For his commencement address at Notre Dame University the other day, President Reagan abandoned plans for a big foreign policy speech in favor of reminiscences on the Gipper and conventional reflections on the future awaiting graduates. All but lost in his remarks, however, was a remarkable statement on foreign policy:

"The West will not contain communism," the president declared. "It will transcend communism. We will not bother to denounce it. We'll dismiss it as a sad, bizarre chapter in human history whose last pages are even now being written."

Not contain communism? Not denounce it? What way is that to talk to impressionable young people at a time when the U.S. government is embarking on a mammoth military buildup? How do you square those "last pages" of a "sad" chapter with the administration's portrayal of the Soviet Union as an irrepressibly expansionist and militarily superior force, dangerously inciting terrorism and exploiting revolution on a global scale?

The apparent contradiction presents no problems to Secretary of State Alexander Haig. At a Syracuse University commencement, he spoke of the Soviet system's "spiritual exhaustion," of Moscow's "unenviable present and an extremely gloomy future." He listed "formidable problems . . . ranging from the hostility of China to the difficult Polish situation, from economic failures to ideological sterility."

This, incidentally, is no new line for him. At his confirmation hearings he called the Marxist-Leninist system a "profound historic failure" and an "agricultural basket case."

But these weaknesses, he explained at Syracuse, could be bad news: "A state as powerful and ambitious as the Soviet Union may be more dangerous because of its weaknesses. . . . That is why the first task of American leadership and the Atlantic alliance is to establish new restraints on Soviet behavior."

That is a perfectly respectable argument, shared by a good many Soviet experts outside of government, as well as within. In almost any gathering of Kremlin-watchers nowadays you hear increasing talk of a historic turn in the Soviet Union's internal fortunes, of the first, faint signs of a "cracking up" of the empire. The reform movement in Poland is only Exhibit A.

Many of the experts also share the administration view, as Defense Secretary Caspar Weinberger explained it to NATO defense planners in Brussels, that the "current and prospective leaders of the Soviet Union may be impelled by lack of success inother fields to turn instead to the one field where they have both confidence and capability: stark military power and military threats."

This is a hard proposition, however valid, to put across both at home and abroad to this country's allies. Here, as well as in Europe and Japan, there is a natural impulse to treat the "bad" news of Soviet weakness and failure as good news -- and as good reason for not having to shoulder the increased burden of defense spending that the Reagan administration's analysis of the Soviet threat would require.

Administration officials themselves admit that their weakness-is-strength, good-is-bad assessment of the Soviets is not only hard to sell but harder still to translate into a comprehensive strategy. "We're working on it, with interdepartmental committees, but it probably won't crystalize until later this year," says one, adding: "The crystalizing is reflected in what Haig and the president have been saying in the past few days."

All this is by way of saying that while the administration had laid down reasonably clear lines of action in terms of initiating a big defense buildup and identifying what it sees as the critical flash points and trouble spots (the Persian Gulf, El Salvador), it is as yet without settled answers to questions of general policy.

Questions such as what precise combination of carrot and stick; how fast a pace on arms control as an offset to missile deployment; how permissive an approach to the European yearning for detente, expressed in free and flourishing East-West trade; how big a Persian Gulf presence and how much sharing of that burden to demand of the Europeans; how much encouragement to "devolutionary" forces in Poland and elsewhere in the Soviet bloc at the risk of a Soviet crackdown?

How, in short, to exploit perceived Soviet weaknesses, while restraining and containing Soviet strengths?

"The test of a first-rate intelligence," F. Scott Fitzgerald once wrote, "is the ability to hold two opposed ideas in the mind at the same time, and still retain the ability to function." Given its assessment of America's foremost adversary, that capability is likely to be the test of an effective Reagan administration foreign policy as well.