How do you know you are not now dreaming?
Any test you might perform, you might be merely dreaming that you are performing.
How can you get outside experience to verify that things are, at least once and for all, the way they seem to be?
This is philosophical skepticism in the potent and daring form that comes down to us from René Descartes.
But there are other equally challenging styles of skeptical argument. Take the whole question of other minds. Each of us knows his or her own mind. We experience. We judge. We perceive. We want. But the minds of others? How can we ever know how things are, internally, with another. Her experience? His feelings? His intentions? These are cut off from us, hidden, as they are, behind a veil of mere behavior, or mere words (which is just a fancy variety of behavior).
That skepticism is not alone the concern of academic philosophy is demonstrated by the steady stream of skeptical storylines in our most popular movies. In Blade Runner, for example, we are asked to take seriously the thought that so-called replicants lack thought and feeling, lack personality and worth, and for the simple reason that they are artificial. But what the movie drives home is the fact that the basis of our belief that replicants think and feel is no better and, crucially, no worse, than our confidence that any of our friends or loved ones really think and feel and experience.
And The Matrix simply plays a variation on one of Descartes' skeptical themes — that we are induced to hallucinate a false reality by an evil manipulator bent on deceiving us.
Skeptical worries are not confined to science fiction. In Angel Heart, we experience the world through the eyes of a fundamentally unreliable first-person guide; and in Gaslight it is the person we think we know and trust the most that, it eventually turns out, is bent on our brutal exploitation and injury.
Despite the vibrancy of skeptical ideas and themes in popular culture, as well as philosophy, it is almost universally acknowledged that the practical implications of skepticism are utterly negligible.
We may not have sufficient reason to believe in the minds of others. But you'd have to be mad to seriously entertain the possibility that your loved ones are not, like you, really conscious beings.
And if you really thought that the existence of a world outside your head, altogether, were, at least for all we know, a mere figment, it is hard to see how you could ever carry on.
And anyway, as Scottish philosopher David Hume said, even the skeptical philosopher leaves his or her skepticism behind when he or she leaves the study. Skepticism is a live rational possibility, but not a real practical one.
I thought of this relation in connection with the predicament facing this country. It's news from nowhere to be reminded that there is a deep political divide in America today. Many people I know are anxious. We watch lips move. We hear the words. But it doesn't add up. It is hard to really even comprehend the thoughts or feelings of people on the other side. It is as if our political culture has gone the way of skepticism.
Our alienation from each other is so great that we stand to each other in something like the ways that skeptical philosophers have always supposed: We are confined to the outward marks, the mere behavior — and that's just not enough to know another, to really understand him or her, or to trust.
Real skepticism is a dangerous proposition. If we lack the common ground even to know each other, then what are the prospects of peaceful resolution of our conflicts?
Alva Noë is a philosopher at the University of California, Berkeley, where he writes and teaches about perception, consciousness and art. He is the author of several books, including his latest, Strange Tools: Art and Human Nature (Farrar, Straus and Giroux, 2015). You can keep up with more of what Alva is thinking on Facebook and on Twitter: @alvanoe