Tuesday, March 19, 2013

Hans Reichenbach's Cubical World and Elliott Sober's Beach

In his 1938 book, Hans Reichenbach imagines a "cubical world" whose inhabitants are prevented from approaching the sides. Outside the world, birds fly, whose silhouettes show on the translucent ceiling of the world. A "friendly ghost" has arranged lights and mirrors so that identical silhouettes also appear on one wall of the world, so that any time a silhouette on the ceiling flaps its wings so also, in perfect correspondence, a silhouette on the wall flaps its wings, etc.

Here's Reichenbach's diagram:


The inhabitants of this world, Reichenbach says, will eventually come to infer that something exists beyond the cubical boundary that causes the shadows on the ceiling and wall. So also likewise, he says, can we infer, from the patterns of relationship among our experiences, that something exists beyond those experiences, causing them.

It is crucial to Reichenbach's argument that the inhabitants of this world ("cubists", let's call them) infer the existence of something beyond the walls that is the common cause of the pairs of corresponding silhouettes. If the cubists could reasonably believe that only the shadows existed, with laws of relation among them, no external world would follow; and so correspondingly in the experiential case there might only be laws of relationship among our experiences with no external common cause beyond.

Unfortunately, it's obscure why Reichenbach thinks the cubists couldn't instead reach the conclusion that the shadows on the ceiling directly affect the shadows on the wall or vice versa, e.g., by the transmission of invisible and unblockable waves through the interior of the cube or simply by action at a distance. (In Reichenbach's mirror set-up, height has no influence on the bird's ceiling position but it does influence position on the wall; and the reverse holds for horizontal position; but direct-causers can posit hidden-variable explanations or similar.) Reichenbach addresses this worry with a single sentence: Within the confines of cubical world, he says, the cubists will have found that "Whenever there were corresponding shadow-figures like spots on the screen, there was in addition, a third body with independent existence", so they'll reasonably regard it as likely that the same is true on their walls (p. 123).

There are two serious problems with this response. First, it cannot be straightforwardly adapted to the sensory-experience/external-world case, which is of course the real aim of Reichenbach's argument. Second, it is false anyway: We can readily construct cases where one spot on a screen causes another on a separate screen without a common cause behind them, e.g. by using a mirror to reflect light from one screen onto another or by making the first screen sufficiently translucent and staging the second screen directly behind it; this is no less natural than the friendly ghost's arrangement.

In a 2011 article, Elliott Sober (the Reichenbach professor at UW-Madison!) notices the weaknesses in Reichenbach's argument and offers a new approach in its place. Call it Sober's beach.

Sober imagines sitting on the beach, noticing the correlation between visual experiences of waves breaking on the beach and auditory experiences of crashing waves. The two types of experience cannot be related as cause and effect because he can stop one while the other continues: When he closes his eyes he still hears the crashing; when he stops his ears he still sees the breakers. Presumably, then, there's a common cause of both.

So far, so good. But to establish an external world beyond the realm of experience, we must establish that this common cause is something outside the realm of experience. Sober responds to this concern by considering one solipsistic alternative: the intention to go to the beach. He then argues that this intention cannot serve as an adequate common cause because the visual and auditory experiences are correlated beyond what would follow simply from taking the intention into account. So he challenges the solipsist to produce a more adequate common cause. He suggests that this challenge cannot be met.

But it can be met! Or so I think. The common cause could be my first beach-like experience. This experience, whether auditory or visual or both, then causes subsequent beach-like experiences. That takes care of the correlation. If I have an experience as of closing my eyes, the auditory experience at time 1 causes the auditory experience at time 2 and also the visual experience at time 2 conditionally upon my having an experience as of opening my eyes; analogously if I stop my ears. The solipsist can either play this out with the first experience causing all the subsequent ones until conditions change, or she can have each experience cause the next in a chain. On the chaining version, if I have my eyes and ears simultaneously closed, my opinion that I will soon have beach-like experiences then does the causal work. (There are imperfections in these regularities, of course, e.g., I might seem to myself to have booted up an an audiorecording of waves, but to take advantage of those imperfections is contrary to the spirit of the toy example and would cause trouble for Sober's model too.)

I agree in spirit with what Reichenbach and Sober are trying to do -- and Bertrand Russell, and Jonathan Vogel. The most reasonable explanation of the patterns in my experience is that there is an external world behind those experiences. But the argument isn't quite as easy as it looks! That's why you need to read Alan Moore's and my paper on the topic. (Or for short blog-post versions of our arguments, see here, here, and here.)

Revised 4:42 pm.

13 comments:

D said...

The most satisfying answer to this problem to me was in an essay by David Chalmers. He essentially said, it may be the case that the illusion of the external world is created by subconcious processes in the mind, or an evil demon, or a computer simulation, or what have you. But the complexity of interactions in whatever it is we observe are real, and they're the only basis we have for defining the term "external world" anyway. So whatever the actual nature of the world, it can correctly be referred to as an external world.

Eric Schwitzgebel said...

Thanks for the comment, D! I'm not trying to address Chalmers here. I do think there is promise in his approach, but I think it goes too far to think that radical skepticism is thereby refuted. More on this in a future post, I expect!

D said...

I see from reading your paper that I am taking for granted that my memory is pretty much reliable, that what seems to be logically valid to me really is logically valid, and other aspects of my own person are what I believe them to be.

Eric Schwitzgebel said...

Yep! Alan and I are taking on solipsism specifically, rather than the entirety of skepticism. We have that in common with Reichenbach and Sober. Chalmers has a somewhat different target.

Arnold Trehub said...

In your philosophical argument, Eric, is the inviolability of logical inference taken to be a property of nature, or is it taken to be something apart from nature?

Eric Schwitzgebel said...

Arnold, I am utterly torn about that question! For these arguments, I am only assuming (I think!) that logical inference is justified, on some grounds or other.

Arnold Trehub said...

It seems to me that the opinions of the solipsist can only be discounted on pragmatic grounds, not in terms of the logic of solipsism. Science, not philosophy is decisive. What do you think?

Eric Schwitzgebel said...

I agree that once you've found yourself in the solipsistic quandary, getting out is better treated as a matter of good scientific judgment rather than as a matter of finding proofs with the force of deductive certainty. I'm not sure whether this makes it "pragmatic", though. Maybe it does, in a sense, but the term has so many different uses.

Scott Bakker said...

The thing I always find so interesting about these kinds of exercises is the original attribution of *activity* to the subject. Solipsism is not a 'natural' assumption, but only comes about as the result of a certain kind of metacognitive deliberation on experience that makes it seem 'natural' subsequently - namely the intuition that everything is somehow a product of 'me.' Why me? Why not 'it'? The skepticism that drives it doesn't actually seem radical at all when you begin asking these kinds of questions. The internal becomes as difficult to 'know' as the external.

This is what Arnold's question touches on, the 'access' as opposed to the 'phenomena' side of the equation. Have you ever tried tackling the issue from this general angle?

Eric Schwitzgebel said...

@ Scott: Once one makes the somewhat unnatural reflective move of thinking that what I experience is some combination of "me" and "it", then the question arises how much "me" vs. "it" -- and solipsism is the radical answer on the "me" side. One thought that both flows from and enables the shift toward "me" is the possibility that there are many unknown aspects to me (un-"accessed", maybe inaccessible). All this fits, I think, with what you're saying, Scott?

Scott Bakker said...

@ Eric. So a kind of 'other' me? Only somehow more in charge because it seems to 'come before' I do? This seems to get us part way to Berkeley at least! Or better yet, Freud. Neurotic solipsism...

Eric Schwitzgebel said...

Indeed! Actually, I think that's where solipsism breaks: From a solipsistic point of view, it's difficult to justify thinking of such non-conscious regulators of my experience as part of "me". This is my planned next step with Alan Moore, if we get our proof of the external world off the ground.

Scott Bakker said...

How does Sartre's cogito go again? 'I think, therefore I *was*' ... I think.(It's sad when you're too lazy to even google.) Anyway, maybe all you need is a stopwatch. The solipsism of the present seems to be obviously incoherent. How hard is it to go from an 'external time' to an external world?