Having nothing really new to say about South Africa, President Reagan's speech has only sharpened the argument he sought to temper.

But the reaction is so ill-tempered that it can hardly be based on what he said or failed to say, or on any very sure grasp of the political realities in South Africa. The citadel of Boerdom seems to be viewed, on both sides, as a sort of enlarged Mississippi, awaiting U.S. enlightenment.

Consider, for instance, the president's description of apartheid, the South African system of racial segregation, as a ''feudal institution.''

Apartheid, it is essential to know, is a very modern institution, a typical product of 20th-century rationalism. In this, it is rather like the anti-Semitic Nuremberg Laws of Nazi Germany. Some of apartheid's more objectionable refinements actually date from after 1948, the year of the Afrikaner takeover, and are the handiwork of Dr. Henrik Verwoerd, an e'migre' intellectual who was not even a native Boer.

Why does it matter to know this? Because apartheid is very different from our now discarded Jim Crow laws, as are the available avenues of attack upon it. Parallels there are, but the crucial differences outnumber them.

Once U.S. courts began to dismantle ''separate but equal,'' beginning in the 1940s, the might and prestige of the federal government could be mustered to support change. In South Africa, the Afrikaner government -- the only center of power -- is the custodian of apartheid. What it chooses not to do about it will not be done, short of revolutionary violence.

In the United States, moreover, the press provided an open window on black discontent. In South Africa, authorities shut the window at will. Gruesome scenes in the black townships penetrate the white consciousness only when the government chooses to let them. And the government so chooses only because the Afrikaners like to think of themselves, now and then, as adherents of ''Western,'' i.e., North Atlantic, norms of law and civility.

It's a rare P. W. Botha speech that fails to harp on the theme. But it is a crucial link that should not be idly tossed away by a barn-burning strategy on our part.

The tragedy is that South Africa's ''Western'' connection, apart from anticommunism, is a fragile thing of fits and starts. The Afrikaners are descended for many generations from tough pioneering stock, long remote from the North Atlantic community, whose legendary exploits of endurance make our own frontier look soft by comparison. This has had its cultural effect. Their traditions of self-reliance and insularity, their paternalistic and authoritarian family structures, their bleak evangelical Calvinism (with its stress on private virtues and relative neglect of the public ones), their lingering resentment of the British colonial presence: all these ingredients go to make a tough and stubborn tribe.

Well, you may say, we knew that. Why are we reluctant to draw the bleak conclusion that it limits our leverage? On the evidence of his speech, Reagan believes that a reasoned appeal to political conciliation and moderation will gradually do the trick. What we know of the Afrikaner character raises doubts.

But it may be an even greater miscalculation to imagine that the Afrikaners will easily be pressured out of their course (which they think of as reformist!) by barring their airliners from Europe and the United States or by freezing their bank accounts in New York and London. The better bet is that these trifling harassments will mainly hurt and embarrass the South Africans who identify with our political standards. They are unlikely to faze the Boers.

These unpleasant but intractable conditions suggest that our preening and ill-tempered argument over how to deal with South Africa is largely an irrelevance. Like the sorcerer's apprentice, we can probably set some processes in motion, but with little prospect of controlling the outcome. Maybe that's enough -- gestures that give us moral self-satisfaction, regardless of effect. But gambling is a poor substitute for statecraft.

The Reagan-Thatcher course is preferable, unsatisfying as it is. Far more stubborn white nationalists than P. W. Botha are waiting in the wings, people who regard Botha a traitor to Boer values. When and if that crowd comes to power and unlimbers its arsenal, Armageddon would be next.