At the moment there is a huge argument going on about the cause of AIDS. Most people in this country -- but by no means all -- believe that the disease is caused by a virus known as HIV, the Human Immunodeficiency Virus. In Africa, however, heads of governments have flatly stated that they don't accept this. They blame a variety of other factors, from diet to climate to genetic disposition.

The available scientific evidence ought to be the same for everyone. So how can there be such vast differences in what people believe?

Part of the reason is what we might call the "Clever Hans" effect. Clever Hans was a horse who lived in Germany early in the twentieth century, and he seemed to be smarter than many of the humans around him. He could answer arithmetic problems by tapping out the correct answers with a fore-hoof, and give yes or no answer to other questions -- Is London the capital of France? -- by shaking or nodding his head, just like a human.

His owner, a respected Berliner named Wilhelm von Osten, was as astonished as anyone by Clever Hans' abilities. There seemed no way that he would commit fraud, particularly since Clever Hans could often provide correct answers when von Osten was out of the room, or even in a different town. The Prussian Academy of Sciences sent an investigating committee, and they too were at first amazed by the horse's powers. True, there were inconsistencies in the level of performance, but those could often be explained away.

Finally, almost reluctantly, the truth was discovered. Clever Hans could not do arithmetic, and did not know geography and history. He was responding to the body language of the audience. Most observers, including members of the investigating committee, wanted Hans to get the right answers. So they would instinctively tense at the question, and relax when Hans gave the right answer. The body movements were very subtle, but not too subtle for Hans. He really was clever -- clever at reading non- verbal cues from the humans around him.

We are no different from the groups who met Clever Hans. We all want certain answers to be true. Given a mass of evidence, we tend to notice the facts that agree with our preferences, while explaining away the inconvenient ones that would tell us otherwise. And AIDS is a disease so complex and so widespread that you can find what appear to be exceptions to any general rules about its cause, spread, or inevitable effects.

That, however, is only half the story. The other reason there can be such intense arguments about AIDS applies equally well to half the things -- or maybe today its ninety-nine percent of the things -- in our lives. We have actual experience in certain areas: boiling water hurts; you can jump off a ten-foot ladder but you can't jump back up; the moon will be full about once a month; it's colder in winter than in summer; coffee with salt instead of sugar tastes terrible.

But there are a million other things in everyday life for which we have no direct experience and explanation. Can you tell me how a digital watch works? Why is a tetanus shot effective for ten years, while even with an annual flu shot you are still likely to get the flu? What does that computer of yours do when you switch it on? How does e-mail from your computer travel across the country to a friend on the opposite coast, or halfway around the world? Just what is plastic, and how is it made? How does your refrigerator work? When you flip a light switch, where does the electricity come from? It's not like turning on a faucet, where we know that somewhere a huge reservoir of water sits waiting to be tapped. So how come the electricity is there just when you need it?

I can give answers to these, in a hand-waving sort of fashion, but if I want any sort of details I have to go and ask questions of specialists whom I trust. And most of the questions that I've just asked are not new, or even close to new. The refrigerator was patented in 1834. The first plastics, like our electricity supply, go back to the beginning of the twentieth century.

Good answers are available to every one of my questions, all we have to do is seek them out. But what about the newer areas of research, for which AIDS forms a fine example? When the experts themselves are still groping their way toward understanding, and still disagreeing with each other, what chance do the rest of us have?

Not much, provided that we insist on direct evidence. Every one of us must decide for ourselves who and what to believe. We, like the audience of Clever Hans, are going to believe what we want to believe until evidence to the contrary becomes awfully strong.

And maybe even after that. We, as ornery humans, tend to go on believing what we prefer to believe.

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