 Now it is evident we have no motive leading us to the performance of promises, distinct from a sense of duty. If we thought, that promises had no moral obligation, we never should feel any inclination to observe them. This is not the case with the natural virtues. Though there was no obligation to relieve the miserable, our humanity would lead us to it; and when we omit that duty, the immorality of the omission arises from its being a proof, that we want the natural sentiments of humanity. A father knows it to be his duty to take care of his children: But he has also a natural inclination to it. And if no human creature had that indication, no one could lie under any such obligation. (Treatise, 3.2.5)