Lacan’s famous Graph of Desire – to be more precise, a certain part of it- tells us about a the point where language fails. No matter what our parents tell us about LIFE (pure existence, that is) and SEX (act of copulation) we won’t feel sartisfied with their explanations. We feel there is something missing. This part is when a failure is designated with the Lacanian formula of S (barredA)[S(bA) from now on] (and is marked with S (ø) in the picture)
It is a paradox situation. Why? Precisely, because the process of signification fails here- our parents are unable to utter, to linguistically formulate the essence of LIFE and SEX, so this crucial point is when a chain of signification (i.e what our parents are about to tell us) bears inadequacy to us. Paradoxically, to refer back, a signifier is what signifies the failure of signification for Lacan.
For example, if a patient comes to a session with dream, where he fails to make a symmetrical form out of some assymetrical object; this dream can be the very association of unsettled family power relations. S(bA) is what marks this point of impossibility.