Like all good plays, it's not certain that you can easily 'cut to the core' of it too easily: it's about lots of things all at once, and it has a variety of interests and themes.
The main one, in many ways, is
Vivienne is dying of cancer in a ward. And as she dies, she talks to the audience, remembering her life thus far and the decisions she's taken, and her life as an academic. She's specialised in John Donne's poetry, and one of Donne's primary anxieties is what happens after death: what death means.
Vivienne reflects on her cancer and how it changes the way people see her and talk to her - and in fact, how it changes her own opinions and self-knowledge.
Wit (and theatricality)
And Vivienne is funny. Wit, of course, is a key ingredient of drama and of (in a meta-theatrical way) Edson's play - as well as of Donne's poetry and Vivienne's character. Often, aspects of all three themes I've mentioned so far are discussed together:
I have been asked, "How are you feeling today?" while I was throwing up into a plastic washbasin. I have been asked as I was emerging from a four-hour operation with a tube in every orifice, "How are you feeling today?" I am waiting for the moment when someone asks me this question and I am dead. I'm a little sorry I'll miss that.
Vivienne speaks directly to the audience, with a dry, sardonic wit - but she's speaking about her cancer, her imminent death, and the distance between her own self-perception and the people who surround her in the hospital. And - perhaps surprisingly - she's funny.
There are three of the most central themes. Yet - and it's worth giving this a try - in most good plays, the opposite of key themes are always dealt with simultaneously. Might Wit also be about life, health and lack of wit (or the moment when wit needs to stop)?