 In contemporary analytic philosophy, the preferred nominal form of the term is "indexical" (rather than "index"), defined as "any expression whose content varies from one context of use to another ... [for instance] pronouns such as 'I', 'you', 'he', 'she', 'it', 'this', 'that', plus adverbs such as 'now', 'then', 'today', 'yesterday', 'here', and 'actually'.This exclusive focus on linguistic expressions represents a narrower construal than is preferred in linguistic anthropology, which regards linguistic indexicality ("deixis") as a special subcategory of indexicality in general, which is often nonlinguistic; see above.Indexicals appear to represent an exception to, and thus a challenge for, the understanding of natural language as the grammatical coding of logical propositions; they thus "raise interesting technical challenges for logicians seeking to provide formal models of correct reasoning in natural language."