MY SON TALKS a great deal about his time machine. He will make it when he grows up, he says, and then he will take it back in history to see how things really happened. He wants to see, for instance, what Christ looked like and what ancient Rome was really like and he has more than a passing interest in the cowboy and Indian era.

When he makes the machine I want to hitch a ride and get off at the time and place where the Cold War started.

I'm not sure quite where that is. It might be Potsdam where Truman toyed with Stalin, hinting we had a terrible new weapon we could use on the Japanese and, by implication, on anyone else. It might be the Greek civil war, or maybe the Russian threat to Iran. Or was it in Fulton, Mo., and the Winston Churchill speech with the throw away line about an "iron curtain"? Maybe it was the terrible winter of 1946-47, the one when a weary Britain almost collapsed, passing the burden of leadership to the United States.

I don't know. It's always hard to tell when one era ends and another begins. Now one is ending and it was called Detente and another is beginning and it is called the New Cold War of the End of the Vietnam Era or something like that. I have a name for it. Maybe we should call it The Time When Thinking Stopped.

Somehow things that were once perceived complex are suddenly perceived as simple. Soviet-U.S. relations are now a matter of bad guys and good guys. And the Russian people, just recently perceived as human beings with about as much interest as we have in avoiding Armageddon, are not cardboard characters out of some primitive propaganda film. It is all too simple. It is all too neat and all too silly.

But it is happening anyway. Some politicians are coming up with global strategies and others are saying we ought to do things like not sell the Russians anything. In the Congress, politicans can't wait to throw out the Watergate-era reforms that applied to the CIA and everyone is talking big bucks for defense.

We have come to talk in terms of good and bad, right and wrong, forgetting that these are not easy qualities to define. What is wrong to us is right to them and it would be good every once in a while to wonder how it looks to them.

This is why I wonder about the Cold War and its origins. Everything seemed so clear back then, too. The people back in the Cold War era had the advantage of a monster in Stalin, one of the great killers of all time, who personified the evil that was thought to be the Russians. What it usually came down to in those days was that we were right and they were wrong and that, in case you had the nerve to ask, was because they were bad and we were good.

It does not tilt the scales of justice to say that we sometimes overreacted, that we thought, for instance, that we had a God-given right to string air bases on the perimeter of the Soviet Union and that they had some sort of obligation to be gracious about it.

We thought we could talk about "liberating" Eastern Europe and "rolling back" the Russians -- standard terms in the lexicon of John Foster Dulles -- and that the Russians would somehow understand that (1) we really didn't mean it and, (2) we were waging a political campaign and not, as they now say, the moral equivalent of war.

So now when it is happening once again, when the world is suddenly growing colder and darker, it once again seems lopsided and it very well may be. But there are mitigating circumstances -- the plans to put new NATO missiles on German soil, for instance, and the whittling away of SALT in the Senate.

None of these are enough to account for the invasion of Afghanistan and nothing, really, excuses it.But what would really be inexcusable would be if we stopped looking for signs from them, if we gave up entirely on detente, if we hunkered down into our old "they are bad and we are good" attitude and turned down the lights of the world more than they need be.

After all, some day some kid might take a time machine and look at what we have done. He might have to conclude that a very funny thing happened. The Soviets invaded Afghanistan and the Americans stopped thinking.