T 3.1.1.19 n69

As a proof, how confus'd our way of thinking on this subject commonly is, we may observe, that those who assert, that morality is demonstrable, do not say, that morality lies in the relations, and that the relations are distinguishable by reason. They only say, that reason can discover such an action, in such relations, to be virtuous, and such another vicious. It seems they thought it sufficient, if they cou'd bring the word, Relation, into the proposition, without troubling themselves whether it was to the purpose or not. But here, I think, is plain argument. Demonstrative reason discovers only relations. But that reason, according to this hypothesis, discovers also vice and virtue. These moral qualities, therefore, must be relations. When we blame any action, in any situation, the whole complicated object, of action and situation, must form certain relations, wherein the essence of vice consists. This hypothesis is not otherwise intelligible. For what does reason discover, when it pronounces any action vicious? Does it discover a relation or a matter of fact? These questions are decisive, and must not be eluded.