When Joseph Bazalgette laid out London’s sewers in the 1850s and 60s, he worked out how wide each pipe would need to be for the most crowded streets he could imagine, and then, as the story is usually told, doubled it, on the grounds that they were only going to do this once. The Institution of Civil Engineers reckons the narrower pipes would have been overwhelmed by the 1960s. Nobody walking above them has ever thanked him for the extra width, and nobody could: the best kind of foresight leaves no trace that a decision was made, only that the drains still go. A good deal of what we inherit is probably like that, decided once, out of sight, by someone allowing for strangers they would never meet.