 Such a notion of basicness—and the hierarchical picture associated with it—is intuitively natural and theoretically useful, in this context and others. It has classical roots in Aristotle’s notion of priority in nature, and has branched into the contemporary program of sparse ontology, in a way that has proven fruitful in understanding a wide range of issues. For instance, the physicalist holds that physical entities are basic, and that mental and moral entities are derivative. For the abstract objects of pure set theory, it is natural to think that the empty set is basic, and that the other pure sets are founded on it. With respect to holes, one might hold that the material host is basic and that its holes are formed by it. And with respect to objects and properties, one classical idea is that objects are basic and properties inhere in them as dependent abstractions (modes). Some are skeptical of these locutions. But for better or worse, such talk has rapidly become  ubiquitous.[33]