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David Lewis, equivalent worlds and knowledge


To: Kristian D.
From: Geoffrey Klempner
Subject: David Lewis, equivalent worlds and knowledge
Date: 16th November 2011 11:31

Dear Kristian,

Thank you for your email of 8 November, with your original essay, 'Equivalent Worlds and Knowledge' for the University of London BA Epistemology module.

You've really thrown down the gauntlet with this effort, which leaves me in no doubt that you are capable of gaining a 1st class honours degree. However, I think that you will need to temper your enthusiasm with a dose of common sense. It is rather easy to be carried away by the beauty of an idea or a theory, to the extent that one loses ones grip on plain old boring reality :-)

First off: the wild 'metaphysical' cases such as Russell's five minute world (commemorated at -- the image is the grave of my mother's first husband who died in WWII, assuming that the universe is older than 5 minutes) or brains in vats etc. are not the only challenge to the closure principle. It's much easier than you seem to credit to question closure, as David Lewis in his contextual account of knowledge has amply demonstrated.

In my Answer to Demetreus I offer the following example:
'Is Bob cheating on Sue?' 'Yes, I saw him together with Mary.' 'Does Bob have a twin brother in Australia?' 'Search me if I know.' 'If Bob had a twin brother, wouldn't it be possible that it was his twin brother on a visit from Australia you saw with Mary?' 'Yes, I suppose so.' 'In that case, would you like to revise your statement?'
All it takes is a question. The question doesn't have to be, 'How do you know you haven't eaten 14 Pound. of hallucinogenic salmon?', or 'How do you know you're not a brain in a vat?'

OK, let's just look at the metaphysical cases and forget closure. I can see why you're tempted by the idea that when I say, e.g., 'Max Mann died in WWII', that is kind-of true even if the world is only 5 minutes old. I can visit the grave. There are dusty documents stored somewhere in an archive. Everything I will ever experience in regard to the question of the unfortunate death of Max Mann will be just as it is regardless of whether a mischievous demon is playing tricks on us or not.

There's actually a similar -- and really metaphysical -- point to be made here regarding theories which are truly non-empirical (as this is not, as you concede, since the mischievous demon knows the truth). Berkeley in his theory of immaterialism claimed to be defending common sense, which seems crazy but in a Berkeleian world everything is as you would expect it to be from an empirical standpoint, and all that's missing is Cartesian doubt about an 'external world'.

In between the truly metaphysical case, where arguably knowledge claims are unaffected (I know there's a tree in the quad even if 'the tree the quad' is just an idea in God's mind) and the example of Bob, Sue and Mary, there are various levels of generality, in some of which, depending on the reach of our powers of investigation, the truth will never be known. However, even if we take your quasi-metaphysical cases, I don't think that it is true that we just don't care about the 'real' truth (as opposed to the experiential 'truth').

Let's say that there's a clever conspiracy which I will never know anything about (because it's so secret and clever) to mock me and make fun of me behind my back, while all the people I meet face to face or communicate with by email show me the greatest respect. I glow in the admiration of my students and peers, while in reality I am despised. Or let's say I live my entire life believing that my wife adores me when in reality she has enjoyed an long-running affair with my best friend and only married me for my money.

I don't want the experience of being respected and admired, or the experience of being loved. I want the real thing. The fact that the example has been set up in such a way that I will never discover the truth does not alter that fact. As Aristotle would claim, I am not a 'happy' man even though I sincerely believe myself to be happy. The test is how I would think if I did happen to stumble upon the truth.

It occurs to me that a generous interpretation of your view of knowledge might put it quite close to Lewis's view, and also to the kinds of claims that the pragmatists (such as James) made about truth. I'm not ruling out entirely that there could be a useful theory here about the precise 'meaning' of knowledge claims, how some claims to knowledge are very much like tools which we use to deal with the world, while others have layers of added 'meaning'.

The question that is really calling out to be answered here is why we have a term for 'knowledge'. Maybe there would be sufficient justification to coin hybrid semi-factive propositional attitudes with built in elasticity to cope with the kinds of cases which you describe. Then I could say, with perfect accuracy, say, that I Gnow that Max Mann died in WWII regardless of whether an evil demon is playing tricks on me or not.

All the best,