President Reagan clearly means to sell his "tax simplification" plan the way nice parents, 30 years ago, administered cod-liver oil -- by lacing it heavily with orange juice and, where possible, even calling it that.

No one disputes the merit, in principle, of tax simplification. In its present complexity, the tax code extends some unproductive, even antisocial, "tax preferences." Indeed, the last "reform" effort -- the Reagan tax cuts of 1981 -- afforded big openings for seekers of loopholes.

The key question is, simplification at what cost? At the cost of equity? Of trampling the principles and comities of federalism?

Consider equity first. One reason Budget Director David Stockman got taken to the woodshed when he told all about the 1981 tax-cut battle was that he was breathtakingly candid about the administration's intentions. The major objective of the cutting spree, Stockman confessed to William Greider in The Atlantic, was to reduce the topmost marginal rate for individuals to 50 percent. The rest was window dressing -- or, as he put it, "a Trojan horse."

And indeed, the notable result of the Reagan cuts of 1981 was a sharp reduction in the overall progressivity of the individual income tax. As written, the Reagan simplification plan would reinforce the same cause -- the shift away from the principle that the more you earn the larger percentage of your income you pay. It would work like the blades of a scissors. The top rate would drop to 35 percent, but meantime the ever-rising Social Security tax (applicable to all incomes up to the $40,000 mark) would ensure that the relative taxation of lower-bracket taxpayers would remain steeper than that of upper-bracket taxpayers.

If, as is not uncommon, the same high-bracket earner derived an occasional windfall from capital appreciation, the proposed cut in the top capital gains tax would accentuate the shift. So would the taxation of employer-paid health insurance benefits at a much lower threshold than proposed in Treasury I.

Disguise it as one may in "populist" rhetoric, the flattening of personal income tax rates, largely for the benefit of high-income taxpayers has been a cardinal goal of the Reagan fiscal revolution -- and continues to be. Second only to the grand strategy of decimating the revenue base so as to force Congress to cut social spending, it is of the essence of Reaganism.

Which is to say that whatever Ronald Reagan thinks he is, or poses as, he is no "populist" in taxation. He is a Mellonite, a devoted acolyte of old Andrew Mellon, the Coolidge-era Treasury secretary, an unsentimental fellow who never hid his belief in orange juice for the few and cod- liver oil for the crew.

It is never quite clear how much of the fine print the president bothers to master before he launches one of his great economic crusades. But there is no special evidence that his bias in favor of regressive taxation springs from conscious "selfishness" on Reagan's part. As his gushing invitation to one and all to go out and start a new Apple Computer company shows, he remains giddily romantic about creativity in a world of low taxes and reduced government.

Whether he also shares the view of his attorney general that progressive income taxes are "immoral" as well as economically stunting, he has been too discreet to say. But one has one's suspicions.

As for the structural impact of the Reagan "simplification" plan, it is again clear who's to take the medicine: the states and cities.

The plan is "revolutionary" all right -- in the centralizing direction. It is Washington the tax hog to a fare-thee-well. The removal of the deductibility of state and local taxes will deal a harsh blow to the financing of local services, especially welfare payments and schools. This, again, seems to accord with Ronald Reagan's functional philosophy of government. It is by picking up an estimated $34 billion a year for the Treasury here that the president's plan would mainly fund the new round of handouts for upper-income taxpayers. It is a strange twist on the "New Federalism."

It suggests, with other parts of the president's plan, that one man's orange juice may turn out to be another's cod-liver oil.