Fig. 1

Theory of language: a fourfold table made up of the vertical axis humanistic versus biological; and the horizontal axis historical versus systemic. Influential works are cited within each type. Type 1 (see “Type 1: humanistic‒historical” section) approach is humanistic‒historical; Type 2 (“Type 2: humanistic‒systemic” section) is humanistic‒systemic. Type 3 (“Type 3: Biological‒historical” section) represents a biological‒historical theory of language; and, Type 4 (“Type 4: Biological‒systemic” section), a biological‒systemic theory