I answer that, We speak of merit and demerit, in relation to retribution, rendered according to justice. Now, retribution according to justice is rendered to a man, by reason of his having done something to another's advantage or hurt. It must, moreover, be observed that every individual member of a society is, in a fashion, a part and member of the whole society. Wherefore, any good or evil, done to the member of a society, redounds on the whole society: thus, who hurts the hand, hurts the man. When, therefore, anyone does good or evil to another individual, there is a twofold measure of merit or demerit in his action: first, in respect of the retribution owed to him by the individual to whom he has done good or harm; secondly, in respect of the retribution owed to him by the whole of society. Now when a man ordains his action directly for the good or evil of the whole society, retribution is owed to him, before and above all, by the whole society; secondarily, by all the parts of society. Whereas when a man does that which conduces to his own benefit or disadvantage, then again is retribution owed to him, in so far as this too affects the community, forasmuch as he is a part of society: although retribution is not due to him, in so far as it conduces to the good or harm of an individual, who is identical with the agent: unless, perchance, he owe retribution to himself, by a sort of resemblance, in so far as man is said to be just to himself.
It is therefore evident that a good or evil action deserves praise or blame, in so far as it is in the power of the will: that it is right or sinful, according as it is ordained to the end; and that its merit or demerit depends on the recompense for justice or injustice towards another.