Ultimately, unless we scale back entitlement programs far more than anyone in Washington is now seriously considering, we will have no choice but to increase taxes on a vast majority of Americans. This could involve higher tax rates or an elimination of popular deductions. Or it could mean an entirely new tax, such as a value-added tax or a carbon tax.

To be sure, the path ahead is not easy. No politician who wants to be re-elected is eager to entertain the possibility of higher taxes on the middle class. But fiscal negotiations might become a bit easier if everyone started by agreeing that the policies we choose must be constrained by the laws of arithmetic.