Butler observes in the Preface to the Sermons that the subject of morals can be approached in two different ways: “One begins from enquiring into the abstract relations of things: the other from a matter of fact, namely what the particular nature of man is, its several parts, their economy or constitution; from whence it proceeds to determine what course of life it is, which is correspondent to his whole nature. In the former method the conclusion is expressed thus, that vice is contrary to the nature and reason of things: in the latter, that it is a violation or breaking in upon our own nature. Thus they both lead us to the same thing, our obligations to the practice of virtue; and thus they exceedingly strengthen and enforce each other.” In making this observation Butler raises the problem of the nature of moral obligation, and of the criteria by which the existence of a moral obligation can be known. He does so by calling attention to the divisions of opinion which existed on this issue in his own days. Samuel Clarke, the fashionable moralist of the period, sought the roots of moral obligation in the “nature and reason of things”: for an agent to know that an act is his duty is to know that it is fitting or suitable to the circumstances in which it occurs. Shaftesbury, on the other hand, and many of the adherents to the doctrine of Natural Law, like Grotius and Pufendorf, sought the roots of moral obligation in the nature of agents: for an agent, to know that an act is his duty is for him to experience a special motive to do it. Butler recognized the fundamental difference between these two approaches.