This year will go down in history as the year a person became a trillionaire for the first time—on paper, at least. Elon Musk’s net worth catapulted to this unprecedented height thanks to the spectacular initial public offering of his company SpaceX. Putting the moral, social and economic consequences of a single person amassing so much capital aside, how we conceptualize Musk’s wealth reveals humans’ flawed sense of numbers. Very few people have an immediate grasp of the immense size of a...