Ronald Reagan made a very generous gesture the other day. He promised that if elected president he would have the White House physician examine him periodically, and that he would resign the office if serious evidence of senility or mental deterioration were detected.

This is the first time any presidential candidate has made such an offer, and Mr. Reagan should be commended for it.

The problem as I see it is: How would any doctor know?

I asked a psychiatrist friend about this and he admitted it would certainly be a hard thing to prove.

"First of all," he said, "you can make a case for the fact that anyone who wants to be president of the United States does have mental problems. A person who thinks he can solve the nation's woes and deal with all the other crazy leaders in the world could easily be certified and put away if he weren't running for the highest office in the land."

"I never thought of that," I admitted. "We do lock up people for a lot less."

"Secondly," he continued, "when a man becomes president he does things that no normal person would do. If Lyndon Johnson hadn't been president, there would have been some questions as to whether he had all his marbles. I cite his strange behavoir of pulling a dog up by its ears, showing everyone his scar, and driving across the ranch in a jeep scaring the hell out of his cattle, not to mention putting 500,000 men in Vietnam, as things a psychiatrist might consider abnormal behavior."

"What you're saying, if I hear you correctly, is that what could be construed as abnormal for someone in your family is accepted as normal behavior once the person becomes president."

"That's correct. The same rules concerning senility and mental deterioration do not apply when someone lives in the White House, as opposed to a nursing home."

"If Nixon had agreed to do what Reagan has promised to do, is it possible that his White House doctors would have asked him to step down?" I asked.

"It's hard to say. We know Nixon talked to the paintings in the White House, and he was terribly paranoid about people who opposed him to the extent of making up an enemies' list. And you might wonder about a person who wanted everything he said put on tapes, and who then decided not to destroy them because it would prove he was innocent. But I'm not certain that the White House doctor would have said to him, 'Mr. President, please put on this white jacket. We're going for a ride in the country.'"

"Jerry Ford was sane, wasn't he?"

"It depends. Medically, I would say yes. But after he pardoned Nixon, everyone said he was crazy."

"How about President Carter?"

"We psychiatrists are very suspicious of anyone who says to the American people, 'I'll never lie to you.' And any person who keeps telling you that the worst of the recession is over should be watched very carefully. But I wouldn't recommend hospitalization yet."

"Then you think Reagan's promise to step down if his doctors believe he is senile or deteriorating mentally is pure hyperbole?"

"Frankly, the fact that he even mentioned it bothers me. It puts ideas into people's heads. As soon as he makes his first State of the Union speech and advocates tax cuts and at the same time demands a balanced budget, everyone's going to say 'Okay, Ronnie, it's time for the funny farm.'"