My understanding is that it's not "before we developed language" - but rather, before we developed modern culture, which silences these voices ("imaginary friends") during upbringing.
However, Jaynes argues that various conditions (schizophrenia, non-schizo "voices") are a non-silences version of that natural tendency, albeit often harmful - the revenge of the forgotten imaginary friends, if you will.
However, Jaynes argues that various conditions (schizophrenia, non-schizo "voices") are a non-silences version of that natural tendency, albeit often harmful - the revenge of the forgotten imaginary friends, if you will.