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.