It is a bit strange when the press gets schoolmasterish about a president's facts. Not even a president of Napoleonic mendacity could dole out as much misinformation in a lifetime as the press, given the hazards of its calling, does in a day.

Still, the press shares the general prejudice that our betters, especially presidents, ought to be informed. One who stumbles as badly over historical matters as Ronald Reagan did at a press conference last month is sure to chase reporters to their reference books.

Reagan said--unnecessarily, since the question did not require the excursion--that there were two "separate countries" in pre-colonial Vietnam (there were three) and that John F. Kennedy sent the first U.S. combat Marines there (it was Lyndon Johnson). It was a field day for those who claim that Reagan's memory lies mainly in his 3x5 file cards.

Recent presidents could be divided between those with genuine historical grasp (FDR, Harry Truman and John Kennedy certainly, Dwight D. Eisenhower probably) and those for whom history, even American history, seemed at best a convenience of polemics.

According to his aide John Ehrlichman, Richard Nixon, when trying to thrust a controversial Supreme Court nominee on a doubting Senate, sent his staff to research the confirmation fights over Brandeis, Hughes and John J. Parker. Armed with their findings, Nixon later seemed to claim that controversiality itself is in some mysterious way a guarantee of judicial capacity.

Similarly, battling Congress over impoundment, Nixon seized upon the story of Thomas Jefferson's impoundment of gunboat appropriations. But he managed to tell (or garble) the story differently every time he trotted it out.

Ronald Reagan is no sly manipulator of history. But to judge by his recent performance, his acquaintance with it hardly goes beyond a polite nod. Whether that will affect his performance depends on how serious the deficiency is, and what it tells us about his grasp of current issues.

The issue here--how much history should a president know and when and how should he know it--lends itself to flapdoodle. It isn't simple. But those who think the country is on the whole safer in the hands of a history-conscious president usually cite Santayana. It was his now well-worn adage that those who forget the past are condemned to repeat it. (He overlooked the episodes that might be worth repeating, but we know what he meant.)

Following the Santayana rubric, some foreign policy sages in the Vietnam period often cited, as a warning example, the Munich agreement of September 1938. At Munich, Neville Chamberlain ceded German-speaking parts of Czechoslovakia to Hitler in the hope of avoiding war. Munich became the great instance of the foolishness of trying to resist aggression by piecemeal concessions. But Munich was also part of the legacy of the Versailles Treaty, whose instability sprang from the neglect of powerful currents of nationalism. That was also pertinent in Indochina, but little mentioned.

Indeed, the complexity of the Munich example suggests that, for a statesman, the value of knowing history goes far beyond the collection of horrible examples. It is important to know our Munichs. It is more important to have that special curiosity about the past, on its own terms, that would lead to full and continuing study.

After all, history is a humanistic study. The difference between a useful sense of history and mere example-collection is about the same as that between being able to quote a few Shakespearean lines and knowing what the plays are about and what they might teach about the human situation.

When a useful past is part of the equipment of a president's mind, it has to condition his judgment, in ways that involve more than knowing what to avoid. Or so we hope. The political opinions of distinguished historians are not always good advertisements for the claim that history is the key to sound statecraft.

President Reagan could hardly be said to be ignorant of history. But when he is confused about Vietnam or about the origins of the New Deal, he leaves one wondering if some essential perspective is lacking. He sometimes reminds you of a good novice chess player who hasn't studied the classic games (which the chess masters call "book").

Masters don't always win. But they all know their book. "Reagan is no sly manipulator of history. But his acquaintance with it hardly goes beyond a polite nod."