Imagine a pencil, not one of the fancy mechanical pencils of today, but a standard pencil: a bit of wood surrounding a lead core. As you write, it gradually diminishes to nothing. First the point dulls, eventually to the point where you need to re-sharpen the pencil, one way or another. Gradually there is less and less pencil, until there is barely enough left to hold as you write. But the pencil never does fully diminish to nothing, for eventually it becomes too small to write with, and then you must take up another pencil. A pencil thus gets close to becoming nothing, but never quite arrives.
This, I think, is Beckett’s model of human life.