Predictions about the consequences of a country’s behavior usually take time to come to pass. The chickens don’t come home to roost from one day to the next.

Not so in the case of Gilad Shalit, the Israeli corporal held hostage in Gaza by Hamas for the past two years. Just a month ago, Israel swapped convicted a Lebanese terrorist, Samir Kuntar, who was serving a life sentence for the 1979 murder of an Israeli father and his little daughter, for the dead bodies of two Israeli soldiers, Eldad Regev and Ehud Goldwasser, abducted by Hezbollah in an incident that sparked the war of 2006.

Critics of the swap attacked it for being one more example of the disproportionate price that Israel has grown used to paying in its prisoner exchanges with Arab terrorist organizations. Dead bodies, they said, should only be paid for with dead bodies. To do otherwise would again give the signal that Israel is easy prey for extortionists — a signal that has time and again jacked up the cost of freeing the next Israelis to be bargained for. Precisely the fact that the men in question were dead (which the Israeli government knew all along, although it refused to admit as much in public), so that failure to strike a deal would not have been tragic, should have made it easy to hang tough.

The critics were ignored. The Regev and Goldwasser families had (perfectly legitimately from their point of view) done all they could to rally public opinion behind them and the government could not, or did not want to, resist the pressure. Israel, it said, owed it to its fighting men to bring them home dead or alive — and indeed, on the day the exchange was carried out, the Israeli press proclaimed that Regev and Goldwasser had “come home” as if they had walked across the border on their own two legs rather than been transported in coffins.

A month has gone by — and the Egyptian government has now let us know that its efforts to help free an Israeli corporal, Gilad Shalit, from Gaza, where he has been held since his abduction by Hamas two years ago, have failed. Has Egypt blamed Israel for this, always the easiest thing for any Arab country to do? It has — but only indirectly. The immediate problem is Hamas. Ever since the Kuntar-Regev-Goldwasser exhange, the Egyptians say, Hamas has raised its already exorbitant demands for freeing Corporal Shalit, so that an agreement that might have been possible before then is no longer so now.

Surprise, surprise!

You can’t fault Hamas’s reasoning. Gilad Shalit is alive. If two dead Israeli bodies are worth one live Arab murderer, a live Israeli body is surely worth dozens or hundreds of murderers.

If there is any good news, it is that it is not too late for Israel to redraw the line where it should go. Corporal Shalit will of course have to be freed first. Whether 500, or 1,000, or 2,000 Palestinian-Arab prisoners will be let go in return, and whether 50, or 100, or 200 of them will have “blood on their hands,” hardly matters. The price will be outrageous whatever it is. But the moment Corporal Shalit is home safe and sound, Israel’s government should issue a statement that says:

“Gentlemen, the rules have changed. From now on, there will be no more bargaining over prisoners or hostages. There will be a fixed price — and it will be one of absolute parity. For one dead Israeli, you get one dead Arab. For one live Israeli, one live Arab. For any multiple of that, you get the multiple, no more and no less.”

Were such a statement made, it would sooner or later, of course, be put to the test. One or more Israelis would again be abducted — and the release of hundreds or thousands of terrorists would once again be demanded in return for them. If it did not wish to make a permanent mockery of the country and people it represented, Israel’s government would then have to stand firm and say: “There is nothing to negotiate about. You know the price we’re ready to pay. If you want us to pay it, we will. Meanwhile, just to make sure you realize that you don’t have forever, we intend to imprison one more member of your organization for every week that it takes you to decide.”

It would never work? No terrorist organization would believe that an Israeli government could behave in such a way? No Israeli government would be allowed to behave in such way by the Israeli public?

Maybe — and in that case, no Israeli government should bother trying. But it would work if it, or something similar, were tried with any measure of determination. And once it worked the first time, it would work far quicker the next time — if, that is, there even were a next time, because abducting Israelis would soon be abandoned as an unprofitable way of doing business.

“For the sake of repairing the world, one must never ransom hostages for more than their worth,” says the Talmud. Israelis can be excused for thinking that a Jewish life is worth more than an Arab one, but when it comes to swapping prisoners, they need to learn to be more egalitarian. One-for-one is the way it should be done.

Mr. Halkin is a contributing editor of The New York Sun.