Where does one start with a theory of man if the theory of man as an organism in an environment doesn't work and all the attributes of man which were accepted in the old modern age are now called into question: his soul, mind, freedom, will, Godlikeness?
There is only one place to start: the place where man's singularity is there for all to see and cannot be called into question, even in a new age in which everything else is in dispute.
That singularity is language...