Dr. Boutros Ghali is the Egyptian minister of state for external affairs and a veteran of the hard diplomatic slogging leading to the Camp David Accords. Now high up in the foreign policy-making hierarchy of President Hosni Mubarak's new government, he is a fellow to talk to about the Palestinian issue, U.S.-Egyptian relations, Egyptian geopolitics.

But Ghali, you discover, wants to talk about Egyptian demography. And so, I suspect, will Mubarak, in his own way, when he sits down with Ronald Reagan on his first visit as Egypt's president. What the dictionary defines as "the statistical science dealing with distribution and density of populations" is unlikely to be on the agenda in quite those terms.

But it will be on Mubarak's mind for the same reason it is on Ghali's as "Egypt's greatest single problem." The crushing, smothering impact of a huge and growing population bears down on every aspect of Egyptian daily life--including, by Ghali's logic, foreign policy.

For some 3,000 years--Egypt's best, you might say--the population ranged upward from 3 million to about 7 million, a number the Nile valley could comfortably sustain. (About 96 percent of the total area of Egypt was then and remains uninhabitable desert.) At the turn of the century the population had grown to 12 million; that's about where it was when Ghali was in high school, he recalls.

Now it is a staggering 42 million, and growing at the rate of 1 million a year. Over one-quarter of the populace is crammed into Cairo, whose continuing sprawl is at the cost of critically needed arable land. An expensive effort is under way to irrigate and make fertile now-useless desert.

But for every acre of arable land thus gained, two are being lost along the banks of the Nile. Why? Because that priceless topsoil is the stuff of which bricks are made to build houses for the expanding population. It's a losing game, which is why Egypt has to spend several billion dollars a year for imports to meet over half of its food needs.

Some 40 percent of Egyptians live "below the poverty level," according to one authority. Families forced off the shrinking farmland flock to the cities, where there are not nearly enough jobs even for college graduates. Cairo is an eye-popping study in sharp contrasts: donkey carts and Mercedes, large pockets of destitution and filth a few hundred yards away from the water- skiers and eight-oared shells skimming the Nile.

The strain on such services as water, sewage, transportation and telephones is unimaginable. A recent government report estimated that employed Egyptians spend 16 percent of their time commuting and only 4 percent at work --the exact reverse of the figures for the average working American.

There are no real rush hours in this city--just one, all-day traffic jam. One reason: telephone communications are so overburdened that people find the quickest way to do business is by personal visits.

It comes down to a huge and growing surplus of Egyptians in Egypt: more than can be housed or fed, more than Egypt can live with indefinitely, even in cruel conditions.

And more, almost certainly, than even Mubarak's inward-turning preoccupation with domestic programs will be able to handle, however successful his promised priority for "economic and social stability."

Which brings us back to the connection between demography and foreign policy: at least a partial solution--a way of easing the pressure--says Ghali, would be for Egypt to "export" Egyptians to other Arab nations. It is not as far-fetched or desperate a remedy as it sounds. Already some 3 million Egyptians work overseas: 1 million in Iraq; 500,000 in the Persian Gulf; as many in hostile Libya; tens of thousands more scattered around in Sudan and elsewhere.

What has come naturally, Ghali would institutionalize, on a far greater scale. In the best of all Arab worlds, he envisages countries that are land-rich (or investment capital-rich) absorb the population surpluses of the land-poor. You can put it down now as no more than a dream. But the demographic driving force is real enough to make it one more reason why Egyptian rapprochement with the Arab world is high on the list of Mubarak's foreign policy imperatives.