"THAT'S A DECISION that has not to be made yet." You all got the idea. President Reagan, in taking the last question at his press conference, was responding to the question heard all over America: will he run? It's a natural thing to ask about a president who will turn 77 in his final year in office if he seeks, wins and serves a second term.

Mr. Reagan is under no obligation to give an answer soon, and he has good reasons to be coy. An early announcement that he will run might put an extra political taint on his acts, and an early announcement that he will retire would reduce his influence. A formal announcement of candidacy might have adverse legal consequences. His political opponents and especially his political allies might be convenienced if they could learn his plans early. But he is under no obligation to accommodate them.

White House watchers have noted in recent weeks that the acts of the president's top assistants and of the president himself suggest that he has decided to run. He tells a group of supporters about the unfinished work of his administration. He says that if he does run, he will certainly want George Bush to be his running mate again. A top aide compares him to a horse in the starting gate at the Kentucky Derby. What's fascinating is that these slight hints have so changed the guessing: a great many people who thought four months ago that Mr. Reagan would retire are now sure that he'll run.

Some day the president may report when he made his decision and why. Maybe he already has made it. But we're inclined to believe him when he says he hasn't, and even if he thinks he has, his mind might change because of events that have not yet occurred. Whatever, there is something cheerfully humbling about the fact that the most expert of Washington insiders today knows no more about the answer than any reasonably well-informed person elsewhere in America, and far less than the least well-informed person will know not too many months from now. In the meantime, surely it makes more sense to concentrate on how the president is doing his job.