Everyone makes mistakes. Every president does. You have to grant, however, that President Reagan gets by with more than his fair share. The occasions for error are multiplying, moreover, as the president increases his public appearances in the fall campaign. Following his Sept. 28 press conference, in which the president volunteered several pieces of misinformation, Democratic National Chairman Charles Manatt called Mr. Reagan "the Great Prevaricator" -- a takeoff on the "Great Communicator" tag bestowed on him by his admirers.

The standard reaction by White House aides is to say that the errors are what you could call de minimis -- a Latin phrase meaning things too small and inconsequential to worry about. When asked about the general impact of presidential misstatements, David Gergen, assistant to the president for communications, says, "In terms of pure impact, I think that this question does not matter to most of the country. People are much more interested in his performance as president and the performance of the economy."

It sounds, at first, like a dodge: the people don't much care even if the details are wrong; they can be fooled and they won't mind. But it's really a reiteration of the de minimis argument. And in one sense, it's true. In 1984, the president and his party will be judged on the overall success of his program -- not on the accuracy of the statistics he cites off the top of his head in press conferences or in spontaneous remarks at public appearances.

The de minimis argument is not the only one the White House makes. It also questions one of the premises of some of Reagan's critics: that most of his off-the-cuff comments are in error. "Over the years he's come out with many facts and figures," Gergen says, "and far more often than not, he was right."

And there's another argument: the press gets the facts wrong, too. Gergen claims there were two errors in a Post story critiquing the press conference. And, referring to charges that Reagan in an earlier press conference had his facts wrong on the Vietnam War, he says that the questioner's premise was factually incorrect. "I say when we get that kind of argument about a factual situation 20 years old, I say it's spinach, and I say the hell with it."

Quibbling over obscure facts can get ridiculous. Partisan critics of the president can succumb to a temptation to criticize even the most harmless deviation from verifiable fact. But much of the criticism has made one of two much more serious points: (1) that Reagan's misstatements tend to paint a false picture, that they deliberately or ingenuously mislead voters; or (2) that Reagan's errors show that the president has an inaccurate picture of what is happening in America today, that he makes policy from fundamentally incorrect premises.

When you start looking at specifics, it is not quite so easy to dismiss all of Reagan's misstatements as de minimis, essentially accurate, or no more inaccurate than the questions or stories of the press. For example, Gergen argues that some of the press conference mistakes were attempts to refer to an accurate statistic that would have made just about the same point. The president may have meant to speak of four months of the government's mixed bag of leading economic indicators rather than four quarters of GNP, about auto sales increasing over 10 days rather than over several months, unemployment rising earlier during the Carter years rather than at the end. Yet, in each case, the accurate statistic would clearly have had less impact than the inaccurate one.

You can say that a president, who needs to keep 10,000 facts on the tip of his tongue, is entitled to such lapses. Yet none of these statistics were in direct response to a reporter's question. They were volunteered. And there seems to be a selectivity in the slips -- like balancing your checkbook and finding $300 worth of errors in your favor and $2 in favor of the bank.

And then, of course, there are the famous anecdotes: the man who bought vodka with his food stamps change, the English criminals said to be hanged for possessing guns, the black Navy mess-attendant at Pearl Harbor whose heroism supposedly led to the integration of the armed services. Gergen does not defend the accuracy of the stories but says, "Presidential storytelling is a time-honored sort of folk art in American politics. These stories tend to have a parable-like quality to them; he's trying to tell us how society works." Or, at least how he thinks it works.

What procedures does the White House have to avoid such errors, or correct them? The Reagan White House, like others, has a team of researchers attached to the speech-writing staff. It is led by Misty Church, 24, a public relations major while attending college, who worked her way up from a volunteer post in Reagan's Illinois primary campaign to a White House job. Every draft of a presidential speech and talking paper goes through her office, and every fact must be checked, preferably through a primary source. Departments are queried on facts relevant to them. Church, despite her lack of economic training, finds herself dealing with disputes on statistical matters between the Council of Economic Advisers and the Office of Management and Budget. She is frank to say she is proud of the results, and the fact is that the complaints about presidential inaccuracies arise almost completely from Reagan's unrehearsed remarks, not from his prepared texts.

Church occasionally receives calls from the president himself. Sometimes the speech-writing office will call the president's office to find out the source for something he has added to a manuscript. Church stresses that the president "reads everything," and she cites, as examples of his good memory, his recall of how many rifles, tanks and other materiel items American industry produced during World War II.

But her fact-checking is designed to guarantee that there is some basis for a fact -- not necessarily that it is fair proof of the more general point the president wishes to make. She showed us a draft of a speech she had worked on with her sources handwritten in the margin. One paragraph made the point that mortgage rates reached as high as 17.5 percent in 1980. She said that, although Treasury and OMB economists, in reviewing the speech, had noted that mortgage rates didn't get that high until 1981, she was able to find a weekly FNMA rate that hit 17.5 in 1980. The task of determining whether a statement is misleading is the responsibility of others -- ultimately of the president himself.

Criticized for misstatements during afternoon press conferences, the president decided to hold them at night, in prime time and, in Gergen's words, "let the public judge for themselves."

Of course, the public does not keep government statistics close at hand, and cannot judge immediately the accuracy of what the president is saying; and if his facts are not merely inaccurate but misleadingly inaccurate, the public may be confused. When they sense that is happening, as they do now, the Democrats will fume, and some journalists will grouse.

But in the long run there is something to be said for Gergen's let-the-public-judge approach. A president -- or any politician -- who consistently makes misleading statements will, sooner or later, pay a price. Just as arsenic accumulates in the body until there is enough to cause sickness and death, so inaccuracies -- inaccuracies significant enough to mislead -- can accumulate until a politician lacks credibility even when his fact-checker can substantiate every comma.

Reagan will be forgiven slips of the tongue and de minimis factual errors: the voters understand that a presidential press conference is not a quiz. But if he uses inaccurate statistics, even innocently, to paint a picture of America that consistently fails to jibe with what the America voters see around them, the voters will reach a judgment accordingly. And as Gergen says, it is the voters rather than the Washington press or the Democratic politicians whose judgment really matters.