THE OTHER DAY Andrew Young pronounced the South African government "illegitimate" and, when corrected by the State Department, insisted still that that government was "unrepresentative" and that it was only the "agency in charge." Our United Nations ambassador then set up a series of private engagements in South Africa on his own - that is, with a little acknowledgement as possible of Pretoria's authority within its own borders. Unsurprisingly, South Africa now does not wish to admit Mr. Young on his chosen terms. Rather, it wants him to go through channels in a manner compelling him to accept its authority. Thus has a test of wills been joined.

Put aside for the movement the familiar question of whether Mr. Young was acting for his government or for himself. Id his tactic of undeclared psychological warfar the best way, or a proper way, for the United States to advance its goals in southern Africa? We do not dismiss this approach out of hand. Many Americans, and no doubt many black South Africas (and some whites), will be gratified by his open display of contempt for the South African government and by the propaganda defeat that Pretoria will almost certainly suffer regardless of how its confrontation with Ambassador Young turns out. There is a school of thought holding that it is precisely through such foreign-applied shocks that the apartheid system can be weakened and set up for domestic dismantling. By this theory, Mr. Young's affront to protocol is a small price to pay, and the government's anger and discomfort and proof that the strategy is working.

We have grave misgivings, which extend considerably beyond the question of how the United States would have reached it, say, Moscow and Peking had privately arranged to send delegations to Watts after the riots - the analogy is not all that farefetched. The question we ask is how best can the United States gain the cooperation of South Africa for 1) bringing majority rule promptly to Rhodesia and Namibia and 2) hastening tolerably orderly change within South Africa itself. Is it better to regard the government of South Africa as a partner, however reluctant and unreliable in one or both of these enterprises or as an adversary? Can we have it both ways, demanding Pretoria's diplomatic cooperation even while making it clear that its payoff is not a certain understanding of its domestic difficulties but a more insistent pressure to resolve them? Granted, it would be nice to have our cake and eat it too. But given the seige mentality that dominates the ruling Nationalist Party, is there not a risk that such pressure will squeeze out what admittedly few moderate tendencies exits within it?

In fact, this incident illustrates not so much Mr. Young's idiosyncratic style, by now almost a bore, as the Carter administration's failure to date to articulate or, as far as we know, to compose a coherent African policy. Since Vice President Mondale is about to meet South African Prime Minister John Vorster in the administration's first policy-level contact with Pretoria, the lag is a serious one. The South African scene, after all, is incredibly complex and volatile. Good intentions alone are of scant value. Ill-considered involvement of a sort that promoted more rather than less violence could have devastating consequences, of which American embarrassment might be one of the smaller. The effect on the welfare of those millions of South Africans whose racial relations the United States is trying, at long range, to reorder must be the dominating consideration.

In other areas of foreign policy, the administration had tried hard, and with considerable success, to gain advance public understanding for its purposes in negotiation with foreign countries. It had not done so in respect to South Africa. Rather it has permitted the impression to be spread that it is allowing individuals such as Andrew Young to improvise policy on the basis of personal predilections. Handing Mr. Mondale the Vorster brief does not solve the problem. The Vice President does not seem to have communicated even to the administration's own foreign policy bureaucracy what it is he is setting out to do. There is a real danger here of running off the rails if the administration does not slow down, figure out a policy tht is at once responsible and feasible and likely to secure a solid domestic underpinning, and explain it clearly, first at home.