Main idea extraction

What is life? One of our greatest difficulties in answering a question like this arises from language. We use words, and are inclined to think that a thing must correspond to every noun. Now some nouns stand for things; for example, bricks, water and coal - gas are things. Others are more doubtful. For example, a wave moves over the sea. We say that it is the same wave now as five minutes ago, but the particles of water in it are quite different. A tune has even less claim to be called a thing. It may be being played in several places at once, or nowhere. And some nouns like greenness or cleverness stand for qualities which no one, except a few philosophers, suppose to have any existence of their own. Where does life belong in a classification of this kind? When a man dies, we may say that he has lost his life, or that life has gone out of him. Is that just a metaphor? Is death the loss of something, or merely a change of state, as when a snowman melts, or a pattern is disarranged? The first people of whose ideas on this subject we know anything thought that life was the same as breath. But we know that breath consists of gas, which can be made into a solid or liquid, and also that many living things do not breathe. Life is certainly not a kind of matter. When a man or an animal dies he does not lose or gain in weight. Nor is there any measurable loss of energy. The heat gradually leaves the body, but is doing so throughout life. A dead body cools because no more heat is being generated inside it, not because anything measurable leaves it at the moment of death. Our ancestors thought that anything which moved itself was alive. And before the days of machinery that was quite a good definition. But a machine such as a motor-car or a steamship moves itself, and as soon as machines which moved themselves had been made, people asked, "Is man a machine?" the philosopher Descartes thought that both men and animals were machines, but that the human machine was partly controlled by the soul acting on a certain part of the brain, while animals had no souls. And some scientists think that life is just a very complicated mechanism.