PRESIDENT REAGAN said it in a cheerful,

enthusiastic tone: "We go forward with the reduction in tax rates, and I have no plans for increasing taxes in any way." The question had come up toward the end of his news conference last week, and he had been giving a series of cautious, close- grained answers on subjects like Poland, Libya and Israel. He seemed to seize the tax issue with pleasure and relief at the chance, at last, to offer a simple, unqualified answer.

Except, of course, for the melancholy truth that in tax policy there is no such thing as a simple, unqualified answer. The president had hardly left the room before his press staff began anxiously telling the world that his statement did not affect, of course, the administration's previously announced plans to seek $22 billion in various kinds of tax tightening here and there.

The president also seemed to rule out, once again, any tax on natural gas. But wait just a minute. It's pretty clear that Congress won't pass a decontrol bill unless it includes a windfall tax on the sudden leap in gas prices. What if the gas producers themselves, and their dearest friends in Congress, should urge the president to accept the tax as the price of faster decontrol? It appears that the administration's position may not be quite so firm as it appeared at the press conference.

The real meaning of the president's statement is that he's proud of last summer's income tax bill with its gigantic reductions, and he has no intention of reversing himself on it. There are a lot of economists in the administration, and fiscal experts in Congress, who are deeply concerned by the looming implications of the coming drops in revenue. Some of them have been trying to persuade him to order a tactical retreat. The press conference demonstrated that they have not succeeded. Yet Congress says that there's very little more spending to be wrung out of the budget, and deficits are rising. That makes it increasingly possible that the next stage of the Reagan administration's tax strategy will be a swing toward excise taxes--taxes, like sales taxes, on things that people buy rather than on the income that they earn. Regarding excise tax increases, the president's press conference is hardly likely to be the last word.