Both sides exaggerate, of course. And in any event, whatever doubts one has about an iron curtain that would screen out the illegals, it's hard to doubt that an iron curtain is there in Congress that says, DO NOT STOP ILLEGALS, UNDER PENALTY OF LAW (NO DETENGA ILEGALES DEBAJO LA LEY). There are about 7 million illegals. Using figures for legal immigrants and extrapolating them, we deduce that one-half are Mexicans, the other half shared by Indians, Chinese, Filipinos and Vietnamese. The big question reduces almost always to Hispanic immigration.

Why can't we enforce the law? The INS analyst has a point, that the long, permeable boundary does not permit a wall, plus checkpoints, as in divided Berlin or the West Bank. Such analysts are prepared to treat immigration law as in the nature of Prohibition: You can't enforce it.

Next is the argument that U.S. employers would not tolerate genuine sanctions. "Si monumentum requiris, circumspice," says the inscription near the tomb of Christopher Wren, the l7th-century architect whose splendid achievements include St. Paul's Cathedral, where he is buried. If you want a monument to Wren, all you have to do is look about. That translates, in modern idiom: If you wonder where illegals are working, look about. They are everywhere.

Some commentators tell us that if illegals were actually shipped home, there would be nobody here to till the fields or sweep out the offices -- i.e., that illegals are necessary to the functioning of our economy by acceptable measures. Sure, if we were starving to death and there were no Mexicans to do the agricultural work, it would still get done, but a head of lettuce might cost $30, if tended by Ivy League graduates.

The next vantage point is of course political. Theoretically, illegals do not vote. In fact, many do. And they certainly influence the voting patterns of resident legal immigrants, whose vote is very heavy in California, New York, Florida, Texas, New Jersey and Illinois. They tend to vote in blocs, and are sensitive to any proposed legislation that they deem xenophobic in spirit.

Finally, there is the allied question: How do you treat illegals once they have got into the country? This is not by any means disposed of by saying simply that illegals are non-persons. You run into moral-philosophical questions: If the pregnant lady coming to term is an illegal, is there a computer mechanism by which that petitioner can be ignored, or shipped quickly to a Mexican-bound airline? If the child is hungry, is he fed?

Now these complications are arresting, when thus stated. The increase in the Mexican-illegal school population in California is marked, and getting the child and parent aboard a southbound bus is a doable thing, but it is also a work-intensive bureaucratic proceeding, involving lawyers, immigration police, transportation requisitions. A slightly removed way of putting on that kind of pressure is to harass the illegals. That is happening in California by use of the supreme sanction in the civilized world. Withholding a driver's license comes just this side of dumping the offender into a pit of hissing cobras.

The primary use of the driver's license, obviously, is to permit that which almost all Americans absolutely require. But that license does not bring only mobility; it is a key instrument for banks, credits cards, insurance and voter registration. Governor-elect Schwarznegger is committed to withdrawing driver's licenses from illegals. What will they do? Return to Mexico? That is unlikely. And if pressures mount, sufficient to repatriate a half-million Mexicans, can we simultaneously keep another half-million from seeping in?

A market pressure not mentioned among possible sanctions is the tax. Suppose an employer were taxed $5 for every hour an illegal put in? If enforced, that would hugely change the economic picture, requiring employers of illegals to pay much more -- and to charge more for their services.

Would the U.S. public then vote to repeal such a tax? We'd see how far the rectifications of the market would actually take us.