That even a thin strand of hope remains for Ronald Reagan's sputtering Mideast peace "initiative" owes much to a meeting that didn't come off -- for a sad and unrelated reason, Israeli Prime Minister Menachem Begin was unable to stop last Friday at the White House.

It was not to have been an official meeting between Begin and the president in any case. But seemingly insoluble points of conflict would have had to have been discussed, and there was little or nothing that the two men could have said afterward that might have been helpful at this point:

1)4 Begin had a laundry list of objections to the Reagan plan. It only began with the Reagan preference for solving the Palestinian problem by some ultimate federation between the West Bank and Gaza with the Kingdom of Jordan.

2)5 Still hanging out there in tatters is that portion of the Reagan plan that calls emphatically for a freeze on settlements as a way of arresting what amounts to a creeping de facto Israeli annexation of the West Bank. Begin immediately shredded it by announcing plans for at least 20 new settlements over the next year. Only some evidence that Begin takes Reagan seriously could have restored some sense of purpose and progress.

Yet the administration was not going to press Begin for an agreement on a freeze. "His government wouldn't last for more than a day if we did," says one administration official.

3)4 That brings us to the worst of what-might-have-been: in the absence of a clear signal that the United States is capable of "delivering Israel" on something, almost anything -- the modest movement now evident on the Arab side would have come to a grinding stop.

It is easy enough to disparage the Arab movement of recent weeks -- the more so for those critics who would have Jordan's King Hussein risk his reign, for the umpteenth time over more than two decades, by simply doing what he was told by Camp David to do. He was told to join a peace process he had not been consulted on in advance.

Given his own shaky situation, and the almost universal opposition in the Arab world to Camp David, Hussein couldn't do it then. But the way Reagan framed his September initiative has given Hussein running room. In The Wall Street Journal the other day, he hailed the Reagan initiative as "the most courageous, realistic stand . . . taken by the U.S. since 1956" (when Dwight D. Eisenhower forced the Israelis to withdraw from the Sinai after the Suez war).

But the king stated the obvious: Arab concessions on participation in negotiations on the West Bank's future are impossible unless the United States can arrange a settlements freeze to prevent the future from being unilaterally foreclosed.

Through the Egyptians, the PLO is testing the terms on which it might work its way into the Camp David process. But the PLO is severely split. Neither it nor Hussein can be expected to make conciliatory noises for very long without at least some conciliation in response.

That was the message that Egyptian Foreign Minister Kamal Hassan Ali brought to President Reagan and Secretary of State George Shultz last week -- a message Hassan Ali himself had received from PLO representatives in Paris on his way to Washington. "Now or never," is the way one of the foreign minister's aides summed it up.

If Hussein and the PLO are out on a limb, so are the Saudis, whose financial leverage counts heavily. And so is Egypt's President Hosni Mubarak. Egypt's relations with Israel, far from flowering in constructive ways with the coming into full force of the peace treaty in April, are withering fast.

Now it is easy enough to sit back in judgment on the Arabs' feckless ways. But that will not alter the realities. The Arabs see themselves vulnerable to religious extremism. They see what befell Iran's shah. And they see an unresolved Palestinian grievance as a deadly weapon in the hands of whatever extremist forces threaten their stability. If the United States is prepared to indulge Israeli domestic political imperatives, it would seem reasonable to take into account those bearing on the Arabs, as well.

The question is how. One priority is the removal of all foreign forces from Lebanon. The Egyptians insist they cannot sit down to Camp David talks until the Israelis have withdrawn. But no less a priority is the settlements issue; before long, the Israeli presence will have hardened beyond the point when there is anything to negotiate. The trick will be to avoid sending the wrong signals until the right signals can be arranged.

That is why the cancelled meeting between Reagan and Begin last Friday should be looked upon as something to celebrate -- as you might celebrate stepping off a curb and being narrowly missed by a bus.