 While this example may seem clear enough, what exactly is it that distinguishes true accidental generalizations from laws? This has been the subject of a great deal of philosophical discussion, most of which must be beyond the scope of this  entry.[2]  For reasons explained in  Section 1,  Hempel assumes that an adequate account must explain the notion of law in terms of notions that lie outside the modal  family.[3]  He considers (1965b) a number of familiar proposals having this  character[4]  and finds them all wanting, remarking that the problem of characterizing the notion of law has proved “highly recalcitrant” (1965b: 338). It seems fair to say, however, that his underlying assumption is that, at bottom, laws are just exceptionless generalizations describing regularities that meet certain additional distinguishing conditions that he is not at present able to formulate. In subsequent decades, a variety of criteria for lawhood have been proposed. Of these the so-called best systems analysis (Lewis 1973) is probably the most popular, but no single account has won general acceptance. Finding an adequate characterization of lawhood is thus an ongoing issue for the DN model. 