The problem with a recent essay from economist Mark Thoma for CBS Moneywatch is evident in the title: “What’s a fair tax rate?”

The word “fair” is unhelpful in discussions about public policy. It’s an extremely subjective word, and the idea itself is hard to either define or refute.

The more appropriate question is “what’s a reasonable tax rate?”

But Thoma is quickly bogged down by “fairness” in his essay.

“How progressive should the U.S. tax system be?” he asks. “Answering this question requires an assumption about what’s fair in terms of tax burdens across income groups. But people differ widely on what they consider fair. Therefore, fairness isn’t something economic theory can address. Instead, a principle of fairness must be assumed.”

Here’s where it gets ridiculously complicated.

“Under the principle of ‘absolute equity,’ everyone pays exactly the same percentage of income, no matter what his or her total income is,” he writes. “In this case, taxes would be flat rather than progressive.Under the ‘benefit principle,’ an individual’s tax burden is equal to the value of the government-provided goods and services the individual consumes (which is very difficult to calculate).”

But neither of those is the premise behind federal income taxes in the U.S.

“The most popular justification for progressive taxes relies on the ‘ability to pay’ principle,” he writes. “Under this definition of equity, the ‘pain’ that each person experiences when paying taxes is the same.”

There are other principles to consider.

First is the social contract. We pay taxes because it’s part of the deal we have with society. The social contract begins with a cessation of hostilities — I won’t hit you and you won’t hit me. But it has developed into much, much more. It’s an agreement amongst ourselves that children should be education, laws should be enforced, and trade should be as free as possible. We support the social safety net because we’re decent people.

The social contract is why taxpayers, for the most part, send in their checks willingly. But we’re not dumb, and we’re not gullible. When government starts taking on matters we don’t feel are part of the social contract — say, funding controversial artists — taxpayers start grumbling.

Then there’s the principle of investment. We pay taxes, in part, to help make our cities, our states and our country better places to live. We’re all in this together. Where this principle falters is when taxpayers perceive others and not being invested alongside them. Mitt Romney’s ill-considered “47 percent … who are dependent upon government” remark reflects this feeling.

The idea of fairness is hard-wired into the human psyche; ask any 3-year-old.

But it’s not a useful way to talk about taxes. A better way to think about and discuss taxes is in light of the idea of reasonableness. What’s a reasonable tax rate?

In France, recently, a 75 percent tax on the wealthy was considered “fair.” It was unreasonable, and it was repealed.

We’ll never all agree on what’s fair. But we can get closer when we talk about what’s reasonable.