The naturalness of a distinction is assumed to be an all-or-nothing characteristic and not a matter of degree. A distinction is natural just in case it corresponds to a difference in the mode of perceiving, processing, storing, or accessing data in the brain, such difference in turn depending on specific features of the brain’s physical structure. It is assumed that only distinctions of this kind can be candidates for primary grammaticization.
- Bickerton (1981), a pag.280