Helen Simpson has written four acclaimed short story collections with no longer work in sight. The American writer Lydia Davis, whose collected stories are published next month, can sum up a lifetime in one paragraph. What follows is the whole of her story “A Double Negative”: “At a certain point in her life, she realises it is not so much that she wants to have a child as that she does not want not to have a child, or not to have had a child.”