REAL KINDS

Substances are those things about which you can learn from one encounter something of what to expect on other encounters, where this is no accident but the result of a real connection. There is a reason why the same or similar properties characterize what is encountered. We can begin with examples of substances that are kinds. I will call these substances “real kinds,” contrasting this, as is traditional, with “nominal kinds.”

Most of the various definitions currently offered of “natural kinds” capture real kinds of one sort or another. Sometimes, however, the term “natural kind” is used to refer merely to a class determined by a “projectable” property, that is, one that might figure in natural laws. Then “is green” and “is at 32° Fahrenheit” denote “natural kinds,” predicates projectable over certain classes of subjects. What I am calling real kinds, on the other hand, must figure as subjects over which a variety of predicates are projectable. They are things that have properties, rather than merely being properties. That is why Aristotle called them “secondary substances,” putting them in the same broad ontological class as individuals, which he called “primary substances.” True, unlike the Aristotelian tradition, in modern times concepts of stuffs and real kinds have traditionally been treated as predicate concepts. That is, to call a thing “gold” or “mouse” has been taken to involve saying or thinking that it bears a certain description. One understands something as being gold or a mouse or a chair or a planet by representing it as having a certain set, or a certain appropriate sampling, of properties.