Here’s a nightmare for you. You leave your classroom, and the minute you turn your back, the desks vanish. Just as you walk back in through the classroom door, poof: desks again. And what goes for desks goes for walls and floors too; indeed at every moment, whatever you don’t perceive stops existing. Things don’t have an independent life of their own; like daydreams, they depend for their existence on you.

David Hume, the great 18th-century philosopher, didn’t believe we actually live in this nightmarish world, but he did believe that if we think about the nightmare, we’ll have to draw a dismaying conclusion about ourselves.

To see how Hume reached his conclusion, start by asking yourself what our basis is for our beliefs about the world. Hume thought it is our five senses. We see desks; we feel them when we pound them; we hear them scrape as we push them across the floor. Even our most sophisticated scientific knowledge ultimately has a basis in perception. (Think what we learn from seeing the dials and meters we encounter each day.)

But our big picture of the world—the sum of what we think we know about it—includes some pieces that sense perception just can’t have supplied. Part of our picture of the world is that the things we perceive will continue to exist when we’re not perceiving them—that their existence is independent from us. But we can’t perceive that this is so; we can’t perceive that desks and walls and floors continue to exist when we’re not perceiving them.

This means that our experience could be exactly the way it is now, and yet the nightmare could be true. But if our experience can’t tell us whether the things we perceive are independent objects, then nothing can, because our knowledge of the world ultimately rests on our sense experience.

What on earth should we do in face of this dismaying skeptical discovery?

I say we should check our work: check for flaws in our reasoning. And in one way or another, Hume’s successors—among them Kant, Hegel, and Wittgenstein—said the same, though with huge disagreements about where the flaw lies. But Hume thought his conclusion was correct, and so he had to face the question of how to live his life in light of what he’d discovered.

One answer has a nice, brave ring to it: if you have baseless beliefs, then just give them up, once and for all. What else can a self-respecting person do but abandon all belief in real desks and walls and everything else populating a world independent from our experience?

Hume did say that his own immediate reaction to his discoveries was to abandon all his beliefs: “The intense view of these manifold contradictions and imperfections … has so wrought upon me … that I … can look upon no opinion even as more probable or likely than another.”

But exhorting yourself to give up all your beliefs for the rest of your life would, for Hume, be like exhorting yourself in mid-jump to stay hovering in the air. We can jump, but powerful forces pull us back down to earth. “Nature, by an absolute and uncontrollable necessity, has determined us to judge as well as to breathe and feel.”

We can recognize the worst about our fundamental beliefs, but that won’t make them go away. So the nice, brave answer can’t tell us how to live, because we’re humanly unable to live in accordance with it. What, then, shall we do?

Maybe we just close the skeptical chapter and get on with life. Hume certainly got on with his own life, writing books and essays, working as a diplomat, cultivating friendships. Even in the Treatise of Human Nature, from which I have been quoting, he launched new investigations within just a few pages of his description of the “intense view.”

Some readers of Hume think really all he has to say about post-skeptical life is: “Get on with it.” Going further, many readers these days think that Hume never really endorsed the skeptical conclusion in the first place.

I disagree. I think Hume endorsed his negative conclusion and then sought a way to lead a life in which his skeptical discovery made a difference. And I think he found that his dismaying discovery registered, not so much in what he believed, as in how he believed.

After reaching his skeptical conclusion, he described himself as “careless” (meaning “unconcerned”) and “diffident.” He warned his reader that he would be saying things like, “’tis evident, ’tis certain, ’tis undeniable,” but he added, “I here enter a caveat against any objections, which may be offer’d on that head; and declare that such expressions were extorted from me by the present view of the object.”

In the aftermath of his skeptical discovery, Hume went on believing everything that the rest of us believe about desks and walls and all the rest, but he was somehow different: careless, diffident, full of caveats.

I think Hume was suggesting that we can develop an attitude of detachment toward our own beliefs. It’s as if we accompany each belief with the thought, “Well, I would believe that, wouldn’t I?” If that’s right, then there’s an analogy that helps explain how Hume thought radical skepticism can change life.

Suppose you’ve realized something terrible about yourself: you’re critical about people for all the wrong reasons (their clothes, their furniture), and your critical beliefs come from aspects of your nature that you just can’t change. Then two seconds after you reach this dismaying realization about yourself, you find yourself thinking what a jerk that guy over there must be if he can stand to wear a shirt like that.

It looks as though your insight has made no real difference to the way you lead your life. But what if it has enabled you to add, as you judge the man in the shirt, “Well, I would think that, wouldn’t I?” Then you’d have achieved a kind of detachment from your judgment; and if this became a reflex for you, your life might change in other ways too. You might become less huffy, or more ready to abandon speculation about what makes your own shirt superior.

Of course, Hume was talking about “all belief and reasoning,” and not just judgments in one arena of life. But as I understand him, he thought that in the wider sphere, too, we might achieve detachment from our own beliefs, even as we declared them “evident … certain … undeniable.”

And detachment might help us become less pig-headed and more ready to abandon speculation about God, the afterlife, and whatever else lies entirely beyond the bounds of sense perception. If this in turn helped us to turn away from religious fanaticism, bigotry, and enmity, then our lives would be so much the happier, Hume thought.

In 1776, knowing that he would soon die, Hume wrote an essay about his life. He described his declining health but remarked on his continued good spirits, adding, as if in explanation, “It is difficult to be more detached from life than I am at present.” I believe Hume reached a dismaying skeptical conclusion about human cognition yet found that life can be different, and happier, for knowing this sad truth.

Janet Broughton is Dean of Arts and Humanities at UC Berkeley.

This article can be found in the September 2007 newsletter.