Visitors to Moscow say a theme there is that President Reagan is a manipulated cipher within his own government. Some members of his government probably think so, too, and hope to prove the point by stuffing their agenda into any communiqu,e issued jointly by the two sides at the conclusion of the summit.

Such a communiqu,e is utterly optional and obviously unwise. There was none at the conclusion of the Gorbachev-Mitterrand summit. Francois Mitterrand successfully avoided having an "arms control summit." He constantly raised issues of human rights, including lists of specific cases. This moral tone and concreteness prevented the antiseptic and anesthetizing arcana of arms control from producing numbness, that absence of feeling that U.S. diplomats confuse with 'an era of good feeling."

Mitterrand understands that commumocratic and totalitarian leaders must be vapid, to the totalitarians' advantage. Such communiqu,es are tissues of muzzy formulations falsely suggesting harmony and moral symmetry.

No summit is short enough, but all are short. Divide the time (in Geneva, two days) in half to allow for meticulous translations. Then subtract time for stilted niceties. Obviously summits must be primarily ratifying occasions, unveiling work done elsewhere. Now, imagine the draining and flattening of language being done today by U.S. diplomats toiling to reinvent d,etente in a concluding communiqu,e designed to eviscerate Reaganism.

Diplomatic boilerplate often proves that even the platitudinous can be injurious. If a joint communiqu,e asserts, contrary to reason and history, that tensions yield only to dialogue, it trivializes this century's great division between freedom and its armed enemies. What is needed is not more dialogue but less Soviet misbehavior.

Larry Speakes, reflecting an inanity loose in the administration, says Reagan's first job is to convince the Soviets that "we don't plan to run over them." Oh? Does Soviet aggression flow from insecurity, which U.S. policy has caused and now must assuage?

Add to that loopy idea the State Department idea that "frictionless" is a synonym for "good" in the phrase "good U.S.-Soviet relations." Then add Reagan's craving for a "fresh start" in relations. Stir well and you will get a communiqu,e designed to blur edges, bleach colors and put both sides on the same moral footing.

This is a recipe for diplomatic junk food, for mental cholesterol that clogs intellectual arteries with absurd propositions, jointly affirmed, such as this: both sides equally value adherence to agreements.

That is rubbish, and if Reagan returns to Washington having endorsed rubbish, he will have negated his recent U.N. speech, in which he denounced "numerous" Soviet violations of "all" agreements of the 1970s. He also will have repealed his five-year record of objections to Soviet violations of arms agreements and the Helsinki Accords concerning human rights.

Speaking of arms agreements, how would a joint communiqu,e deal with Soviet violations of the pact forbidding use of the sort of chemical weapons the Soviets are using in Afghanistan -- the sort the Soviets' Vietnamese allies are using in Indochina? The Soviets' preference is for Goebbels-like denials. They constantly call for a "comprehensive ban" on the kind of crimes they commit. The State Department wants to be agreeable ("frictionless") and adores agreements, so it probably is lobbying for communiqu,e language that treats both sides as equally committed to banning such weapons.

Speaking of human rights, the Soviets may soon perfume the Geneva atmosphere by settlinga trivial number of human rights cases. So imagine a joint communiqu,e pledging both sides, as moral equals, to work to resolve human rights problems without "interfering" in the "internal affairs" of one another. That would mean the Helsinki Accords on human rights do not apply to the internal affairs of any country. They protect human rights in . . . Antarctica?

How about a paragraph painting both sides as equally horrified victims of terrorism. Never mind that the Soviet side is the foremost funder and organizer of terrorism.

Finally, a "fresh start" requires tossing some murders down the memory hole. A pledge of mutual efforts for improved air safety in the North Pacific would obliquely communicate the lie that the massacre of Korean Airlines Flight 007 was the result of a procedural flaw, not Soviet brutality. Another pledge for "both sides" to adhere to clear rules in Germany would intimate that the murder of U.S. Army Maj. Arthur D. Nicholson, who slowly bled to death, was the result of another misunderstanding.

Were Reagan to leave Geneva enveloped in a cloud of such verbiage, he would demoralize those who for 20 years have taken his quite different words seriously. And he would be disdained by those who would have successfully manipulated him. Such Geneva words would retroactively discount his public life, and would confirm Moscow's current theme.

Communiques issued jointly by democrats and totalitarians generate myths of moral equivalence. In Geneva, Reaganism requires reticence.