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.