If arguments acquired cogency from vehemence, the case for sanctions against South Africa would be made. Vehemence abounds, as does right-mindedness -- the belief that morality consists of voicing certain sentiments.

In the name of capturing ''the high moral ground'' (that real estate was last in the news when Mondale said we lost it by liberating Grenada), moralists are demanding denial of landing rights for South Africa's airline. That may be done, just as soon as we finish reestablishing landing rights for the Soviet airlines.

It will cost the University of California $118 million in commissions and other administrative costs to divest itself of holdings in companies that do business in South Africa.

Political hygienists in the British Commonwealth have pronounced themselves too fastidious to participate in the Commonwealth Games with the British because Margaret Thatcher opposes sanctions. These hygienists include Zimbabwe's president (the state of emergency begun under white rule continues under Mugabe; see ''Torture in Zimbabwe'' by Amnesty International) and the current reincarnation of Ghana's popular will, Flight Lt. Jerry Rawlings, and whoever is now supreme in Uganda.

Ethiopia's communist regime has engineered a famine that kills more people every day than die in violence (much of it black against black) in South Africa in months. People up there on the high moral ground calling for sanctions should study a new word: ''Ethiopianization.'' That may be the fate awaiting 80 million black Africans in the six southern African nations whose economies -- in some cases, barely subsistence economies -- are dependent on South Africa's economy.

These nations depend substantially on South Africa for transportation, electricity and jobs for itinerant workers. Those nations will be forced to collaborate with South Africa in defeating sanctions (by transshipping material, and other assistance) or face ''Ethiopianization.''

Today 1.2 million black workers from the north are in South Africa illegally and 350,000 are there legally. The money they send home to their families in their much poorer countries is crucial to those countries. The flow of money will slow if South Africa's economy slows and will stop if sanctions cripple South Africa's economy so much that these workers are expelled.

''Which side are we on?'' asks The New York Times, assuming there are only two sides, that the choice is obvious, and that the Reagan administration has chosen immorally. The Times is thrice wrong. Obviously America is not ''on the side'' of white oppressors, having condemned apartheid as immoral and called for release of Nelson Mandela, its foremost opponent. What is ''the'' side of the blacks? The leader of the largest black group, the Zulus, opposes the sanctions The Times supports.

It is not in the U.S. interest to contribute to the creation of a Lebanon -- a war of all against all -- in South Africa. American interests include pluralism and prosperity in southern Africa, neither of which will be advanced by deepening South Africa's isolation.

The social and cultural as well as economic dynamism that accompanies capitalism is the surest solvent of superstitions and irrationalities like apartheid. So South Africa needs more of what sanctions would diminish. It needs foreign capital operating under rules of racial justice written in the nations from which the capital comes.

And South Africa also needs something the London Times advocates, something incompatible with a policy of isolation. It needs a Marshall Plan targeted at educating, housing, training and capitalizing blacks.

The pounding of South Africa by moralists at a safe distance is not softening the regime. Support for the Botha regime from English-speaking South Africans, the more liberal element, has recently risen sharply. Even if all the ''English'' whites emigrated, there would be 3 million Afrikaners armed with the products of a domestic arms industry that grew in response to Western sanctions -- an arms embargo. The industry produces fine aircraft, tanks, howitzers, projectiles.

Will the regime use these? The example of the shah, Somoza, Duvalier and Marcos may suggest that oppressive regimes -- at least noncommunist ones -- melt away rather than use military force against their populations in order to hold power. But the well-armed Afrikaners are descendants of the ferocious Boers who fought the British Empire at the peak of its power.

They will fight, if it comes to full civil war. They probably will lose, but not before a million blacks have lost their lives. Will the West, having disengaged, have clean hands?

In the years before the American Civil War, ''abolitionists'' wanted slavery abolished. But that could not be done immediately, so some of them favored abolishing the Union lest they be sullied by further association with slave states. That would not have helped the slaves, but helping the slaves was not their primary concern. A sense of purity -- right-mindedness -- was.