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In this thesis, I examine the biological relationship of language and thought with a view to elucidating the relationship of syntax and semantics in linguistic theory. Part I (Chapters 1-4) grounds a notion of 'meaning' in terms of evolved properties of organisms, while Part II (Chapters 5-8) explores how linguistic meaning is thereby cognitively related to the structure of language.

In Chapters 1-2, I combine intentional systems theory (Dennett 1971) with Peircean semiotics, so as to describe meaning as a relation between the internal state of an organism and its environment. I then develop this description for those meanings that are particular to communicative dynamics and I show that this enables us to unify models of nonhuman signalling and human pragmatics within a single evolutionary theory of communication.

In Chapters 3-4, I argue that language is not fully explained by communication theory, given how its formal (syntactic) structure uniquely varies in tandem with compositional (semantic) meaning. I establish that a compositional Language of Thought (Fodor 1975) must precede language in phylogeny and ontogeny, and explaining its linguistic representation requires communication theory to be supplemented by a theory of Universal Grammar.

In Chapters 5-6, I explore how the co-variance of form and meaning allows us to probe the nature of compositional thought by studying the functional structure of language, particularly within exo-skeletal syntactic theory (Borer 2005a,b). However, I argue despite this that the form and meaning of a sentence cannot together be the output of one and the same generative process, contrary to an axiom of generative theory since Chomsky 1965.

I claim that there can be no such thing as a syntax-semantics interface, as the operation, Merge, only generates compositional thoughts that are independent of language. Sentences must be produced separately in a way that only resembles generation by Merge because they express Merge-generated meanings, though they only acquire those meanings when used, thanks to a syntax-pragmatics interface that is the real substance of Universal Grammar.

In Chapters 7-8, I propose a cognitive model to reconcile this Wittgensteinian contextualism with a generative Language of Thought and I present a comparative analysis of expletive constructions and verb movement to show how core insights of generative theory can be adapted and maintained.