Now, clearly, the IRS is not going to do any of these things, nor would Congress allow it to do so. But because tax analysts implicitly accept the Haig-Simons definition of income, even though it appears nowhere in law, there has been a long-term tendency for the IRS to push the limit of what can be considered taxable income. Now, a federal court has said there is a constitutional limit.

One area where I would like to see the court go further has to do with the question of whether interest constitutes income. To economists, some portion of the interest we receive on our savings is merely compensation for loss -- loss of the immediate enjoyment we would receive if we consumed our income today instead of saving it.

Think of it this way. Would you be satisfied receiving your paycheck a year from now instead of on payday? Of course not. You would be suffering a real loss if you had to wait a year to get paid for your work. But if you were offered, say, 10 percent more in a year, you might say that was OK. Collectively, our willingness to put off consumption today for greater consumption in the future is what determines the pure rate of interest.

But in the view of many great economists, such as John Stuart Mill, the future interest one receives is merely compensation for the loss of immediate satisfaction. Therefore, it is not income, but more like an insurance settlement that simply makes us whole. Now, obviously, market interest rates are more than simply a discount between present and future, as my example implies. A lot represents a return to risk and an adjustment for expected inflation. But in principle, some portion of interest is compensation for loss and therefore not income.

Given the logic of the Murphy decision, it is quite possible that the risk-free, inflation-adjusted rate of interest could also be excluded from taxation on constitutional grounds. Following through that logic consistently would revolutionize taxation and eventually lead to a pure consumption tax, which most economists today favor.

I'm not predicting that the Supreme Court will follow this logic. But it does open an interesting possibility that tax analysts will follow with interest.