The Art of the Full Stop

It was a September afternoon in Ghent, and a boy weighing two summer jobs asked me, half as a joke, how many he ought to look at before he simply chose one. I started to give the brisk answer a teacher gives — *use your judgement, weigh the pros and cons* — and then stopped, because I realised I knew a better one. Mathematicians had answered his exact question decades earlier, and the answer was stranger and more useful than either of us expected. Look at about the first third, I told him, then take the next one that beats them all. He went away satisfied. I did not. I had spent thirty years teaching mathematics as a way of seeing the ordinary world more clearly, and I had just noticed that the clearest, most useful thing my subject had to say about ordinary life was a thing I had never once taught.

We make a particular decision dozens of times a day and almost never notice making it: the decision to *stop* — to stop looking and choose, to stop holding on and let go, to stop searching and commit. When to take the flat, leave the job, sell the house, end the evening, call the work finished. And we make it badly, most of us, most of the time, because no one ever taught us how. We are drilled, relentlessly, in how to begin and how to persevere — *never give up* is stitched on every cushion — and we are taught almost nothing about how to stop well, which is the harder and rarer skill, and the one this book is about.

The strange thing is that the knowledge exists. It has existed for decades, worked out properly by careful people, sitting in journals under names like the secretary problem and the prophet inequality and optimal stopping. It is real, and it is tested, and its central finding is genuinely cheering: for a great many of the decisions that torment us, there is a right moment to stop, and you can know roughly where it is — in advance, without seeing the future, and without doing any arithmetic worth the name. The mathematicians did the hard part. What never happened was anyone carrying their plain answers out of the journals and into the hands of the people standing in supermarket aisles and estate agents’ offices, frozen, not knowing the trouble even had a name. That carrying-across is the whole of what I have tried to do here. I discovered none of it. I have only translated it, into the plain classroom language I spent a career speaking to people who hate formulas.