Listen to just about any liberal speak about the budget and taxes these days, and it won't be long before you hear the phrase "The rich need to pay their fair share." This is one of those standard lines in politics that has become so universal and commonplace in the progressive world that it's barely noticed any more. It's a mental checkbox that everyone on the Left ticks without thinking about it. Of course the rich should pay their fair share: only a sociopath would disagree with a statement like that, in the abstract anyway.

But there's the rub: the statement is only widely acceptable because everyone gets to define it however they like. "What's fair is fair," we sometimes say. Well, not really…

Frankly, for a while now, the call for "fair share" taxation has been making me vaguely uncomfortable every time I hear it. Whenever some liberal icon like Paul Krugman or Bernie Sanders would repeat it, I'd give the obligatory mental nod, but somewhere in the back of my mind I'd feel a tiny tug of rebellion—and then I'd dismiss it and move on with whatever I was reading or listening to. To the extent that this feeling received conscious articulation, it was along the lines of "They need to come up with a better argument than 'fair share'."