Finally President Reagan has provided a basis for understanding his relatively relaxed and indulgent view of South Africa. To an Atlanta radio station, he has given a little snapshot of his picture of reality in that quarter. He finds it a place where important progress is being made through the happy incidence of a reformist local government and a persuasive American administration: where blacks really have very little to complain about; anyway the Soviet Union is stirring the pot. No wonder he resists any policy change.

Except it is all -- or most of it -- quite untrue. From his comments it is evident that the president does not have the foggiest notion of what apartheid is actually about. As Post reporter Glenn Frankel makes plain in today's paper, even in the limited areas where President Reagan points to progress, there is not all that much to boast about. One could hoot at the president for his ignorance if it were not so painful. He altogether omits from his picture, furthermore, any hint of the larger and grosser aspects of apartheid: the humiliation and harassment of the pass courts, for instance, the forced removals, the brutalization of family life. He shows no awareness of the fundamental question of political power -- that is, of the disenfranchisement of blacks under the current system and of the quality of repression organized by whites to deny blacks political power. Has anyone told him that a state of emergency is currently in effect and that blacks are being arrested and killed practically every day?

The president did make his ritual use of the adjective "repugnant" to describe apartheid. By what else he said, however, he erased any impression that the system truly troubles him. It was not simply that he was at pains to excuse Jerry Falwell from the burden of his unfair and unfeeling criticism -- since only partially retracted -- of the Nobel peace laureate, Bishop Tutu. It was the particular explanation Mr. Reagan offered in so doing. South Africa, he said, is "a combination of minorities. There are at least 10 tribal divisions there." This is a painful echo of the official South African rationale for the large structure of apartheid: that blacks are not the rightful citizens of a unitary South African state but are members of distinct tribes who are to be consigned, unconsulted, to separate "homelands."

What Mr. Reagan has said goes far to explain the most objectionable aspect of his approach to South Africa: his stunning lack of moral energy and commitment to the cause of justice. The South Africa he depicts exists only in his mind. But the South Africa the United States must deal with exists on the ground.