In articulating his theory of natural law, St. Thomas Aquinas asks about the unity of its norms in application to cases. To answer this question, he borrows a distinction from Aristotle’s physics between what is universal and necessary, and what is particular and subject to chance variation. Applying this distinction, he says the basic principles of natural law are the same for all, but at the concrete level the proper principles which guide action hold for most cases, yet because of special conditions can be subject to exceptions (S.t., 1–2, q. 94, a. 4; cf. q. 100, a. 8). Some dissenting theologians quote or cite this one article of Thomas to support their view that one must decide in each case whether a received moral norm must be fulfilled or is overridden by other considerations.
The first thing to notice about this argument is that the point Thomas is making can be true with respect to most specific moral norms, which must be applied and limited by basic moral principles. The next thing to notice is that Thomas in his actual moral thinking—as distinct from his reflection upon natural law—holds that there are norms which do not admit of exception (e.g., S.t., 2–2, q. 41, a. 1; q. 64, a. 6; q. 70, a. 4; q. 100, aa. 2–3; q. 153, a. 2, with q. 154). In any case, Thomas does not articulate a proportionalist theory.
Beyond these considerations, I think it must be said that this particular position of Thomas is a mistake and that his Aristotelian argument for it is fallacious. The structure of the existential domain is not exactly parallel to that of the natural world. The whole morally significant content of one’s action must be intelligible, since one is responsible only for what one understands. For this reason, proposals one adopts by choice never are morally particularized by unique, unrepeatable, material, contingent factors. The really unique aspects of one’s action make no difference whatsoever to the morality of what one does.
Hence, Thomas is confusing the specificity of moral acts with the uniqueness of physical particulars. Although one’s dog Fido has individual traits which no branch of science ever studies, one’s morally significant act of mistreating Fido on a particular day due to particular irritations has no morally relevant features which will not be considered by a complete Christian ethics. For one’s morally significant act will include only what one deliberately chooses to do and permit—that is, what one understands about what one is doing—and one’s practical understanding can be wholly determined by moral principles.