"according to the Philosopher, it is what is harmful to other men [(qui est aliis hominibus nocivus)] that is properly [(proprie)] evil, and on that basis he says that the prodigal is not evil, because he harms no one but himself [(nulli alteri nocet nisi sibi ipsi)]. And so too with all the other acts that are not harmful to the neighbor [(de omnibus aliis qui non sunt proximis nocivi)]. But we are saying here that evil is generally [(communiter)] whatever is repugnant to [right] reason [(omne quod est rationi rectae repugnans)], and on that basis every individual [human] act is good or bad, as has been said."
Thomas Aquinas, Summa theologiae I-II.18.9.ad 2, trans. Ralph McInerny (Thomas Aquinas: selected writings, ed. & trans. Ralph MacInerny (London: Penguin Books, 1998), 580), boldface mine. Latin from the Leonine edition of 1891, as reproduced in Corpus Thomisticum.
I leave aside the further (and probably irrelevant) question of whether such a harm must be capable of being felt.