BOSTON -- The two men have now come down from the summit. For three days we lived off the heady air of their high-altitude politics. But such an atmosphere can dissipate as quickly as helium from a balloon.

With pomp and circumstance, Reagan and Gorbachev cut 2,000 weapons from the nuclear arsenal. But on the plains of reality, we still live with 48,000 more, acres full of redundant annihilators. Even if the START agreements succeed, we will have more than 30,000 nuclear weapons.

The real high of the summit didn't come from these modest cuts. It came rather from the sudden openness to new possibilities, the glasnost to change.

Suddenly, a whole category of weapons is to be removed. Suddenly, we can go into each other's forbidden zones to verify. Suddenly, the terms of what we ''need'' for national security are rewritten. Assumptions are shifting, and questions that were considered hopelessly naive are again forming on our lips.

At the end of the line of these questions are perhaps the only ones worth asking. Might it be possible, finally, to eliminate war? Or is there a heart of darkness in human beings that makes war inevitable and all our efforts futile?

These are not questions arms controllers like to ask. Serious people are supposed to crunch numbers; wondering is left for philosophers. Yet our beliefs about the nature and behavior of human beings inform our attitudes about arms policy.

Nuclear weapons have already altered the nature of war. For most of history, great powers fought to become top dog, or top turtle in the imagery of Dr. Seuss. That sort of war is no longer a rational option. It is a war without winners.

A series of "smaller" wars has gone on under the nuclear umbrella. Korea, Vietnam, Iran-Iraq, Afghanistan. But as MIT's Kosta Tsipis points out, not one has ended in victory as it was once defined. These, too, have been wars without winners.

Now some say a reduction of nuclear weapons would only ''make the world safe for conventional war'' between superpowers. If indeed we build up conventional forces, we could fall into a disaster that mimics the opening of World War I. But our knowledge of a nuclear ending makes a deliberate move incredible.

Nuclear weapons have created a new concept of limited war. Brutal and without victory. And perhaps even archaic.

Randall Forsberg, a hard-headed idealist who founded the Freeze movement and now heads a disarmament think tank, shares a hopeful belief that ''nuclear weapons are teaching us how not to be violent. We have learned that you don't use everything you have. You choose not to. It's only in the nuclear era we have begun to practice this, not just in terms of nuclear weapons but in conventional war.''

Forsberg makes a controversial but intriguing case that violence -- both interpersonal and cultural -- is slowly diminishing. ''The ability to be violent exists in all of us,'' she cautions, but it isn't uncontrollable, inevitable. In human relations we now regard violence as aberrant behavior. We may eventually regard violence between nations as equally aberrant.

''I think this society has developed a dual standard in this half century,'' she says. ''We have a very strong ethic that there's only one situation in which it's legitimate to use violence. If you're defending yourself against aggression. That works at every level: for me personally, for society at large, for the international system.

''The other standard is that might makes right, Realpolitik, power politics backed up by force. It exists, but it's become archaic.''

It's not a coincidence that, after World War II, after Hiroshima and Nagasaki, we changed the name ''War Department'' to ''Defense Department.'' The next question is whether in concert with the Soviets we can reduce our arms -- nuclear and conventional -- to a truly defensive posture.

Nuclear arms symbolize our inability to end war. We don't know how. We doubt human nature, worry about that heart of darkness. We have to scare each other out of war. Yet it's also possible that we have begun a long, uncertain, fragile and not at all inevitable process of ending war.

In 1946, Albert Einstein said that the atom bomb ''has changed everything except our way of thinking.'' The best legacy of the summit of 1987 is not in the provisions of this modest treaty, but in the hope it raises that we may, at last, be changing our way of thinking.