Grammaticalization explained how grammatical complexity could emerge, and how grammar was continually in flux.
The answer comes from the fascinating phenomenon of grammaticalization: the strange process by which words with concrete, specific meanings transmute into the grammatical machinery of language. The idea of grammaticalization (and research on language change more broadly) was a complete revelation for both of us because we had been trained in the generative approach to language (Chomsky’s “principles and parameters” and its many rivals), which sees language as a vastly complex but immutable mathematical object. Grammaticalization explained how grammatical complexity could emerge, and how grammar was continually in flux.