Israel's invasion of southern Lebanon has, if nothing else, demonstrated the contradictions of the Arab world - especially the weaknesses of those who take the toughest line against Israel and its now faltering peace negotiations with Egypt.

Yasser Arafat, leader of the Palestine Liberation Organization, lashed out at his former allies yesterday, telling a meeting that the invasion "revealed Arab feebleness" and saying that his guerrillas and Lebanese leftists "are fighting all alone in South Lebanon." He did not name the Arabs whom he felt had let him down.

Syria, however, would have to be put at the top of the list of those who have been exposed for falling short of their rhetoric. Damascus, it is recalled here, once critized the Lebanese army for not protecting the Palestinian guerillas against, reprisals here.

In the present crisis, Syria studiously has refrained from taking action against the Israel.

Admittedly, despite recent massive Soviet arms shipments, Syria is still military too weak to help the Palestinians without risking embroiling itself in unequal battle with Israel.

Yet the spectacle of arch-enemy Israel accoupying a tenth of a neighbouring country with virtual impunity is scarcely calculated to improve the standing of Syrian President Hafez Assad either at home or in the rest of the Arab world.

On the other hand, of all the major Middle East players, Syria has the most to gain from a stabilized southern Lebanon. It would deprive israel of its present capacity to cause trouble there which could undermine the Damascus government.

The Israelis' hope, analysts here suggest, is that the Syrians would go along with isolating the guerillas in an area north of th Litani River up to a line running south of Sidon.

As for the Palestinian guerillas their spokesmen have preferred to accentuate the positive. They argue that the main forces remain intact and that the invasion was not an Israeli walkover.

They also contend that Saturday's Palestinian terrorists attack in Israel, which triggered the invasion, was justified because they stood up to gain nothing from even appearing to be moderate.

Over the months they had lost whatever initial hope they had pinned on the Carter administration and its ability to bring pressure on Israel.

They were not surprised when Israeli Prime Minister Menachem Begin dug in his heels over Israeli settlements in occupied Arab territories or sought to exclude the West Bank of the Jordan from lands to be evacuated in any peace negotiations.

Nor did they have anything to gain from encouraging Egyptian President Anwar Sadat's peace initiative from which they in effect were excluded.

Nevertheless, the Palestinians are faced with the short-term probability that in losing the south they have lost their last area of autonomous maneuver in the Middle East.

They argue that if Israeli troops stay inside Lebanon that proves Israel's expansionist policy. But past experience indicated that the Israelis couldn't care less what others think about them if they are convinced that a given policy is in their own interest.

Nor do analysts agree with the Palestinians' insistence that they can operate as before in the south if Israel withdraws and entrusts the long border area to its thinly stretched rightist Lebanese Christian allies.

Oddly enough, Lebanon is perhaps the one country that has learned to live most easily with the anarchy in the south. After a decade of guerilla raids, Israeli reprisals, fleeing refugees, ineffective government, warring Christians and Palestinians and Israeli refusal to allow the Syrian peacekeeping force in Lebanon to police the border area, Lebanese have grown cynical.

"The Lebanese stomach has digested the south," a prominent Christian Politician recently remarked, ""To the point no one cares who occupies it since it certainly isn't us."

President Ellias Sarkis weak government is credited with wanting to recover the land in the south, but not if the price is continued loss of effective sovereignty.

That is not likely since the Israelis are proposing that the south be policed jointly by their Christian allies and elements of the new Lebanese Army emerging from the ashes of this country's civil war.

Lebanese officials insist that no Lebanese government - and certainly not the present weak Cabinet - could survive under such a deal that would be sure to raise the hackles of the Moslem half of the Lebanese population.

Nor is it certain that Israel itself - not much less its Christian allies - really favors a return to law and order in the south despite their official statements to that effect.

The most extreme Christians still dream of provoking a crisis that could bring Israel not only into a relatively narrow border strip, but farther north in a joint operation to drive all Palestinians from Lebanese soil.

Any deal stablizing the south would end such dreams since Israel could scarcely invoke guerilla activity there to participate in a drive northward.

Analysts have not ruled out Israel having its own second thoughts about its demand for sterilizing the border.

Lebanese politicians are not alone in accusing Israel of deliberately encourage turmoil in the south because of its refusal to allow Syrian peacekeeping troops there at the end of the Lebanese civil war in 1976.