In a long ''conversation with Henry Kissinger, a private citizen,'' the weekly Economist shows just how much light an old Middle East hand can shed on the prospects ''after Lebanon.'' For all the controversy surrounding the former US secretary of state, he knows the territory. When Kissinger says the circumstances for progress are ''the best I can remember,'' anybody would want to say tell me more.
How to capitalize on these circumstances?
See President Reagan's peace plan as a framework not a rigid blueprint, says Kis-singer.Pursue such encouraging elements of it as using Jordan as a negotiating partner and giving the inhabitants of the West Bank a decisive role in determining their political future.
''Yes, we should encourage Israel to negotiate. Israel is more likely to do so, paradoxically, if it feels compassion on our side, maybe even affection, rather than unre-mitting pressure. Where we disagree with Israel on substantive points we must be prepared to express this - strongly if neces-sary. . . .In other words I feel that pressure on Israel should be exerted retail rather than wholesale, if one can put it in such crude terms.''
In view of Israel's virtual annexing of the West Bank through settlements, is it any longer conceivable to talk of West Bank nego-tiations?
''It is not only conceivable; it is imperative. Annexation of the West Bank - overt or disguised - will sow the seeds of endless crises, one of which will inevitably erupt into conflagration. It is not even in the interest of Israel however narrowly conceived.
''The incorporation of Gaza and the West Bank into Israel will sooner or later produce an Arab majority that will destroy the essence of the Jewish state. And if Israel seeks to escape this dilemma by expelling all the Arabs it will lose the moral support of even its best friends. Over an historical period Israel would not be able to withstand the crisis that would result.''
Looking back at previous West Bank au-tonomy negotiations, Kissinger suggests that moderate Arab support could have been gained if President Sadat could have pointed to his method as bringing some improvement where no one else had achieved anything. Instead the talks dragged on inconclusively.
So Kissinger says new negotiations must be concluded in, say, 12 to 18 months. ''If they grow very prolonged, a moderate position will be cumulatively difficult to sustain and the domestic support of the participating governments will erode. Reagan's initiative must yield some tangible results while the impact of the PLO's lost military option is still fresh in everybody's mind.''
These are just a few bits from an interview with plenty of points to further the Kissinger controvery. But, when both such optimism and such urgency are expressed by this particular private citizen, the public men behind the Reagan plan must be listening - and, the world must hope, mapping the next step toward those essential tangible results.