Americans are used to having policies. A policy is an opinion harnessed to a program to achieve some purpose. It is recognizable by its teeth and bite. In contrast, other countries are used to having opinions. Indonesia may have an opinion on the Falklands, and Niger an opinion on Zionism. Such opinions are on display at the U.N. General Assembly, where they are all added up on a big board and amount to nothing. Any real business is conducted in the Security Council, where the great powers and, most significantly, the superpowers reside. Superpowers have policies.

The air of unreality surrounding the American debate about South Africa derives largely from the fact that for once we find ourselves debating not policy but opinion. Constructive ngagement, says the administration. Sanctions, says Congress. While not quite Indonesia eyeing the Falklands, the hard truth is that neither ''policy" will have any significant effect on the course of South African history. We are to a large extent irrelevant.

South Africa, like the Soviet Union (to use today's favorite analogy), is a great country: vast, advanced and insular. Its internal dynamics reveal a formidable collection of immovable objects and irresistible forces: a politically conscious and wholly disenfranchised black population; a sophisticated revolutionary vanguard (the African National Congress and the loosely aligned United Democratic Front); an independent Zulu movement, 1 million strong (Inkatha); a dominant Afrikaner tribe with a strong military and nowhere to go; and Anglos, Indians and people of mixed race in varying degrees of opposition and discontent.

These are not the petty warlords of Lebanon -- and you remember how much influence we had over them. In Lebanon, moreover, we brought the New Jersey along for persuasion. To believe that the whispers of constructive engagement or the slap of sanctions will alter the course of South Africa's unraveling is an illusion.

It is an illusion with some appeal. Even Bishop Desmond Tutu will occasionaly give it a wistful glance. After President Botha's bar-the-schoolhouse-door speech of Aug. 15, which left black and white moderates politically marooned, he said that South Africa is "on the brink of disaster unless a miracle intervenes." The miracle? ''A decisive intervention by the international community."

If Tutu occasionally invites a foreign- made miracle, it is because he has little else to hang on to. Men of Tutu's Gandhian nobility know that South Africa's future does not belong to them. In Durban rioters attacked the Phoenix Gandhi Settlement and burned the Mahatma Gandhi Center to the ground. The irony was unintended, the message unmistakable. Tutu threatened to leave South Africa if black-on-black violence did not cease. He then noted that the young radicals are welcoming him to go, so they can ''get on with the revolution" without him.

They are certainly getting on with it without us. Being able to do so little, what should the United States do? Do the right thing. Impose sanctions.

Sanctions, of course, don't work. That is as close to an axiom as one finds in international relations. (Think of the non-effects of the U.S. embargo against the pitiful Nicaraguan economy.) But that is not a reason to abjure them. The United States was morally obligated, for example, to put Poland in default after the crushing of Solidarity not because it would change Jaruzelski, but simply because Americans should not be subsidizing him. Sanctions are most valuable for their didactic, not their diplomatic effects.

Call it moral hygiene if you will, but cleanliness is important. And in South Africa our lack of influence makes the moral choice all the easier. We do not have to agonize long, calculating the reverberating effects of our actions. There will be effects, but they will be marginal. Sanctions will make the lot of blacks somewhat more difficult. They will immiserate the masses and heighten the contradictions, as Marx liked to say, and perhaps hasten revolution by a month or two.

A subsidiary effect will be to make the whites feel their isolation more deeply. Western sanctions can depress the rand and perhaps add a bit to the panic being felt by whites. Will that induce them to reform, as the left says, or to more desperate resistance, as the right pretends? Both sides are being disingenuous: no one knows. And, in either case, sanctions will hardly be decisive.

That is certainly true of the sanctions bill passed by the House, soon to be passed by the Senate and grandly entitled The Anti-Apartheid Action Act of 1985. It bans the sale of Krugerrands in the United States, new bank loans to the government of South Africa (most American banks stopped lending seven years ago), and the sale of computers to certain South African government agencies. No more nuclear technology either. And it threatens to get stern if things don't get better in 12 months.

This is opinion, not policy. Yet even the strongest policy would only minimally affect South Africa. And on something as unambiguously evil as racial oppression, it is important for a country to have opinions. Countries, too, have to be able to look themselves in the mirror.