Ever since the first Homo sapiens emerged from her cave on an idle morning and wondered "Who am I?" the human race has been inventing and re-inventing answers to that question. The latest people to pick up on that have been the scientists, as they start to tackle the great question of consciousness.
Their interest has been caught by dramatists, with On Ego opening last month at the Soho Theatre and Imposters opening tonight at the Union Theatre in Southwark. This surely must be the first London season that two plays have appeared in which a major part is played by the rare and strange Capgras' Syndrome - in which the victim of a brain injury believes their nearest and dearest have been replaced by near-identical imposters. (Meanwhile in New York there's a whole theatre festival on the subject, in which this play is included.)
Yet, when you think about it, what better way to tackle issues of identity than this? Certainly the American playwright Justin Warner has used this as a fruitful way to approach what is a common tale - a family and a marriage under strain when the children have grown up and the holes at the heart of a long marriage are suddenly exposed.
The narrator of the tale is the youngest son, Andrew (Edd Hunter), a young post-graduate science student, alternately cockily sure of himself then consumed by doubt, a characterisation that many will recognise in his class. He's sometimes a bit of an irritating know-it-all, but usually endearing, although lacking in depth. Andrew takes some massive emotional shocks with barely a shrug, and comes to some major life decisions without sign of a tremour.
The real emotional core and heart of the play - and indeed the family - is his mother, Peggy, a fine performance by Joyce Springer, who manages to be both ditzy and deep in a single moment. She's been the victim for 24 years of emotional abuse from her husband, Frank, a frustrated and angry salesman, and her (Catholic) church. Continually told she is stupid, Peggy has learnt to play up the stereotype. The only dream left to her - the summit of her (unachievable) ambition - is to be the hand model in the Palmolive dishwashing adverts.