"...[U]se your moral muscle only very sparingly. My father, a professional philosopher, has a job that involves thinking very hard about very difficult things. This, of course, is an activity that consumes mental resources at a terrific rate.

The secret of his success as an academic, I am now convinced, is to ensure that none of his precious brainpower is wasted on other, less important matters. He feels the urge to sample a delicious luxury chocolate? He pops one in his mouth. Pulling on yesterday's shirt less trouble than finding a clean one? Over his head the stale garment goes. Rather fancies sitting in a comfy armchair instead of taking a brisk jog around the park? Comfy armchair it is. Thanks to its five-star treatment, my father's willpower - rested and restored whenever possible - can take on the search for wisdom with the strength of 10 men.

Although we may not all be able to live the charmed life of the well-paid scholar, the general principle - not to spread our inner resolve too thin - is an important one. If you are about to embark on a big project you court disaster if at the same time your life is cluttered and demanding, or you also commit to draining attempts at self-enhancement. The would-be novelist whose taxing day job exhausts her moral muscle will find it harder to apply the seat of her trousers to the seat of her chair. The dieting philosopher will struggle to keep his attention on a tricky passage of Friedrich Nietzsche.

Where are the students whose self-discipline is constantly worn away by other concerns? Not in the library reading course-notes, photocopying articles or borrowing books. And if they are relying on their smarts to get them to the top of the class then there will be disappointment ahead."