Coming back to your native land after an absence of many years is a surprisingly unsettling business, a little like waking from a long coma. Time, you discover, has wrought changes that leave you feeling mildly foolish and out of touch.

If this book has a lesson, it is that we are awfully lucky to be here-and by 'we' I mean every living thing. To attain any kind of life in this universe of ours appears to be quite an achievement. As humans we are doubly lucky, of course: We enjoy not only the privilege of existence but also the singular ability to appreciate it and even, in a multitude of ways, to make it better. It is a talent we have only barely begun to grasp.

I can't imagine there has ever been a more gratifying time or place to be alive than America in the 1950s. No country had ever known such prosperity.

Even though sugar was very expensive, people consumed it till their teeth turned black, and if their teeth didn't turn black naturally, they blackened them artificially to show how wealthy and marvelously self-indulgent they were.

Every atom you possess has almost certainly passed through several stars and been part of millions of organisms on its way to becoming you. We are each so atomically numberous and so vigorously recycled at death that a significant number of our atoms-up to a billion for each of us, it has been suggested-probably once belonged to Shakespeare. A billion more each came from Buddha and Genghis Khan and Beethoven, and any other historical figure you care to name.

That's the trouble with losing your mind; by the time it's gone, it's too late to get it back.

My favourite fellow of the Royal Society is the Reverend Thomas Bayes, an obscure 18th-century Kent clergyman and a brilliant mathematician who devised a complex equation known as the Bayes theorem, which can be used to work out probability distributions. It had no practical application in his lifetime, but today, thanks to computers, is routinely used in the modelling of climate change, astrophysics and stock-market analysis.