Normal people tend to forget this process unless and until they are reminded of it. We are so familiar with the fact that man ages, that people have for years assumed that the process of losing vigour with time, of becoming more likely to die the older we get, was something self-evident, like the cooling of a hot kettle or the wearing-out of a pair of shoes. They have also assumed that all animals, and probably other organisms such as trees, or even the universe itself, must in the nature of things “wear out”.

　　Most animals we commonly observe do in fact age as we do, if given the chance to live long enough; and mechanical systems like a wound (上发条的) watch, or the sun, do in fact ran out of energy in accordance with the second law of thermodynamics (热力学). But these are not similar or equivalent to what happens when man ages. A run-down watch is still a watch and can be rewound. An old watch, by contrast, becomes so worn and unreliable that it eventually is not worth mending. But a watch could never repair itself — it does not consist of living parts, only of metal, which wears away by friction (摩擦). We could, at one time, repair ourselves — well enough, at least, to overcome all but the most instantly fatal illnesses and accidents. Between twelve and eighty years we gradually lose this power, an illness which at twelve would knock us over, at eighty can knock us out, and into our grave. If we could stay as vigorous as we are at twelve, it would take about 700 years for half of us to die, and another 700 for the survivors to be reduced by half again.

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