Drive by the world's greatest medical research facility, the National Institutes of Health in Bethesda, and you see trouble. The bumper stickers say "Liberate Laboratory Animals," and the signs say "Honk for the Silver Spring Monkeys." The animal liberation front has arrived.

The issue is no longer freeing 15 monkeys from a Silver Spring lab, where, for the sake of understanding strokes in humans, they were subjected to brain-damaging injuries. That fight was won years ago. The monkeys were freed and the research terminated.

This fight is over the proper rest home for these monkeys. NIH first housed them in cages in Poolesville. Under pressure, it then sent them to a more open environment in Louisiana. People for the Ethical Treatment of Animals demands that the monkeys live out their days at a "primate park" (Primarily Primates, Inc.) in Texas. One demonstrator -- "prepared to die" -- went on a 64-day hunger strike over the issue.

I am no expert on retirement homes for monkeys, but this seems to me to be going a bit far. But then again, the animal rights crusade is about going far. Its soldiers want not just to stop the abuse of animals in the laboratory. They want to stop the use of animals in the laboratory. That is what "Liberate Laboratory Animals" means. If they had their way the labs would be shut down, the animals freed, and -- a side effect -- medical science devastated.

The animal rights cause is (I apologize in advance to the mailman for the extra load he will have to carry on account of this paragraph) a form of fanaticism. It places one value, admittedly an important one, above all others. But there are values other than the prevention of animal suffering. One of these is the prevention of human suffering through medical advances, many of which rest indispensably on animal experimentation.

And yet we owe these fanatics a great debt. A scientific lab is a place of romance and power. Even for the best-intentioned, it is an easy place to forget about the value of lesser creatures. The extremists have had a salutary effect. If you work in an animal lab you know that they are outside demonstrating and sometimes inside infiltrating. (That is how the Silver Spring monkeys were discovered.) It makes you doubly careful about how you treat your animals.

"Extremism in the defense of liberty is no vice," said Barry Goldwater in 1964. It will probably be his epitaph, and it is certainly wrong. Extremism in the defense of anything is a vice. A personal vice. Yet for a society, the presence of extremism -- or rather, a mass of contending extremisms -- is a virtue. It helps produce a moderation that would otherwise be impossible.

Madison, of course, was the great theorizer of such a system of contending factions. ("By a faction I understand a number of citizens . . . actuated by some common impulse of passion, or of interest, adverse to the rights of other citizens, or to the permanent and aggregate interests of the community.") As he thought, factions do indeed constrain and moderate each other, and restrain the headlong plunges of majority enthusiasms.

Enthusiasm for nuclear power, for example. Chernobyl is the latest example of Madison's wisdom. Societies where contending extremisms -- i.e., pluralism -- are not permitted are subject to catastrophic headlong plunges, such as the Soviets' crash nuclear power program. Where the nuclear imperative is not constrained by busloads of Diablo Canyon antinuclear fanatics, safety can be ignored.

During Chernobyl, there was much self-congratulation here about American nuclear safety. Yet that was not something that those who believeju (as I do) in the nuclear imperative could take credit for. Credit was due anti-nuclear fanatics, who have argued and demonstrated and litigated and cajoled this society into nuclear safety.

Of course, if they had their way, we would have not safer nuclear power, but no nuclear power. It would be disastrous if they ever won. It would be only mildly less disastrous if they went home. The paradox for a pluralist society is that extremists must be resisted, while at the same time welcomed, even celebrated.

The same is true, for example, of the antipornography fanatics, feminist and bluenose alike. They are a threat to free speech. And yet their critique of pornography (particularly the feminist critique) is a valuable one. It makes us rethink perhaps not what ought to be legally permitted but what ought to be socially sanctioned.

The point is that an extremist is the last person to whom you want to give power, but the first to whom you might want to give the floor. Such is the project of political pluralism. Every fanatic -- whether for monkeys or motherhood -- is granted the power of petition and contention. The result is a brilliant scheme for harnessing the energy that lies at the political extremes and deflecting it to produce, paradoxically, a moderating effect.

Banning experimentation or nuclear power or pornography are terrible ideas. But we need the extremists who believe in them. They keep the rest of us honest.