The UN's new secretary-general comes along just when the United Nations needs a lift. Peru's Javier Perez de Cuellar may have been elected to carry on the quiet, persevering, don't-rock-the-boat approach of Kurt Waldheim. And this is not to be minimized in a large body of diverse interests where conciliation must have first priority. But to counter current pessimism, the UN will have to have fresh ideas and search actively for solutions to international problems.
This does not mean the aggressive pursuit of any one element's goals, as was sought by China and others in pressing for a third-world advocate as secretary-general. China can claim to have saved face in that Mr. Perez de Cuellar literally comes from the third world. But he is politically and culturally a classic Western career diplomat, fully tutored by Mr. Waldheim over several years at the UN.
His selection provides confirmation that the American and Soviet superpowers retain dominance in the United Nations. The North-South relationship of the industrial and developing worlds may be increasingly publicized as vital to preserving global peace in the face of sharp and widespread economic disparities. But such matters still remain subordinate to the East-West situation.
Thus even the most vigorously innovative secretary-general would depend on US and/or Soviet backing to restore the UN to a more central role on the world stage. Today's pessimism is fed by the circumstance that both Washington and Moscow appear to be in a period of return to power politics, leaving the UN on the sidelines. That they want to keep it that way is one reading of their agreement to the prudent Mr. Perez de Cuellar.
But Mr. Waldheim showed more than once that the ''powerlessness'' of his office did not mean abdication of the effort to exercise its moral force. His successor ought to do no less. And, indeed, the superpowers if they are worthy of the name ought to foster this task.