First, “lesson” is not the right word to describe Beckett’s motivation for writing.  He had no interest whatsoever in showing the world how things work.  Endgame is a stage description, in stage language, of Beckett’s view of the Cartesian duality of mind/body.  Hamm and Clov are two sides of a master/slave relationship, or of a thinking being and the physical life he finds himself in.  Any old person experiences the same duality – the “self” begins to dissociate itself from the physical presence the body has occupied since birth, and the “self” – the consciousness, the connection with the senses that makes sense of the physical universe – seems to lie somewhere other than in the immobile, limited body.  Beckett’s depiction of this duality (and see Waiting for Godot’s Gogo and Didi, and Pozzo and Lucky) is an ingenius way of “effing the ineffable” – of stating, in theatre language, the unsayable feeling of the conscious duality.