I don't think that there's a single one of us who hasn't heard the phrase "Go out and play!", at some time in their lives. It wasn't just a way of getting kids out from under foot. It was a way of getting them to avoid sitting on the couch doing nothing. It was a way of getting them to have their exercise and to expend they're nearly inexhaustible supply of energy. And in that little phrase, there remains much wisdom. The trouble is, we don't go out to play. We belong to gyms , we wear fitness trackers, we become slaves to fitness software and apps, we know all there is to know about oxygen consumption, training, caloric expenditure, oxygen use, and any one of a number of pieces of data. Despite that, we're a nation getting fatter and fatter every day. As I said, we don't go out to play. Why? Because we think that the old wisdom is dead, and that the new wisdom must be right. New wisdom that says if we track everything, exercising some...