Prime Minister Menachem Begin left Israel yesterday on the first leg of his peace mission to the Camp David summit talks, which, he said with pointed reserved expectation, he hopes will end in an agreement for further negotiations with Egyptian President Anwar Sadat.

Implicitly dismissing Sadat's view of the summit meeting in itself as a decisive turning point in the Middle East peace effort, Begin pledged simply that Israel will make "every possible human effort so that the Camp David conference ends in agreement which will enable the conducting of negotiations for the signing of peace treaties."

The prime minister's remarks before boarding a regularly scheduled El Al flight to New York seemed at odds with Sadat's reported unwillingness to consider talks after Camp David unless Israel agrees in principal to the notion that the West Bank and the Gaza Strip should be transferred to Arab sovereignty.

Conspicuously missing from Begin's airport remarks - and from a television speech on the eve of his departure - was the euphoric anticipation that marked the days leading up to Sadat's historic peace mission to Jerusalem last November.

Israeli government source said Begin's restrained tone was intentional in that the prime minister is eager to promote a dialogue on a practical settlement to the West Bank-Gaza issue that will continue beyond Camp David rather than end in a declaration of principles at the presidential retreat.

Consistently in recent weeks, Israeli Foreign Ministry officials, including Foreign Minister Moshe Dayan have played down the importance of a declaration of principles - favored by the Egyptians - and have promoted the merits of practical procedural discussions that could lead to a partial solution now and a comprehensive Middle East peace treaty later.

Accompanying Begin yesterday were Dayan and Defense Minister Ezer Weizman, although several Cabinet ministers had urged that Weizman be left behind because of the delicate military situation in Lebanon, where Israeli-supported Christian militias are under attack by the Syrian Army.

Begin, however, reportedly insisted that Weizman join the delegation, not only because matters of national security will be discussed at the summit, but because Weizman along among the group already has established a rapport with the Egyptian president.

Begin is expected to remain in New York until Tuesday talking with Jewish leaders and with journalists. He is scheduled to travel Tuesday to Camp David, where exploratory talks will start on Wednesday.

In his parting remarks, Begin stressed the importance of the talks to the "International prestige" of President Carter, an observation that recognized the political risks to Carter but left unsaid the risks to Begin and to Sadat.

For Begin, a collapse of the summit could intensify widespread perceptions both at home and abroad that the Israeli prime minister has a theological attachment to the West Bank and is nitractable on that issue. Some political observers maintain that a failure at Camp David would also erode Begin's delicate parliamentary coalition and could bring the government down.

Sadat could face intense political pressures at home if the conference fails, possibly forcing him to abandon his much-heralded peace initiative and turn once again to the fold of the Arab confrontation states.

The road over the last nine months from Jerusalem's King David Hotel, where Sadat stayed during his visit to the Camp David retreat has been strewn with disputes over numerous region issues - but none as difficult as the West Bank and Gaza, where 1.1 million Palestinian Arabs live under Israel military occupation 11 years after the end of the Six Day War.

Apart from resolving the technical differences between Israel's 26-point peace plan and Egypt's six-point proposal for the West Bank and Gaza, the summit meeting must bridge an enormous gulf based on fundamental opposite principles.

Begin's basic position has remained essentially unchanged since he came to power a year ago on a platform proclaiming that Israel's right to the West Bank was "eternal."

He starts with the proposition that Judea and Samaria (the Biblical names for the West Bank) are part of the Jewish national homeland liberated in a war of Arab aggression, and that while Israel is willing to discuss the question of sovereignty after a five-year trial period of limited Arab self rule, it will be just as ready to assert its own claim of sovereignty over the occupied areas.

Sadat, on the other hand, begins with the premise that the West Bank and Gaza are historically Arab territories now occupied by force, and that while Israel may have a right to border security, the land must be returned to its rightful owners.

Thus, after months of diplomatic foot-shuffling and trading of nuances by both sides, the spectacle of two people fighting over the same house is as fundamentally unchanged now as it was 11 years ago.

There have been concessions by both sides, and the ultimate fallback positions are still unknown.

Last spring, Israel grudgingly acknowledged that U.N. Resolution 242, which calls for withdrawal from occupied areas, applies to the West Bank as well as other seized territory.

This shift came about tortuously, with Dayan saying in a television interview in April that the U.N. document applies to negotiations between Israel and all her neighbors, which by implication included Jordan.

At the time, it appeared to be a stunning turnabout of policy, but Israeli acceptance of the applicability of Resolution 242 to the West Bank is now so taken for granted that it is merely part of the landscape of the forest of Egyptian and Israeli negotiating positions.

Nevertheless, there are still vastly differing interpretations of the breadth of the resolution in terms of redistribution of sovereignty in the West Bank of Gaza.

Israel still maintains that its own peace plan of limited autonomy for the Palestinian Arabs satisfies the intent of the vaguely worded U.N. declaration, while Egypt argues that the intent of the resolution will never be fulfilled without Israel withdrawal to the pre-1967 borders. In between, the Unites States is still calling for withdrawal from most of the area, with adequate security guarantees in exchange for a peace treaty.

The challenge confronting Carter at Camp David, therefore, will be to narrow the differences of interpretation of 242 even further, and perhaps to produce a substitute declaration removing some of the ambiguity written into the U.N. document, which for 11 years has enabled Israel, Egypt and the United States to interpret it in different ways.

Another modest but significant shift of Israel's position since the Jerusalem talks has been its approach to the final status of the West Bank and Gaza after the five-year period of self rule.

Under Israel's Ismailia peace plan, the current military government in these areas would be dissolved and the Palestinian residents would take over most governmental functions, with Israel retaining responsibility for security and internal order by maintaining military garrisons. The question of the final political status of the areas was deliberately left unanswered.

In an attempt to resolve that basic issue, the Carter administration last April pointedly asked Dayan for an answer and the Begin government finally responded last June 18 with a murky statement that the "nature of future relations" would be taken up after five years. That position - and the Americans' cool response to it - touched off a stormy political debate in Israel and a crisis for the Begin Cabinet.

Begin softened his position later by promising to discuss the sovereignty issue after five years. The government has become so used to the once-taboo principle of negotiating over sovereignty that a recent Foreign Ministry position paper forecast that Israel "believes that it will be possible to agree on a solution to this (sovereignty) issue."

Although Israeli officials invariably deny it, there have been indications that Sadat, too, is more agreeable to concessions than his public statements would indicate. These include indications reportedly made to Weizman at a meeting in Salzburg, Austria, that Egypt might agree to the presence of some Israeli military units in the West Bank, and might allow Jewish settlements to remain there.

Nonetheless, it is considered unlikely that Sadat would be willing to make concessions at Camp David that would be rejected outright by King Hussein or Jordan or Egypt's benefactor, Saudi Arabia. At the least, such an agreement would seem to have to include the probability of Arab sovereignty on the occupied areas.

Since Begin is still cautiously tendering an offer only to discuss somebody's sovereignty as a possibility - and still refusing to mention Arab sovereignty as that possibility - Egypt and Israel will sit down at Camp David still far apart on the crucial issue.