As you read this, you are gradually consuming the oxygen that surrounds you. Don’t worry: it constitutes 21 per cent of the ocean of air we inhabit, and the supply is almost certainly sufficient for your needs. But were the indispensable “fire air” — in which Joseph Priestley noticed that “a candle burns with an amazing strength of flame” — suddenly to be removed, your consciousness, and then your life, would fail almost as fast. We are pathetically dependent on a constant stream of this life-giving gas. It reminds us that, whatever else we may be, we are thoroughly physical systems. We are the matter that composes us: the laws that govern our atoms also govern our lives.
But, of course, you are no mere assemblage of atoms, no mere morass of molecules. You are a needy, vulnerable, hungry, thirsty, breathing, palpitating, sickening, and — I would it were not so — ultimately mortal creature. Were your cells suddenly to falter in their ceaseless but unnoticed processes — as if lightly dosed with cyanide — your life would falter also. So, cataloguing the atoms that compose you does not suffice to understand our nature; we need to know something of life — the intricate network of processes through which we use energy from the environment to grow, maintain and reproduce ourselves. Though we tinker with it, we cannot finally escape the fate that our biology decrees.