Two very large numbers are essential in the American public debate today. They are so big that they are almost inconceivable. The first number is 15.5 trillion. That’s a 14 digit number. $15.5 trillion is the size of the U.S. national debt—more or less the amount the federal government owes individuals, organizations, and governments that have loaned money to it. To put this number in a more human perspective, it’s roughly $50,000 for every person in the United States. $50,000 is more than the U.S. median income, about $44,000. That means if a normal, working American worked an entire year...