Restrictions on possible language types

The first step beyond typology as the classification of types and toward the explanation of the cross-linguistic variation that classification describes is the discovery of restrictions on possible language types. Linguistic theory in any approach, formalist or functional–typological, has as its central question, what is a possible language (§1.2)? This question can in turn be paraphrased as: of the logically possible types of languages, how do we account for what types actually exist?

One of the features that distinguishes the typological method of discovering constraints on possible language types is the empirical method applied to the problem. If a typologist wants to find restrictions on possible relative clause structures, for example, he or she gathers a large sample of languages and simply observes which of the possible relative clause types are present and which are absent. That is, the restrictions on logically possible language types are motivated by the actually attested language types. If there is a gap in the attested language types, then it is provisionally assumed that the gap represents a constraint on what is a possible language, and explanations are sought for the gap. This is the inductive method, which must be used in constructing generalizations from empirical data. In contrast, the generative approach uses a rationalist deductive method, in which it is argued that certain analyses of a single language represent universals of human language because they cannot possibly be learned by a child (the ‘poverty of the stimulus’ argument; §1.2).