The thesis of this paper is that explanation in linguistics must refer to a deductively formulated theory of language couched in terms of semiotic universals. A theory of this kind, utilizing complementarity and markedness as formal universals, is developed and applied as the explanans of linguistic phenomena from Russian, Spanish, English, and Old Balto-Slavic. Two other fundamental notions are exploited with the aim of demonstrating the explanatory importance of the functional relationship between rule components and distinctive-feature hierarchies in phonology, morphology, and morphophonemics. These notions facilitate a coherent, internally consistent examination of linguistic aetiology and permit inferences about the nature of teleological explanation. 1