In Romans 3:8 one finds the well-know precept that one must not do evil in order that good may come of it. Obviously I, as should all who profess to be Christian, accept this precept by the authority with which it was promulgated, but I’m wondering how to prove it, by natural reason alone, to secularists, many of whom are infected with utilitarianism and would not hesitate to do evil if they thought that they would thereby procure a greater good or avert a greater evil. St. Thomas Aquinas does not, as far as I know, answer the utilitarian objection directly (please correct me if I’m wrong and let me know where he does), though he does invoke Romans 3:8 a couple of times in other contexts. Also, in St. Thomas’s treatment of the natural law he notes that
… the first principle of practical reason is one founded on the notion of good, viz. that "good is that which all things seek after." Hence this is the first precept of law, that "good is to be done and pursued, and evil is to be avoided." All other precepts of the natural law are based upon this …So is it as simple, then, as asserting that the first precept of the natural law enjoins the avoidance of evil? If so, then how does one answer the more moderate utilarians who, while agreeing that one should never do grave evil (like what Catholics would call a mortal sin), would nevertheless argue that if the evil were small in some absolute sense (rather than just relative to the good that one expects to obtain), like what Catholics mean by venial sin, then one ought to do it if a greater good were expected to be obtained thereby? I would be interested in, and appreciate greatly, any arguments and/or references that you could provide for me, readers.
(Ia IIæ, q. 94, a. 2,