I THINK IT IS POSSIBLE to track the onset of middle age exactly. It is the moment when you examine your life and instead of a field of possibility opening out, an increase in scope, you have a sense of waking from sleep or being washed up onshore, newly conscious of your surroundings. So this is where I am, you say to yourself. This is what I have become. It is when you first understand that your condition—physically, intellectually, socially, financially—is not absolutely mutable, that what has already happened will, to a great extent, determine the rest of the story. What you have done cannot be undone, and much of what you have been putting off for “later” will never get done at all. In short, your time is a finite and dwindling resource. From this moment on, whatever you are doing, whatever joy or intensity or whirl of pleasure you may experience, you will never shake the almost-imperceptible sensation that you are traveling on a gentle downward slope into darkness.