This article explores some fundamental issues of definition-based lexical semantic research through a critique of the Natural Semantic Metalanguage (NSM) theory of semantic and grammatical description (Wierzbicka 1996, Semantics. Primes and Universals, Oxford University Press, Oxford). NSM is criticized for attaching excessive importance to explanatory definition, for its adoption of the reductive requirement that a definiens be simpler than a definiendum, and for its use of ‘canonical contexts’ to disambiguate meaning. The principle of substitutability, according to which a definition of (...) a term is accepted if it can be substituted for the term itself, is also critically examined, and the theory’s use of syntactic phenomena as evidence for polysemy is shown to be inconsistent. Finally, suggestions that NSM may be a valid analytical method for only a subpart of the lexicon are rejected. (shrink)