It is argued that the source of complexity in language is twofold: repetition, and syntactic embedding. The former enables us to return again and again to the same subject across many sentences, and to maintain the coherence of an argument. The latter is governed by two forms of complexification: the functor-argument structure of all languages and the operator-bound-variable mechanism of familiar formal languages. The former is most transparently represented by categorial grammar, and an extension of this can adequately describe the syntax of variable binders. Both developments have roots within the work of the Lvov-Warsaw School.
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Simons, P. M. (2006). Languages with Variable-Binding Operators: Categorial Syntax and Combinatorial Semantics. In J. Jadacki & J. Paśniczek (Eds.), The Lvov-Warsaw Scool. The New Generation (pp. 239-268), Amsterdam: Rodopi.