/value distinction” by insinuating that morality is purely subjective, insofar as “value” seems to presuppose someone doing the valuing. Aristotelians and Thomists (and other classical philosophers such as Platonists) tend to speak, not of “value,” but of “the good,” which on their account is entirely objective.”
Feser, Edward. Aquinas: A beginner’s guide. Oneworld Publications, 2009.