Mikhail Gorbachev had the sort of audiences a politician once defined as ideal: intelligent, well educated and slightly drunk. Drunk, that is, on de'tente.

This December is springtime for de'tente, the catalyst of which is arms control. Concerning that, consider this paradox: if the Soviet Union has suddenly abandoned its aspiration for world hegemony -- if the regime has discarded the Leninist basis of its legitimacy -- arms control is not urgent. If the aspiration persists, the Soviets seek arms agreements as its instrument.

President Reagan says that although Soviet leaders have repeatedly seen ''the future in a one-world communist state,'' Gorbachev ''has never made that claim, but is willing to say that he's prepared to live with other philosophies in other countries.'' ''Prepared''? Of course Gorbachev will live with pluralisms that are -- so far -- beyond his power.

Stalin, Khrushchev and Brezhnev all espoused forms of ''peaceful coexistence.'' It was a facet of building ''socialism in one country''; it is a necessity for a nation condemned by its economic irrationality to parasitism off Western capital and technology; it expresses the fact that in the nuclear age the climactic spasm of East-West competition should not be war.

Gorbachev says Lenin never even thought like a Leninist, never even thought of ''imposing communism throughout the world.'' One wonders, then, what Lenin was thinking when he wrote: ''As long as capitalism and socialism exist, we cannot live in peace; in the end, one or the other will triumph -- a funeral dirge will be sung over the Soviet republic or over world capitalism.'' And: ''As soon as we are strong enough to defeat capitalism as a whole, we shall take it by the scruff of the neck.''

Scholars have expressed in various ways the importance of Soviet ideology. Leonard Shapiro said Leninism was not a blueprint but a compass. Alain Besancon says communism is not the instrument of the Soviet state; rather, the state is the instrument of communism. In Robert Conquest's formulation, the problem is not that Gorbachev reads Lenin nightly, any more than Richard Coeur de Lion went around reciting the Athanasian Creed. Rather, the problem is that the Soviet elite are ''men whose attachment to the Leninist attitudes is part of their whole personality rather than a matter of the 'opinions' they hold.''

De'tentists say that privation, stagnation, tyranny and irrationality -- afflictions of the Soviet masses for 70 years -- suddenly are intolerable to the ruling elite. That change, say de'tentists, either is because of, or is the cause of, Gorbachev. Reagan seems to think Gorbachev is something that Marxists say is impossible -- a man who wrenches history onto a new path.

But the Soviet regime is not the radical evil Reagan has said it is if it can be transformed by the coming to power of a particular individual.

A defining trait of a totalitarian regime is the ability continuously to replicate the ruling elite. It does so through a thorough socialization process that prevents anyone unreconciled to the regime's ethos from approaching the pinnacle of power. The pool of candidates from which the Politburo rises is composed of character types shaped by the regime and suited to its continuation.

With American sweetness, Reagan says Gorbachev ''sincerely believes'' what he says about our ''shortcomings'' and would be mellowed if he could ''see what there is to see in this country.'' See Gorbachev. See him see our supermarkets. See Spot run.

But in Leninist categories, America's ''shortcoming'' -- what a word! -- is that it is doomed by inexorable laws of history to a losing conflict with socialism. Leninists do not ''believe'' this, they know it. It is a scientific matter.

When talking about human rights, Reagan talks, as Americans are wont to do, like a schoolmarm. There is teaching to be done. He says ''we've got to make them see'' that the Helsinki undertakings should be fulfilled. And ''maybe we could make them see'' that expanded freedom would decrease desires to emigrate. And ''maybe we could help'' Gorbachev understand that emigration for religious reasons would decrease ''if they simply would repeal the restrictions'' on religion. ''Simply''?

Gorbachev's warm reception by Americans was preceded by his NBC appearance, during which he spoke in the old clanging categories of Leninism about human rights, the Berlin Wall, Afghanistan and other matters. After that, Reagan said arms control would not be contingent on any improvement in Soviet behavior anywhere. So the lesson of this summit is that the Soviets can have de'tente whenever it suits them, cost-free, not even paying a rhetorical price.