In Natural Kinds and Genesis: The Classification of Material Entities, Stewart Umphrey raises and answers two questions: What is it to be a natural kind? And are there in fact any natural kinds? Umphrey agrees with scientific essentialists that there are natural kinds, but he argues that most of the chemical, physical, and biological kinds posited in current theories are not natural kinds in the primary sense of the term. The natural-kinds realism he affirms is thus quite restricted: it requires the existence of enduring things which closely resemble one another in virtue of their essences, and such things exist, apparently, only if they have come into being, or emerged, in the course of symmetry-breaking events.