SIMPLY AS AN offhand opinion, President Reagan observed that he finds it hard to justify the corporation income tax. But there is only one justification that really counts: the federal government needs a certain amount of money to run itself, even after all those cuts in food stamps. You can't pay for nuclear aircraft carriers with buttons, and even with the present corporation tax, the Reagan deficit is now coasting toward $200 billion.

Did Mr. Reagan have anything serious in mind-- like abolishing the corporation income tax? White House spokesmen anxiously assure everyone that he did not. He was simply offering a congenial thought to some businessmen to whom he was speaking in Massachussetts. It is very much like the two congressional Democrats who had been sniping at the Social Security compromise the day before. They didn't have anything specific in mind, either. It's the kind of gesture that public figures sometimes make to show that their hearts are in the right place.

The corporation income tax has attracted a lot of attention over the years from the philosophers of taxation. The accusation is that it subjects investment income to double jeopardy, first when the corporation earns it and again when the dividends go to shareholders. That's true enough, but the remedies turn out to be not so simple. The central issue is the profits that corporations retain and reinvest. Does that seem like a narrow technical point? It isn't. Without a tax on retained earnings, a corporation becomes a tax-free savings account for its shareholders--an IRA on a grand scale, with earnings compounding merrily beyond the reach of the Internal Revenue Service. They would be taxable only when finally withdrawn, and then at the very low capital gains rate. People have occasionally tried to devise solutions to this riddle, but each has brought with it new and complicated defects.

The corporate income tax belongs in the large category of institutions that are not entirely satisfactory but on balance are greatly preferable to any alternative. But that is increasingly a matter of purely theoretical interest, since the corporation income tax is rapidly shrinking. Over the years, Congress has provided one loophole after another, and in 1981 it expanded depreciation benefits in ways that will reduce it to an irrelevance for many companies by the end of the decade.

Mr. Reagan need not concern himself with the abolition of the corporation tax, since under present law it is already proceeding rapidly. The things to worry about are the inequities being created by its erosion--not to mention the government's need for more money.