The syntax of a language is its grammar, or the way its expressions may be put together to form sentences. A syntactic study is one that is not concerned with sentence-meaning, but with the purely formal aspects of word combination in a language. In studying formal languages (see logical calculus) the notion of a well-formed formula is purely syntactic, as is that of proof, since each is defined without regard to the interpretation the sentences of the language are intended to have. See also model theory. It is a doctrine of Chomskyan linguistics that the syntax of a natural language is so complex, yet picked up by the learning infant so quickly, that we have to postulate an innate universal grammar, or disposition to select only certain forms as grammatical out of the theoretical possibilities. However, see Bayesian nets.