Parsing the transcript of President Reagan's extemporaneous mini-press conference the other day was scary enough. But what must have triggered my nightmare, as best I can reconstruct it, was a background briefing the same day by a senior European diplomat (you don't become senior by going public) on the utility of the upcoming summit meeting of the seven Western industrial nations at Williamsburg.

"There is huge benefit," he argued, in the opportunity for "practical politicians" to meet even briefly by themselves with no aides "shoving notes in front of them"; to let their hair down and "take each other's measure."

That did it. It was the thought of Ronald Reagan having his measure taken by six sharp-eyed, seasoned veterans of parliamentary rough-and- tumble, with their keen ear for nuance and their necessary sensitivity to every twist and turn in U.S. policy--and of him talking the way he talked to those six reporters in the Oval Office.

I am not even thinking of the quickly corrected slips of the tongue about Salvadoran "freedom fighters" or the muddled presentation of his Nicaraguan policy, which attracted most of the notice. Rather, my nightmare vision is of polite coughs, fleeting exchanges of glances, and perhaps abrupt changes of subject (nothing rude) as the president takes off on one or another of the fundamental flights from reality to be found in the press conference transcript. For example:

China: He was "not at all sure" that Jimmy Carter had "added anything" to what "a previous president to the previous administration" (that would be Richard Nixon) had already accomplished. What Carter added, of course, was the hard part--the 1979 "normalization" agreement establishing full diplomatic relations with the People's Republic, at the expense of downgrading relations with Taiwan to the level of "unofficial." In Carter's time, a military aid relationship was also struck up with Peking, while limitations were imposed on such aid to Taiwan.

Does the president not remember the mess he got into making such a big deal about this in the 1980 campaign? Does he not recall charging that Carter "made concessions that were not necessary and not in our national interest"?

The Mideast: Conceding that Jimmy Carter "started" the Camp David agreement, he argued that the Reagan administration has "gone a step beyond (by) trying to have an overall peace in the entire area--that had never been proposed."

The reality: it has been proposed in the so-called Rogers Plan under Nixon, and regularly by Henry Kissinger; it is the bedrock of the Camp David Accords (which Reagan takes as the basis for his own Mideast "initiative"). They are subtitled "A Framework for Peace in The Middle East." The preamble states: "The parties are determined to reach a just, comprehensive, and durable settlement of the Middle East conflict."

The Alliance: "I do not believe that the NATO alliance has ever been more solid than it is now, or that there has been a better relationship between us and our allies." If Reagan tries that in the free-form shoptalking sessions, his best hope would be that it would be taken as a joke to break the ice so apparent in the summit preparations on East-West trade, on Western economic relations, on arms control, on assessments of the Soviets.

These are fundamental misapprehensions. The familiar post-mortem rationalizations--that the president was "having an off day" or perhaps didn't quite catch the question--don't persuade. White House aides, conceding a problem, talk about the need for better briefing, more structured formats, tighter wraps, all of which helped Ronald Reagan out of trouble in the 1980 campaign.

But that's only a formula for cleaning up the act. It can have the effect of improving the performance. But it can't alter the evidence that to "let Reagan be Reagan" is to invite the conclusion that his attention/retention span for foreign affairs is short--that, left to his own devices, he is either out of date or out of touch.