  In Part 5 of the Discourse on the Method, for instance,  Descartes identifies the ability to use language as one of two  features distinguishing people from “machines” or  “beasts” and speculates that even the stupidest people  can learn a language (when not even the smartest beast can do so)  because human beings have a “rational soul” and beasts  “have no intelligence at all.” (Descartes 1984: 140-1.)  Like other great philosopher-psychologists of the past, Descartes  seems to have regarded our acquisition of concepts and knowledge  (‘ideas’) as the main psychological mystery, taking  language acquisition to be a relatively trivial matter in comparison; as he puts it, albeit ironically, “it patently requires very  little reason to be able to speak.” (1984: 140.)