It is an uncommon achievement for any man to know who he is. Ronald Reagan knows who he is, and this is perhaps his most compelling qualification for the presidency.

In the 16 years I have been involved in presidential politics, I have known over 50 men whose accomplishments made it reasonable for them to at least think of running for president. You would think that men who stood within reach of the most powerful office in the world would be marked by a confidence and self-assurance that could only result from self-knowledge. Nothing could be further from the truth.

There is something about the presidency, or more likely the pursuit of it, that consumes men, destroys their perspective and leaves them less well off for the experience of brushing up against it. In a strange way, a man must already have survived some experience that was more important to him than his presidential ambition in order to achieve the inner strength necessary to make a run for it. You've got to be able to take it or leave it alone; part of you has got to honestly not want it; now and then you've got to hope you lose; then you might be a good president.

Harry Truman was a failure for the first 50 years of his life; it never bothered him that he might fail as president. He's already failed; he knew what it was like. He wasn't afraid of it.

Franklin Roosevelt lost the use of his legs to a child's disease. He crawled on his belly until he could hoist himself up on braces. The presidency never had a chance of beating him; he dominated it. He had it licked every time he got up on his feet.

Abraham Lincoln never even had a proper education. Every time a ray of happiness seemed ready to shine on him, he'd lose an election, fall into debt or someone close to him would die. The politicians laughed at him. But by his essential patience and devotion, by the very manner of his life and even of his death, there was no greater human being than Lincoln. John Wilkes Booth didn't assassinate Lincoln; he was only the instrument. It was the presidency that conspired to have its revenge, for Lincoln, this country bumpkin who had known such misery, bent the presidency entirely to his own persuasion. We don't really remember President Lincoln; we remember Lincoln; Lincoln knew who he was. The presidency wasn't big enough for him.

I remember President Eisenhower saying to me that nothing he did in the eight years he was president was as difficult as giving the final command to launch the D-Day invasion. The weather was choppy in the English Channel and there was some suspicion that German intelligence had discovered the location of the invasion beaches. Thousands of men would give their lives, so a delay would have been justified if better weather or one more review of the plan could have improved the chances of success. In listening to Eisenhower, it was easy to see that the presidency was an afterthought. He was Dwight D. Eisenhower, general of the Army. Yet during his presidency, this country came close to nuclear war three times, troops were sent to a Little Rock, Ark., school in support of integration and a policy of brinkmanship was practiced with the Soviet Union. Small things to a man who had given the order to launch the D-Day invasion.

There is a fallacious attitude that in some way men who are not really capable of being president suddenly grow in stature once the job is theirs. It's simply not true. The great presidents were great men to begin with; we just didn't notice until the spotlight was turned on. But the real reason for their greatness was not that they were brilliant, not that they possessed some genius; we would have noticed those things. Their greatness was in the simplicity that they knew themselves well from having been scared, from having failed, from having suffered despair. The men who never lose at life aren't the great ones; they're just lucky. They never find out who they are. The great men are those who lose and come back from it.

It is not my purpose here to award Ronald Reagan equal standing with the likes of Truman, Roosevelt, Lincoln or even Eisenhower. Reagan has never sent men into battle or crawled on his belly or failed at business. But at least, as of a few months ago, he still knew who he was. He doesn't really enjoy talking about Hollywood or the Screen Actors Guild. He's proud of having been an actor and he's proud of having been the president of a union. In a way, the very fact that he may always be more proud of those things than of being president of the United States gives him a fighting chance to be a good president.

What I am saying is that Ronald Reagan has kept his personal identity. In a curious way, the impression Reagan has created with the press and part of the public of being almost fixated on his time in office as governor of California is misleading. He refers to the Sacramento years repeatedly in public speeches to justify this or that position he now takes or to demonstrate that he has had a broad range of experience -- and success -- in handling "the issues." But this is not his real or instinctive frame of reference, as people who have been close to him know. Again and again in private, spontaneous conversation and musing. Reagan harks back to and draws on his life in the movie industry and as a union official and a politician of the Hollywood years. It is his real identity, the self he has neither lost nor abandoned and from which he truly speaks as a person.

Today is the beginning of Ronald Reagan week in the United States. Before the week is over, Ronald Reagan, son of a shoe salesman from Tampico, Ill., will become one of three men who might be president of the United States. That's an awesome thing to comtemplate for a guy who would have settled for broadcasting the Cubs games 45 years ago. But he mustn't let anyone fool him; he isn't the president yet, and if he wins the election he still won't be the president. He is Ronald Reagan, the actor, the union president. He can't be sensitive about it, he must be proud of it. Bankrupt haberdashers have held that job and done a hell of a lot more with it than have most of the lawyers.

Reagan can easily lose his perspective from here on out. Close associates will encourage him to act differently, he will be treated more deferentially, close friends won't be critical, he'll hear mainly good news. These are the ways the job gets a hold of you. If Reagan needs any proof of the need to remember who he is, he need only look at Jimmy Carter. Carter has no idea who he is; I don't think he has ever known.