Not all of President Carter's Third World policy has crumbled in Iran; only half. It will be crucial in putting a policy back together to understand just which half.

It's the moral half: to a lot of people, the crisis has dashed an airy Carter courtship of third World that began with the post-Vietnam guilties, progressed through the psychodiplomacy of Andrew Young and confronted reality only when our Tehran embassy was invaded on Nov. 4.

Through that period, the implicit premise of policy was that American and Third World interests were indentical or complementary. Interdependence seemed not so much a necessity as a privilege, the revolution in Iran not merely an embarrassment but, on a deeper level, a deliverance. There was a scarcely hidden belief that the Third World's status as the victim of an unjust world system, and the United States' status as the system's leading manager, had put us in the Third World's moral debt.

That web of ideas and conceits now lies in ruin, or at least in shadow, consigned there by a wave of nationalistic revulsion at Ayatollah Khomeini's barbarism. From here on in, it will be awfully hard for any president to persuade the American people that the Third World is fertile soil for either our diplomatic profit or our moral regeneration.

The coincidental handoff of the Rhodesia problem from the Carter administration, which was flailing, to the new conservative government in Britain, which is succeeding, only confirms the point, Andrew Young's resignation, it may turn out, could not have been better timed: it saved him from close personal association with the final frustrations engendered in part by the earlier ascendancy of his line.

Yet the danger is that, after we Americans have had a go at laying off the blame for Iran upon each other, we will fetch up at the conclusion that there is something inherently forbidding in the Third World, a propensity for anti-Americanism, that makes it foolish to try to do business there at all. The tendency to see an intrinsic defining essence in the Third World would be the same but the essence seen would be different: not moral worthiness but political unworthiness. One sees this emerging in the talk about Iran. Campaign debate is likely to aggravate the itch.

It would be a big mistake to let this go too far. We are poorly served be any policy that plays down our own choices and emphasizes seemingly unchangeable qualities of the other guy. That's surrendering to frustration -- not so bad as waving a stick wildly but not very help either.

If it is naive to suppose that Third Worlders stand closer to God, it is self-defeating to suggests that they are animated by spirits -- revenge, envy, obscurantism, perversity, ingratitude for our past aid -- that make them ineligible as a class for a working partnership with other states. Some are, some aren't; the task of policy is to find a way.

Moreover, the other half of the Carter Third World policy is every bit as valid as it was before the roof fell in on the moral half. I refer to the administration's version of realpolitik -- the need for the Third World resources, investments, markets, votes and cooperation in everything from settling political disputes, to combating terrorism, to tending the lights in the maritime channels of the Red Sea.

Underneath its moralism this administration has always played this real-politik card hard. It has suggested that its appreciation of Third World economic power and potential made it more hardheaded than its critics, who were said to have merely the East-West political-power balance on their minds. One can argue about that, but the undisputed fact remains that world economic trends are making the Third World more important, not less to the United States. In brief, strong and enduring considerations of national interest, as traditionally defined, must still be taken into account.

In our anger, the temptation is to define the Iran problem as one of insufficient respect for the United States. The remedies flowing from this diagnosis tend to the martial - and the hysterical. As things calm down, we may be able to define the problem more broadly: insufficient Third World respect for the United States and insufficient American attention to what we need from the Third World.