This paper reasserts the fundamental conceptual distinction between language- particular categories of individual languages, defined within particular systems, and comparative concepts at the cross-linguistic level, defined in substantive terms. The paper argues that comparative concepts are also widely used in other sciences, and that they are always distinct from social categories, of which linguistic categories are special instances. Some linguists (especially in the generative tradition) assume that linguistic categories are natural kinds (like biological species, or chemical elements) and thus need not be defined, but can be recognized by their symptoms, which may be different in different languages. I also note that category-like comparative concepts are sometimes very similar to categories, and that different languages may sometimes be described in a unitary commensurable mode, thus blurring (but not questioning) the distinction. Finally, I note that cross- linguistic claims must be interpreted as being about the phenomena of languages, not about the incommensurable systems of languages.