''The overriding concern is to avoid nuclear war.''
MIT Professor Henry Kendall speaks softly, earnestly. As head of the Union of Concerned Scientists (UCS), he says he deplores the current arms race with the Soviet Union and calls for a nuclear freeze.
From the other side of the Charles River, Boston University President John R. Silber minces no words in response. ''The nuclear freeze,'' he says, ''as it will work in effect, freezes such an imbalance (between the US and the USSR in conventional and nuclear arms) as to amount . . . to unilateral disarmament.''
Between them lies one of the major issues of our time: how to move from the threat of nuclear war to the assurance of international peace.
Like few others, this issue has seized the public imagination. On Thursday the UCS helped organize Veterans' Day ''teach-ins'' at some 500 university and college campuses nationwide - up from 150 teach-ins last year, when the teach-ins began.
And in last week's national elections, freeze referendums passed in 12 counties, 22 cities, and in eight of nine states. On the surface, the language of the referendum questions was generalized. They typically called for the federal government to pursue negotiations with the Soviet Union on a ''mutual nuclear weapons moratorium and reduction, with appropriate verification'' - language with which few thoughtful voters could take issue.
But look deeper. In fact, voters were choosing between two positions so different as to constitute entirely different ways of looking at the world:
* Should the two superpowers (as Prof. Kendall advocates) freeze all testing, deployment, and production of nuclear weapons at current levels, as a first step toward arms reductions that would make war less probable and peace more lasting?
* Or should America (as Dr. Silber insists) upgrade its aging weapons systems - making war less probable by maintaining a daunting defense?
Like most public topics, this one can be argued by two broadly different methods: measuring statistics, or assessing attitudes.
The world is awash with armaments statistics in general. And in nuclearm arms, bean-counters are having a field day. The problems come in part from secrecy: Each side, depending largely on spies and satellite surveillance to assess the other's strengths, worries the counts may not be accurate.
Another problem lies in the difficulty of finding similar things to compare. Case in point: Do the 989 missiles in 83 Soviet submarines constitute a weapons system so entirely different from the 520 missiles in 32 US submarines as to be beyond comparison? Numbers alone suggest that the US is outgunned. But if (as UCS specialists say) the US keeps 60 percent of its submarine fleet continually on patrol, while the Soviet Union can only manage to keep 10 percent active at any one time, the picture changes. Unless, that is, you agree with Dr. Silber that the greater ''throw weight'' of Soviet missiles allows them to deliver far more punch per weapon, in which case the picture shifts back.
So broad are the disagreements, in fact, that each side uses the numbers to prove its case. Prof. Kendall's just-released book from Beacon Press (''Beyond the Freeze: The Road to Nuclear Sanity,'' coauthored with Daniel Ford and Steven Nadis) tries to tote up the balance. Using ''Hiroshima-bomb equivalents'' - the explosive force of the atomic bombs dropped on Japan in 1944 - the authors find that the US has 276,000, the USSR 362,000. Their conclusion, however, sidesteps the imbalance and focuses on the overkill. So overwhelming are the arsenals already, they write, that both nations would ''run out of cities and military bases worth destroying long before they ran out of bombs.''
Dr. Silber disagrees. ''The parity has simply been lost,'' he said in an interview earlier this week in his elegant office in Boston's Back Bay.With materials from the highly regarded International Institute of Strategic Studies in London at his elbow, he noted that the Titan and Minuteman III missiles are ''obsolete,'' that the Soviets have 200 to 300 SS-20 missiles in central Europe (while the US has not yet deployed the Pershing II to counter that threat), and that America's B-52 bombers are ''older by almost 10 years than the pilots who fly them.''
Statistically, then, the question is whether a freeze would ensure balance or institutionalize Soviet supremacy. And that depends on whose count you take. The UCS figures may be oversimplified: Their book purports to give ''straightforward'' accounting of what is in fact an extraordinarily complex power balance. But Dr. Silber's assessments may tilt in a different direction: many of them arise from a US military establishment that could be downplaying its present weapons in order to win future funding increases.
So the numbers are controversial. What about attitudes? Here, the arguments fall into three broad areas: pacifism, intention, and verification.
* Pacifism, which literally refers to any peace-seeking movement or ideology, has come more narrowly to suggest unilateral disarmament and a renunciation of all warlike postures. While each of these academicians wants peace, neither is a pacifist - although they do not always keep the two terms separate.
Prof. Kendall, in fact, told a press conference last week that the UCS is not a ''peace'' group. He frowns on the ''ban-the-bomb, disarmament, universal peace'' movement: That kind of disarmament, he says, is neither wise, safe, nor politically achievable.
Dr. Silber says it even more forcibly. ''The peace movement is the method that the Soviet Union uses to keep the United States asleep,'' he says. He points to Soviet support for recent peace movements in Denmark and West Germany.
With characteristic assertiveness, he notes that ''every peace movement in modern times has led to war.'' What evidence does he cite? The 1922 Washington Naval Treaty and the Kellogg-Briand Treaty of 1928.
* Pacifism might work among neighbors intent on keeping peace. But what is the Soviet intent? To Dr. Silber, intention is discovered by looking at history, not at promises. And the Soviet record, he says, is that of ''the most aggressive, totalitarian, imperialistic power in the history of the world.'' But would they use nuclear weapons? Yes, he says, if their plans for civil defense and their internal military publications on nuclear war strategies are to be believed.
To freeze proponents, two bad intentions do not a good one make.
Prof. Kendall faults the Reagan administration for echoing Soviet logic by suggesting that nuclear war could be ''winnable'' - rather than insisting that nuclear war should be avoided at all costs.
* A treaty among parties who mistrust each other's intentions requires verification. On that point, the UCS book apparently dismisses the need for verification, arguing that the size of both arsenals is so great that ''even major violations'' would be of ''minor consequence militarily.'' Later, however, it asserts that verification could easily be accomplished by satellite and seismic instruments.
Again, Dr. Silber disagrees. He insists that a few days of heavy cloud cover could obscure satellite pictures, and that even heat-sensitive observation instruments could be confused by decoys. He also faults the Soviet record, claiming that treaty provisions banning antiballistic missile systems (widely regarded among arms controllers as one of the real successes of the SALT agreements) have been violated by the Soviets.
Finally, he notes that ''if verification proposals by the Soviet Union were authentic, they would include on-site inspection.''