THE ARABS won't go to a Geneva peace conference without the Palestine Liberation Organization, they've said so far, and Israel won't go with it. But there may yet be a way to solve this riddle, thanks most recently to Egyptian Foreign Minister Ismail Fahmi, who followed Israeli Foreign Minister Moshe Dayan to Washington. Mr. Dayan had suggested Israel could accept, in a Jordanian or pan-Arab delegation, the likes of Palestinian West Bank mayors not plainly identified with the PLO. Responding, in effect, Mr. Fahmi said: "I am not aware, really, of the so-called assumptions that the Palestinians under the occupations, the Israeli occupation, are in the any way different from the PLO." Precisely in this play between the territorially based West Bankers and the organizationally based PLO leadership - Palestinians, all - there is a narrow opening. It could yet be closed by Arab insistence that, at an initial Geneva reconvening, no one without clear-cut PLO leadership credentials be invited. But Egypt, for one Arab country, seems to be ready to leave the way open for an accommodation-by-bofuscation - if everybody else will try to be reasonable, as well.

Why all this dancing on the head of a pin? Why not simply invite the PLO? The reason is simple. To undemanding foreigners, PLO "moderates" often murmur of their reasonableness. But among their own people they cling to the familiar formulas, saying, among other thins, that Israel should be declared "null and void." "We have said," President Carter said last week, "if the PLO would accept publicly the right of Israel to exist and exist in peace, as described under U.N. Resolution 242, that we would meet with them and discuss the future of the Palestinians in the Middle East." Few Israelis are prepared to agree. But we regard this as a principled position, fair to Israelis and fair to Palestinians. Despite this magnanimous and historic American overture, however, and despite efforts by other Arabs to induce the PLO to respond forthrightly to it, the PLO has hung back. It has moved, but not nearly far enough.

Therein lies the basic reason for looking for some other legitimate party to speak for the Palestinians. They should not be deprived of a voice simply because the umbrella organization that claims to be their "sole representative" cannot bring itself into compliance with ordinary standards of international order and civility. Who knows if the PLO, seeing, say, the West Bank mayors at Geneva, would remain as rigid as it's been so far? That's an interesting question. It is unavoidable, we feel, for the United States to persuade the Israelis to accept a Palestinian "entity," in Mr. Carter's useful word. But it must be an entity that enhances rather than undermines Israel's security. Only that, in turn, would give Palestinians a chance for a homeland in which to live in peace.