This chapter approaches the ontological question, “What are natural kinds?” through another, partially linguistic, question. “What must natural kinds be like if the conventional wisdom about natural kind terms is correct?” Although answering this question will not tell us everything we want to know, it will, be useful in narrowing the range of feasible ontological alternatives. The chapter summarizes the contemporary linguistic wisdom and then tests different proposals about kinds against it. It takes simple natural kind terms—like “green,” “gold,” “water,” “tiger,” and “light”—to be Millian terms that rigidly designate properties typically determined by a reference-fixing stipulation to the effect that the general term is to designate whatever property provides the explanation of why, at actual world-state, all, or nearly all, the samples of items associated with the term by speakers who introduce it have the observational properties they do.
The Psychology of Natural Kind Terms