Radical skepticism

Young Earth creationists believe that the earth was created in the past ten thousand years. According to their so-called Omphalos Hypothesis, dinosaur bones and other seemingly-old artifacts were planted there by God as a test of our faith.

As ludicrous as this may strike you, it contains no logical contradiction. In fact, there is no way to disprove that the universe sprang into existence, fully formed, last Thursday. Mathematician-philosopher Bertrand Russell takes it one step further (as mathematician-philosophers are wont to do):

There is no logical impossibility in the hypothesis that the world sprang into being five minutes ago, exactly as it then was, with a population that 'remembered' a wholly unreal past.

But why stop there? For all we know, this may all have appeared freshly in this very moment. While such a claim is not logically impossible, it is certainly very improbable. Right?

Well, how would we evaluate its likelihood?

First, basic physics tells us that fully-formed worlds are very unlikely to just pop into existence. Therefore we should a priori be very biased against this possibility, right? Unfortunately not: we could only have learned about physics in the "real past," which we cannot trust without resorting to circular reasoning.

Okay, what about Occam's Razor? It tells us that we should prefer simpler theories to needlessly complicated ones. But this is a statement about preference, not likelihood. Moreover, it runs into the same problem as before: when, exactly, did we collect evidence that justifies the Razor?

Fine. Suppose you find a picture of something that took place yesterday. Is this evidence of a real past? No: for all you know, it sprang into being just now. In fact, anything that you might use as physical evidence falls prey to this problem. Our sudden-world hypothesis was designed that way.

A final attempt might be to say fine, let's accept that maybe the world sprang into being five minutes ago (or whenever you began this exercise). But since then, you've been collecting evidence that justifies your trust in physics, Occam's Razor, and the rest. This ought to restore your faith, right?

But notice that you can repeat the thought experiment right now. How do you know that this is not the first moment? When you try to work out the answer, you will find that your supposed "evidence from the past five minutes" gets tossed out just like our supposed "picture from yesterday." So this approach fails, too. There's nowhere to get a grip!

Strange as it may seem, we are not rationally justified in saying that a real past is "more likely" than a fake one. It certainly feels like the real past is more rational, but this is an illusion. Here's Caltech physicist and arch-skeptic Sean Carroll on a similar problem:

There is no way to distinguish between the scenarios by collecting new data.

What we’re left with is our choice of prior credences. We’re allowed to pick priors however we want—and every possibility should get some nonzero number. But it’s okay to set our prior credence in radically skeptical scenarios at very low values, and attach higher prior credence to the straightforwardly realistic possibilities.

Radical skepticism is less useful to us; it gives us no way to go through life. All of our purported knowledge [...] might very well be tricks being played on us. But what then? We cannot actually act on such a belief [...]. Whereas, if we take the world roughly at face value, we have a way of moving forward. There are things we want to do, questions we want to answer, and strategies for making them happen. We have every right to give high credence to views of the world that are productive and fruitful, in preference to those that would leave us paralyzed with ennui.

In other words: since evidence cannot help us, we must rely on prior credences—i.e., our a priori assumptions, which are fundamentally untestable. Don't be fooled by the description of some scenarios as "straightforwardly realistic," either: having just proven that there's no way to determine what is real, the word "realistic" cannot refer to anything other than a feeling[1].

Russell came to a similar conclusion:

Skepticism, while logically impeccable, is psychologically impossible, and there is an element of frivolous insincerity in any philosophy which pretends to accept it.

Now let's visit another oddity. 18th-century philosopher David Hume popularized something called the Problem of Induction. Roughly, it goes as follows.

It seems painfully obvious that the past provides good evidence for the future, right? But why do we believe this? Well, it has certainly been true in the past. Okay, but so what? We'd like to continue "... and therefore it will be true in the future," but then we are assuming what we set out to prove. There's that nasty circularity again! So we cannot justify our belief that the past will continue to provide evidence for the future.

Thus, the fact that the laws of physics have faithfully operated for billions of years gives us no reason at all to believe that they will continue to operate even one second from now. This is a very hard thing for beginning philosophy students to accept:

http://stephenlaw.blogspot.com/2012/10/problem-of-induction-explained-simply.html

It’s a good test of whether someone has actually understood Hume’s argument that they acknowledge its conclusion is fantastic (many students new to philosophy misinterpret Hume: they think his conclusion is merely that we cannot be certain what will happen tomorrow.) ... [But] if Hume is right, the belief that the sun will rise tomorrow is as unjustified as the belief that a million mile wide bowl of tulips will appear over the horizon instead. We suppose the second belief is insane. But if Hume is correct, the first belief is actually no more rational. ...

[T]he onus is on these defenders of “common sense” to show precisely what is wrong with Hume's argument. No one has yet succeeded in doing this (or at least no one has succeeded in convincing a majority of philosophers that they have done so).

As before, notice how ridiculous and illogical this feels, despite being impeccably rational. This is a powerful clue, if wielded properly! Unfortunately Hume, too, loses his nerve before fully taking on board the implications:

Should it be asked me whether I sincerely assent to this argument which I have been to such pains to inculcate, whether I be really one of those skeptics who hold that everything is uncertain, I should reply that neither I nor any other person was ever sincerely and constantly of that opinion. I dine, I play backgammon, I converse and am merry with my friends and when after three or four hours of amusement I would return to these speculations, they appear so cold and strange and ridiculous that I cannot find in my heart to enter into them any further. Thus the skeptic still continues to reason and believe, though he asserts he cannot defend his reason by reason.

Once again: "it may be true, but it's too painful to take seriously, so let's not." This is fine for normal life, but it is deeply unhelpful if your goal is to awaken.

[1] Professor Carroll's preference here is physicalism. He also calls dresses it up as "poetic naturalism," and reminds his readers that they, too, can choose this as their religion:

"In particular, there is no supernatural world -- no gods, no spirits, no transcendent meanings. [...] Facebook will allow you to declare [this] as your religion."