But there is something poignant about the Zionist left's continuous attempt to reconcile the irreconcilable. Its criticisms of Sharon hark back to an idealised notion of a Jewish state in which democracy, decency and tolerance are the guiding principles. In moving forwards towards peace with the Palestinians, the left seeks to take a few steps back; consolidating the Jewish state, preserving its Jewish character, withdrawing from the quagmire of occupation and reinstating the values of a democratic and humane society. But to Palestinian ears there is something inherently wrong here: for us, there is a basic and inescapable contradiction between Zionism and democracy. If Zionism means anything, it means a Jewish state with a clear Jewish majority - and in Palestine this has necessarily been at the expense of Palestinian Arab rights.

The question of whether Zionism can be reconciled with democracy has always been at the heart of the debate on the Palestinian problem. But it has dropped off the broader political agenda partly because a majority of Israeli Jews have been resistant to anything that smacks of a challenge to the very premise on which the Zionist enterprise was built, and partly due to the belief (on both sides) that the Palestinian problem is ultimately resolvable via a territorial partition that would separate the mass of Arabs from Jews.

However, a number of recent developments have challenged these assumptions. With an unbridled settlement policy now matched by a "separation wall" that merely consecrates the divide between Palestinians and Israeli settlers within the occupied West Bank, Sharon and his predecessors have all but destroyed the possibility of a viable and sustainable territorial settlement along national lines.

There is also a growing realisation that demographic trends will redefine the Arab-Jewish population balance in the territory of historic Palestine between the Mediterranean sea and the river Jordan. We already have rough parity between the two populations today; by 2020 the balance is likely to be 60-40 in favour of the Arabs.

But that is not all; the moral terms of Palestinian-Israeli debate have also changed. Thus former Mossad head and national security adviser Ephraim Halevy believes that Israel's real post-Oslo mistake has been to accept a trade-off whereby the Israelis acknowledge Palestinian rights (such as that of "statehood"), while the Palestinians merely concede contingent realities which, by implication, they could overturn at a later date. Henceforth, he says, the Palestinians must accept Israeli-Jewish rights, and the moral legitimacy of their presence in Palestine.

But there are no conceivable circumstances in which any Palestinian can concede their own history in favour of the Zionist narrative. It would mean that they would have to accept that for 1,400 years the Arab-Muslim presence in Palestine was transient and unlawful, and based on the false premise that continuity of habitation conferred rights of ownership. Furthermore, the Palestinians would have to accept that the pulverisation of Arab Palestine in 1948, and the 50-odd years of subsequent dispersal and occupation, are the rightful outcome of an illegal struggle against the real owners of the land. Simply put, Halevy wants Palestinians to become good Zionists.

Palestinians cannot confer legitimacy on the Zionist narrative and should not be asked to do so, or vice versa. But if the two-state solution is no longer physically possible, and demography is creating its own inexorable facts, what are we left with that can serve as a framework for a settlement?

A move from the dominance of the territorial struggle to a redefinition of the national struggle, from the discourse of self-determination to that of freedom and democracy, provides one way out. If history cannot serve as a common basis for legitimisation, let us consider doing so on the basis of mutuality and equality. In other words, on the basis of equal political and civic rights in one state, with one-man, one-vote.

One can almost hear the sheer panic in former prime minister Ehud Barak's voice as he argues that the Palestinians may demand not two states for two peoples, but one state west of the Jordan river: "But," he warns, "that single state will have to be in the spirit of the 21st century: democratic, secular, one-man, one-vote. One-man, one-vote? Remind you of something? Yes. South Africa. And that's no accident. It's precisely their intention."

As it happens, having espoused the two states for some three decades, the PLO and the Palestinian Authority are desperately clinging to the fading prospects of a viable partition. But let us suppose that the Palestinians do "demand" one-man, one-vote as in South Africa. What is the argument against?

Barak is not alone in not really having an answer. The best that the influential New York Times columnist Thomas Friedman can do is caution his American Jewish readers that if they think it is hard to defend Israel on college campuses today, "imagine what it will be like when their kids have to argue against the principle of one-man, one-vote".

The truth is that the Zionists want it both ways: a secular political existence for Jewish communities everywhere and equality of rights as the underpinning of global Jewish security, yet an ethnic state in Israel that is built on the converse. As that canny veteran Israeli peacenik Uri Avnery has observed, Israel is not really a "Jewish democratic state" but a "Jewish demographic state".

By positing one homeland for both sides, the one-state solution not only does away with the conflict over history and mutual legitimisation, but has practical political implications as well. Both sides can maintain their "right of return" without this being at the expense of the other, and Israeli settlers would not need to be removed from where they are today. Jerusalem could truly become the shared capital of a unitary Arab-Jewish state.

It would be sheer illusion to pretend that this idealised vision is about to wean Israel away from Zionism. Neither can the Palestinian national impulse be easily melded into some kind of fuzzy warm-hearted version of semitic brotherhood. But if the two-state solution simply is not to be, some truly serious questions must soon be asked: what is more important, democracy, or the Jewishness of the state? A Jewish state, or a homeland for Jews and Arabs alike? What is better: no Palestinian state at all, or a single state that provides them with equal rights alongside Jews?

There are alternatives to the one state/two-state paradigm; a slide towards apartheid, for example, or a drift towards ever-escalating resistance and violence, and the chances are that both sides will pursue them to the bitter end.

But the resilience of the democratic option should not be discounted. Unlike the two-state solution, its viability is not contingent on developments on the ground, but really is a matter of a change of hearts and minds. And yes, we do have the example of South Africa.

· Ahmad Samih Khalidi is a Palestinian writer and former negotiator and a senior associate member of St Antony's College, Oxford