An old man with a one-track mind should not try to write a column about Iranian activists while half his brain is concentrated on a VDT keyboard and the other half is on a television set tuned to a political convention.

The half of me that is in Washington is reading letters from people who are angry about Iranian "students" who engage in disorderly demonstrations.

"Why don't we investigate them and send them home?" one woman asks. It is a question echoed in other letters, and it brings to mind the frontier justice that was meted out to suspected horse thieves a century ago. Vigilantes and members of a sheriff's posse were alwaysstrong advocates of fair play and justice. Their policy was, "Give the suspect a fair trial before you hang him."

If we are going to assume that everybody who is arrested is guilty, we're going to hang some innocent people.

In the present case, the assumption that all the arrested Iranians are guilty could lead to the deportation of one or more innocent students who really spend most of their time attending classes and studying. Perhaps they just happened to attend a demonstration out of curiosity, and when a fight broke out, the cops grabbed everybody who looked like he might be an Iranian, It's possible that a few genuine students suffered that fate.

But it seems safe to assume that mostof those who are being arrested for violence against their political enemies or against American policemen are really activists, not students. They are here to foment trouble, not to study dentistry.

Therefore it seems reasonable and produent to investigate them and find out what the truth is.

We should check official records to ascertain how many classes they have attended and how many they have skipped.We should ask school officials whether each alleged student is maintianing an adequate level of scholastic achievement. We should find out if they are breaking our laws.

Without prejudging them, we should investigate, and then evaluate what we learn in each individual case.

Those who have given us cause to deport them should be deported. Those who have not broken our laws should be permitted to stay.

Superficially, my proposals may seem reasonable enough. But if you agree with them, I must warn you that we may both be wrong. The deportation of several hundred Iranian activists from the United States might touch off heightened hostility among the volatile men who now control Seventh Century Iran. As a consequence, the Americans still being held captive in that country might pay a bitter price for ourshowing our displeasure with foreign troublemakers.

Only the president and his top advisers have the secret intelligence reports upon which policy judgments must be based. As a group, they have the long years of experience needed to evaluate such information. And they have the best "feel" for the situation because they have been dealing with its twists and turns since the day our diplomats were taken captive.

You and I are in the position of backseat drivers. Our hands are not on the wheel, our feet are on neither brakes nor accelerators. It is not our responsibility to react instantaneously to emergencies; it is not in our power to alter the course of events. We're not in the driver's seat. When there is need for an immediate decision, the pressure is on somebody else, not us.

When the emergency is over, we can give the driver the benefit of our hindsight.We can lecture him about what he should have done, or what we would have done had the steering wheel been in our hands. A back-seat driver is never wrong because he has the advantage of letting somebody else make the initial decision. If it turns out well, the back-seat driver can comment disdainfully that a child have handled the matter. If it turns out badly, he can proclaim grandly that his decision would have been far wiser.

Mature people realize that most back-seat driving, even their own, is just hot air. Nevertheless, the TV screen that has been distracting me tonight has also been bringing a messagefrom the back seat to my brain.

People who would like to be presient some day have nattering in the back seat all night about what the president should have done, and what they would have done if they had been atthe steering wheel.

Alas! Political back-seat driving is just as obnoxious and just as irritating as any other kind.

When you're an "out" trying to get in, it's easy to say that if you were president you'd spend 16 billion, 986 million, 347 thousand, 522 dollars and 17 1/2 cents to create a job for every unemployed American.

But when you're the president, you must say candidly, "I cannot make that kind of precise committment. I can suggest a program, but Congress can overrule me. I can suggest a course, but when it comes to money, Congress does the driving."