A dreaming argument describes an alternative world scenario: you're in an elaborate dream, you're a brain in a vat, you're part of a computer program. On these alternative world scenarios, what appears to be an external, material world is a simulated reality. The key premise of a dreaming argument is that we have no sure criteria for distinguishing the one from the other, the material world from a simulated reality. The conclusion is that we must be sceptical: for all we know, we are in an alternative world scenario.
The most famous dreaming argument comes from René Descartes (1596–1650): ‘[I]n our sleep we regularly seem to have sensory perception of, or to imagine, countless things which do not exist anywhere’, and so ‘there seem to be no marks by means of which we can with certainty distinguish being asleep from being awake.’Footnote 1
The psychological effect of dreaming arguments is noticeably similar to the effect of conspiracy theories: suspicious mystification. This is something that I've noticed, in any case. They're repeated in hushed tones with cloak-and-dagger superiority: ‘Psst. I know something that will floor you!’ The common thread with conspiracy theories is getting wise to the deception. The real story is concealed from us by a powerful force, but the savvy person has figured it out and can say with confidence: ‘Toss out the test tubes. The world is a rainbow.’
I want to suggest that this is all wrong. The point to take from a dreaming argument is a rather dull truth about knowledge. No dope for the press.
Berkeley's Alternative World Scenario
For rhetorical power, nothing tops the dreaming argument of George Berkeley (1685–1753). Students find it deadly:
Suppose, what no one can deny possible, an intelligence, without the help of external bodies, to be affected with the same train of sensations or ideas that you are, imprinted in the same order and with like vividness in his mind. I ask whether that intelligence has not all the reason to believe the existence of corporeal substances, represented by his ideas and exciting them in his mind, that you can possibly have for believing the same thing? Of this there can be no question; which one consideration is enough to make any reasonable person suspect the strength of whatever arguments he may think himself to have for the existence of bodies without the mind.Footnote 2
Suspect their strength indeed. Even if you see through Berkeley's lousy arguments for immaterialism, this one really paints you into a corner. Throw your best punch. No good. The floating mind can throw it too. The natural response is to buckle: ‘I guess my arguments for the external world weren't so hot after all. Buddy in the dream world has all the same gear.’ A bit of a run in the stocking.
Turning the Argument Around
A strategy occurs: the ping-pong refutation. Bounce it right back at the sceptic. Let's review. Person Alpha believes that she lives in the everyday world of material objects. Person Beta lives in a dream. Put all of Alpha's evidence in a pile. How big is that pile compared to the pile of Beta? There's not a crumb of difference. Exhibit A, Exhibit B – it's all the same. Every particle of evidence that Alpha has for the existence of an external world is a particle in the world of Beta.
Show the flag? Not today, boys. What evidence does Beta have for living in a dream? Suppose Beta wants to convince his mates that the world is exactly what he believes it to be: a highly organized illusion. Put all of his evidence in a pile. You see how I'm chopping with my paddle: the same pile is available to Alpha. Ping … nice weather we're having … pong!
So what are we to make of this parity between the evidential circumstances of Alpha and the evidential circumstances of Beta? Only this: the evidence does not discriminate – logically speaking – between the world of Alpha and the world of Beta. In other words, Alpha's evidence is logically consistent with the non-existence of the external world. And Beta's evidence is logically consistent with its existence. There is thus no demonstration – in the strict, algebraic sense – that the external world exists (nor any demonstration that it doesn't). NBD. Not quite time to say uncle.
The Crux
Well folks, what's this all about? ‘Populous with wisdom’ is how my colleagues describe me, so consider this a handout. Suppose your roommate returns from the gym. She always takes a shower after lifting. The bathroom has one access and no windows. There you are with your peeps, stationed outside the lavvy. She touches down, gives notice of the plan, and bolts herself in. You hear the water and then the curtain. Is your roommate in the shower? Would you like to place a wager?
Imagine a possible world – make it a Martian classic – identical to ours in every detail but one: as she loosens the hot and cold, an invisible spacecraft beams your roommate out of the tub. She materializes on their bridge, leafless and surprised. Creepy – really creepy. Now here's the payoff. Any argument you've got that Pearl is in the shower can be published – word for word – in the world of the alien invaders. So is it reasonable to believe that your roomie's hosing off? That depends, not on the logical possibility that she's been transported to a flying saucer, but on how likely this sceptical scenario is given what else you know. It boils down to evidence. The arguments for and against the pinching of Pearl – she's in the saucer, she's in the suds – are not evidentially equivalent.
Take an old screwball of an idea like this: we cannot demonstrate that the world is a globe. That's as deep as the ocean, for sure. And what of it? Should I suppose the world is flat? Should I go easy on belief? Obviously not. I should tailor my beliefs to the evidence. (Chimp to chimp: it's round.) Likewise, it may be that the evidence for the existence of a material world is better – even much better – than the evidence against it. It could be a total walkover. The issue has not been addressed.
I've seriously got to get going, so let me shove this bird in the oven. The takeaway lesson is that logical consistency with the evidence is not the only criterion of rationality. It doesn't even begin to tell us what we ought to believe. We're talking minimal constraint on rational belief here – nothing more. That is the point of a dreaming argument: an ironclad guarantee of truth is not required for knowledge. Adiós to science if it were. False alarm, therefore. And back to your books.
