 Both the layer-cake and the mechanistic approach to levels aim at defining levels in the sense of giving at least necessary conditions for what constitutes a level. William Wimsatt (1976a, 1994) takes a different approach and sets out to characterize the key features that levels of organization typically (but not necessarily) exhibit across different instances. His aim is to delineate major structural or organizational features of nature, of which levels of organization are the most salient ones. First, levels are compositional and form nested hierarchical structures, so that wholes at lower levels function as parts at higher levels. Second, levels of organization are a 