This paper is an attempt to summarize as explicitly as possible certain empirical findings of classical biosystematics and modern semantic ethnography which may be considered to represent formal universals of human mental structure. The paper offers a formal treatment of the subject of taxonomy, and an application of the formalism to several problem areas in the fields of semantics, ethnography, and cognition. The structure formalized here, essentially following the notions of Linnaean biology, is a hierarchy of inclusion relations among a collection of named sets of objects. Section one introduces the formal definition of taxonomic structure and sketches the major outlines of this kind of mathematical object. Section two introduces some theoretical problems relating to taxonomy in ethnographic and semantic contexts and show how this forumlation applies to, and clarifies, these problems. In particular, the notion of semantic contrast in the context of taxonomy is examined in some detail. Section three introduces the notion of taxonomy and examines the nature of the mapping which governs the realization of conceptual taxa by lexical items. (Author/AM)