According to the received tradition, the language used to refer to natural kinds in scientific discourse remains stable even as theories about these kinds are refined. Hence, scientists discover, rather than stipulate, that sentences like 'Whales are mammals, not fish' are true. In this illuminating book, Joseph LaPorte argues that mammals, not fish', are true rather than false. Instead, scientists find that these sentences were vague in the language of earlier speakers, and they refine the meanings of the relevant natural-kind terms to make the sentences true. Hence, scientists change the meanings of these terms. This conclusion prompts LaPorte to examine the consequences of this change in meaning for the issue of incommensurability and for the progress of science.