Let us go to the heart of the Lebanese crisis: it exists because of an undigested Palestinian mass--a "state within a state" --in Lebanon.

That mass came to be when, after World War II, the Palestinians were denied the usual and right form of expression for their collective consciousness: statehood. A state was offered them by the United Nations in 1947, but their fellow Arabs kept them from accepting it. Jordan and Egypt could have created one in 1948-67 in the West Bank and Gaza but did not. Israel, in a sense the lesser villain, has denied them one ever since.

The sense of Palestinian consciousness and collectivity, however, has not dulled. At first the active core of it resided in Israel proper, which had the state power to deal with it. It moved in turn to Egypt, Syria and Jordan, which also had the state power to deal with it. Finally, in the 1970s, after King Hussein killed 10,000 or more Palestinians who he felt (rightly) were trying to kill him, it moved to Lebanon.

There, however, contesting Christian and Moslem communities had agreed not to construct a state apparatus that one might use against the other, and so no apparatus has been available to deal with intruding outsiders--either with Palestinians or with Syrians and Israelis who came in their wake. That produced the convulsions culminating in the latest Israeli invasion.

In responding to the Palestinian "state within a state" in Lebanon, Israel has had two very different purposes.

One is to protect its borders and its officials and citizens, wherever they may be, from terror. I might add that it is hardly necessary for Israel to produce courtroom- type evidence in each case. Whatever the PLO's exact role in the attempted assassination of the Israeli ambassador in Britain, for instance, the PLO remains a group that itself condones terror in word and deed and had set up guns for the sole purpose of terrorizing Israeli villages.

I must add further, however, that Israel is daily devaluing its claim of self-defense by the still-expanding scope of its military operations and by the death and destruction it is inflicting upon innocent Lebanese and civilian Palestinians.

The Israeli government would like the discussion to stop with universal acceptance of the Jewish state's right of self-defense. But that is a false stopping place. One must go to the second Israeli purpose in moving against the PLO in Lebanon: to crush the Palestinians.

Crush, indeed. The purpose is not simply to defeat the PLO's armed forces but to decimate its non-military structure, to shatter the sense of Palestinian corporateness and to turn the Palestinians' Lebanese hosts against them ever more bitterly.

The Israelis do not hope to turn the PLO toward peace: that would mean opening to it a path to statehood. They hope to destroy it as an organization, an idea and a cause, leaving "the Palestinians" as a small isolated bunch of individuals on the West Bank who can be intimidated, manipulated and perhaps eventually shuffled across the river into what the Begin government regards as the real Palestinian state, Jordan.

In short, the Begin government is using its legitimate purpose of self-defense to mask its illegitimate purpose of repressing another small, abused and persistent people.

Any effective solution to the Lebanese crisis, even if it gets worse before it gets better, must have three parts. First must come a complete Israeli withdrawal and a Palestinian border truce. Since the Israelis have now destroyed the PLO as a military force, they can have no acceptable hesitation on this score.

There must be a rearrangement of the internal Lebanese factions. Lebanon was headed toward July presidential elections in which a strong tendency was emerging for Christians and Moslems to assert a Lebanese identity and to start removing the basis for any foreign military forces to stay in Lebanon. Those elections must be put back on track, not least so that Israel will not be tempted to rig an artificial political alternative.

Then there must be a renewed quest for a solution--a statehood solution--to the Palestinian question. Since the Israelis have been rendered more arrogant by this crisis and the Palestinians more desperate, it will be up to their best friends to bring them along. The Israeli-Palestinian quarrel is a luxury that no other country-- least of all Lebanon, certainly not the United States--can any longer afford.

Bruce Clark's Canadian exile is over. He signed a multiyear contract yesterday with the New Orleans Saints.

"It's been a long two years," the 6-foot-3, 250-pound all-America middle guard and linebacker from Penn State said.

He was the Green Bay Packers' No. 1 draft choice in the 1980 NFL draft, but opted for the Toronto Argonauts of the CFL.

Clark, 24, played out his option in Toronto. The Packers had NFL rights to him until last April's draft. Since then, he has been free to negotiate with all NFL clubs but the Packers had the right to keep him by matching other offers.

The Saints gave Green Bay their first draft pick in 1983 in return for their rights to Clark. A spokesman said Coach Bum Phillips of the Saints probably will use Clark as a defensive lineman . . .

The Redskins announced that defensive tackle Dave Butz had signed a series of three one-year contracts. Butz led the team's defensive linemen in tackles last season and was fifth overall on the team in tackles. Since coming to Washington in a trade with the St. Louis Cardinals in 1975, he has played in 100 of 103 games. He's started 46 straight.