The story is told, and in enough detail to suggest that it is true, that a reporter once asked the late Nelson Rockefeller if he was going to seek the presidency once again. Rockefeller said "No," and so the reporter returned to his newspaper and wrote a story which began: "Nelson Rockefeller said yesterday he would not run for president, but he didn't mean it."

Now it is Ronald Reagan about whom the phrase "but he didn't mean it" can be applied. After almost every Reagan press conference, reporters troop back to the White House briefing room so they can be told what the president did not mean. In fact, this ritual has become so institutionalized that newspapers can anticipate that for almost every story the press conference produces, there will be a companion story explaining what the president meant to say.

Take the most recent press conference. The president said he had "no plans to increase taxes in any way." By combining "no" with "in any way," it would be reasonable to conclude that the president, who after all is in the midst of preparing his budget, was saying no, no, a thousand times no, to a tax increase.

But no. He didn't mean no, said the presidential apres-press conference explainer, Larry Speakes. He meant something else. He meant, in fact, that there could be tax increases after all, although they are now called in Reaganspeak, "revenue enhancers."

Having come down definitively on both sides of the tax question, the president then proceeded to have his cake and eat it, too, on the subject of affirmative action. He said he was for it. Referring to a 1979 Supreme Court case that upheld affirmative action, the president said: "This is something that simply allows the training and bringing up so that more opportunities are there for them in voluntary agreement between the union and the management. I can't see any fault with that. I'm for that."

But he's not. At least his administration is not. His own Justice Department, speaking through the head of the civil rights division, said it is opposed to affirmative action. In fact, it is seeking a court case so it can challenge existing laws before the Supreme Court.

These are but two examples of issues in which Reagan and Reagan do not agree. There have been others. He has been all over the lot when it came to the Voting Rights Act, to the meaning of the antinuclear rallies in Europe and even, if you will, on the health of the Soviet economy. Reagan had the Russians eating sawdust -- a statement that forced him to eat crow.

Some of these may be simple Eisenhower-like misstatements, and some of them reflect the president's penchant for picking up something and then, without checking, passing it on as truth. But really, what we are dealing with here is the importance of sentiment or feeling or emotion -- call it what you want. The president, for instance, does not want a tax increase. This is not a matter of economic policy. It is gut, visceral. It is just not realistic.

Yet sentiment plays as big a role in the formulation of administration policy as does realism. A good hunk of the economic policy, for instance, is based on the way things should work, not on the way they do. The same holds for foreign policy. It should be true that the Soviets are behind all manner of evil abroad. It should be true, but it simply isn't.

In some ways, this sentiment is a wonderful thing to behold. Compared to the astringency of Jimmy Carter, Reagan is both warm and moving, and his emotions are very close to the surface. When, for instance, at the press conference, affirmative action was put into a certain context, he said he favored it. It seemed fair and Ronald Reagan, above all, sees himself as a fair man. But affirmative action in general is something he opposes, and this was an example of his emotions going one way and his policy another.

Usually, though, policy is tailored to sentiment and the two converge. For all the early talk of think-tanks and intellectuals, the overriding thrust of the Reagan administration is emotional -- a feeling about how things ought to be. This is why there has to be so much explaining after a press conference. Reagan is not like Rockefeller. He believes in what he says. The trouble is, almost no one else does.