Suppose I made you the following wager: "I am going to take a brick, hold it out in front of me, and release it. If the brick falls, I will give you a thousand dollars. If the brick stays suspended in midair, you will give me a thousand dollars." Would you accept the bet?
The first thing that would occur to you, of course, is that I must be trying to con you. Perhaps I will try to fool you with sleight of hand, or use wires or glue or magnets to keep the brick from falling. Perhaps I might even set the brick on a table, and appeal to some lame semantic quibble about what "suspended in midair" means. But let's suppose there is no trick, and that you can know this for sure; let's suppose I am actually betting you that gravity simply will cease to affect the brick the moment I release it, and that it literally will hover in midair in front of the two of us, no tricks involved.
Well, if that's what I proposed, of course you would take the bet. You would take it even if I bet only ten dollars against your thousand: what an easy, risk-free way to make ten dollars!
But your response raises a problem: what justifies your confidence?
Fine! you say. If you don't think my confidence is justified, go ahead and bet against me!
But that doesn't answer the question. I feel the same level of confidence as you do. I would never, in actuality, make such a bet against you. But this agreement in feeling does nothing, in itself, to justify our confidence.
Let's grant, at least, that absolute confidence is unreasonable. Ask yourself whether it is possible, for all you know, that gravity could cease to apply to the brick the moment I release it?
But, you ask, why would it?
But that's not the question. No one is claiming that there is good reason to think it will. The question is, is it possible, for all you know? And the answer to that, surely, is yes. You don't think it will happen, but nothing you are aware of rules out the bare possibility.
But no one has ever seen something like that happen!
No, indeed not. Let's even go further: let's grant—what in practice cannot be verified—that it actually never has happened. Even this does not prove that it won't happen the next time. Maybe such events are so rare that they occur only once every twenty billion years.
But the laws of nature forbid something like that from happening!
It may be that our current best account of the laws of nature forbid the local suspension of gravity, but how do we know that this account reflects the actual laws of nature? Isn't our best account based upon generalization from what we have seen? If this is so, then appealing to the laws of nature is just a roundabout way of saying that no one has ever seen something like this happen—which we just realized that is not a good reason for saying that it never will happen. For all we know, the true laws of nature allow for occasional deviations from the usual way things happen. In fact, for all we know, there could be a built-in time limit to all of the actual laws of nature, so that everything will change tomorrow.
But there are mechanisms underlying the laws of nature, that make them work!
Let's suppose we have access to these mechanisms. How do we know the mechanisms will continue to work the same in the future? Suppose, for instance, that we demonstrate that gravity is driven by the exchange of gluons. How do we know that gluon exchange will not, all of the sudden, have a different effect on the brick I am holding once I release it? Presumably, we have to appeal to the alleged constancy of the laws of nature, at the gluon level, and this ultimately brings us back to square one.
But this kind of reasoning has always worked for me in the past!
Has it really? Have your expectations about the future, based upon rigid patterns in the past, never turned out wrong? But put that aside. Even if that kind of reasoning has always worked for you in the past, to try to justify its future application on the basis of past success is to argue in a circle: you're still extrapolating from the past to the future. Since the legitimacy of such extrapolation is precisely what's at issue here in the first place, you can't make this move.
It seems, at this point, that we have no way to justify absolute confidence that, for instance, the brick I release actually will fall.
Fine! But we can still reasonably be confident, even if we can't reasonably be sure.
Is this true? Again, it is not that I would bet against you; I am just as confident as you are. The question is, what makes our confidence reasonable? Our reasoning seems to go back to the same set of reasons we gave above. But none of those reasons underwrites anything about the future, with any probability.
But one can't live one's life like that, believing it just as likely as not that gravity will cease to work in the next minute!
One probably can't. But, again, that doesn't answer the question. David Hume, who forcefully presented this problem (now known as the problem of induction), agreed that human nature compels us both to generalize from our experiences in certain ways, and to have confidence in those generalizations, the moment we set aside our philosopher's caps. This makes life livable despite the skeptical riddles of philosophy, but Hume also realized that it no more resolves these problems than closing your eyes makes the world disappear.
It appears that we have no reason, on the basis of evidence, to believe, with any confidence, that the brick I release will not hover in the air. This is insane. Surely, there must be a solution, mustn't there?
Last updated: 10 Apr 2010
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