Notes on Falling Leaves, Jerwood Theatre, Royal Court London

What would it feel like if your mind was in the process of predeceasing your body? Wittgenstein said that "death is not an event in life", since we do not live to experience it. But what about those hastening into the death-in-life of senile dementia or Alzheimer's? Not quite so clear-cut there. This is the predicament that Ayub Khan-Din tries to express in Notes on Falling Leaves, a new play that shows him moving from the gusto of the mixed-race-marriage and father-son problems of East is East into the terminal territory where the presiding spirit is Beckett.

In Marianne Elliott's sympathetic, if faintly over-poetic, production the Royal Court's main stage is empty save for a wan carpet of dead leaves and a solitary, peeling park bench. A fiftysomething woman shuffles on, craning forward, handbag over her arm, fingers plucking in vague distress at her floral frock. She contemplates the bench as if confronted by some oddly familiar but baffling conundrum, aims her bottom at it, misses, and sits wide-legged like a child on the floor, whispering to the leaves. Behind her comes a man in his mid-twenties, bearing the kind of spouted drinking beaker you would give to a toddler.

I've never really been comfortable with the idea that it is worse for a once-towering intellect (like, say, Iris Murdoch) to be brought down to this level. "The poor beetle that we tread upon," says one of Shakespeare's characters, "in corporal sufferance finds a pang as great/As when a giant dies." Does not the same principle apply to a suffering mind, a mind struggling to fathom its own extinction? So I applaud the fact that here we are dealing with a very ordinary, working-class woman, heartbreakingly played by Pam Ferris. The council house where she raised her family is about to be cleared and her son (portrayed with an appealing, unsentimental empathy by Ralf Little) has come back from London for a visit.

The cruel, locked-out frustration of his plight is communicated in a monologue where he punishes himself by articulating in graphic detail things he could never have told his mother were she well. For example, he has taken a posh virgin to the old home where they had "dead sex in a dead house", condemned in more ways than one. He envies a friend who can at least commune with the spirit of his dead mother: the inference is that it would be less lonely to live with a ghost than with a husk. All she is now is a "lovely smell on an old coat" she wore as a lollipop lady.

On the words "If you could talk, what would you say?", the woman's voice overlaps his as she tries to describe the experience of being on the inside of the "goneness" where "my me has gone" and "to think the think of things and not do" is part of the impotent indignity and separation of it. Of course, to write such a speech is to risk the charge of presumption. But I don't think that, in its desire to speak up for the losers on both sides, this powerful, distressing play could be fairly indicted of that.