Another in a series on getting the most out of your brain.

People use all kinds of tactics when they argue, be it about gun control or whether the soul is a substance formed in the fifth dimension. One ploy arises so often, however, that it depresses my philosopher's soul, whatever dimension it inhabits. You'll recognize it.

Someone whose position has just been skewered heads for higher ground by huffing, "Well, I'm entitled to my opinion."

This obviously doesn't improve the meeting of minds, but is it a fair statement? Doesn't everybody have a right to his or her opinions?

Frankly, it depends on what you mean.

In this country, we are legally entitled to believe anything we like, though whether we may act on all of our beliefs is rightly another matter. Everyone has a legal right to hold and express any opinion, with remarkably few constraints.

This isn't trivial. In some societies, holding certain opinions can lead to brutal consequences. Most of us, including me, find that appalling, so we might go further and say that even if the law didn't recognize it, everyone has a moral right -- a basic human right -- to believe anything at all.

Most of us in this society don't think that people should be persecuted for what they believe, but typically, when people insist on their right to opinions, they aren't being persecuted. Instead, it's just that they feel themselves losing the argument and have exhausted things to say in their defense.

We need another category -- not legal rights, not moral rights but, to concoct a phrase, "logical rights." These don't come cheaply. You acquire them only by possessing knowledge, evidence and sound judgment. Somebody lacking those has not earned the right to have his or her opinion count in the real world.

Here's an illustration. Recently, physicists reported evidence that quarks, once thought to be the smallest units of matter, actually consist of smaller parts. Other than the brief news report or two that I've read, I know nothing about the evidence and little more about quarks. Am I entitled to an opinion on this issue?

I don't think so. Of course, I shouldn't be shot if I start spouting about quarklets or whatever they might be called. I should simply be ignored. Or better, I should be reminded that I don't know what I'm talking about.

This is a clear case of somebody having no logical right to a view. I'm a college professor who once studied quantum mechanics, but I still would say that I simply don't know enough to have a basis for an opinion on this subject.

This case may be clear, but there is a sliding scale on opinion. Most people recognize that a casual opinion about whether rain fell in London July 17, 1532, is worth nothing. It's a factual matter unyielding to mere speculation.

Most people also recognize that they haven't earned the right to opinions about elementary particles or the number of irreducible representations of the four-dimensional rotation group. These matters call for specialized knowledge.

On questions dealing with people, however, caution is more likely cast aside. I've heard people who wouldn't recognize a chromosome if it belted their jeans offer firm opinions about whether homosexuality has a genetic basis. And when we discuss matters of Ultimate Significance, opinions flow like spoiled gravy. Detailed views about the innermost secrets of the universe are as cheap as eggs and nearly as sturdy.

This isn't really surprising, and it reveals an interesting tension. Our most anxious concerns are human concerns, earthly and cosmic; we can hardly resist the urge to opine. But this very anxiety should help us to see why reasons and evidence remain important.

First, what we think about earthly concerns can affect others. Our opinions on the nature of angels may not matter. But consider less esoteric questions:

Are illegal immigrants a drain on the economy? Do lenient divorce laws increase divorce rates? Will banning discrimination against homosexuals undermine the traditional family?

None of these has obvious answers, but opinions abound. People vote on the basis of these opinions. People give money to causes. People organize, and people act.

But, one might object, most of us aren't experts on issues that influence our votes, so for democracy to work, people must participate. Indeed. Presumably, however, democracy works best when people have well-considered opinions. Furthermore, some opinions are downright vicious.

If you think members of (fill in favorite suspect group) are prone to (fill in suspected evil trait), you probably will act accordingly. And if your opinion is a mere ill-founded suspicion, you are likely to increase the total of human misery for no good reason. Whatever the nature of your "right" to such opinions, it can be plainly wrong to hold them.

Second, even when it comes to secrets of the universe, we care about truth, and the loftier the matter, the less our mere guesses are worth. To the extent that we do care about truth, we need to continue caring about the credentials of our beliefs. Our logical rights serve our deeply felt ends.

Should we withhold all opinions until we know that we're right? If we did that, we would do nothing else. None of us can justify all of our beliefs. Even the best opinions are fallible, and a brilliant conjecture can be worth a dozen dull facts. Not only that, some criticisms aren't worth the trouble of a response, though distinguishing good criticism from bad often is an art in itself.

But what we can do is learn to be more aware of what we don't know. As Winston Churchill said of a "modest" political rival, we have much to be modest about when it comes to our beliefs. When we're caught short in a healthy debate, we often can do better than insist on our rhetorical rights. We can stop and think.

At least, that's my opinion.

Allen Stairs teaches logic and philosophy at the University of Maryland.