The extent to which social categories differ from person to person: We seem to live in a single world, to have attitudes about the same objects and propositions as one another, and to communicate about them. Are categories shared among people in a culture, between cultures, or universally? Intersubjectivity and objectivity are acute problems for mentalistic theories of social categories. In certain views, each person’s private “conceptual scheme” may differ in subtle or stark ways from the schemes of others. Some hold that conceptual schemes may be similar among members of a community, as a result of collaboration and causal feedback, but never identical. Alternatively, conceptual schemes and social categories may be less fine-grained, so that differences in people’s mental states may not yield differences in their categories (cf. Mach 1886, Natorp 1888, Cassirer 1910, Quine 1960, Foucault 1966 [1970]).