According to a celebrated philosophical tradition that has enjoyed prominence for more than a quarter of a century, so-called “natural-kind terms,” like ‘oak’, ‘water’, and ‘mammal’, refer to kinds with theoretically interesting essences. According to the tradition, scientists learn by empirical investigation what those essences are. Scientifically informed conclusions about kinds' essences are discoveries, not stipulations. Scientists do not change the meaning of a term like ‘water’ or ‘mammal’; they simply discover the essence of what speakers past and present have been calling “water” and “mammal,” respectively.

The essence of water, scientists have discovered, is the chemical composition H2O: Nothing could possibly be water without being H2O or H2O without being water, so long as scientists' empirical facts are right. If earlier speakers ever called something “water” that was composed of another chemical, they were wrong. In similar fashion, earlier speakers were wrong to call whales “fish.” Scientists have corrected ordinary speakers about this matter.