To: Alex V.
From: Geoffrey Klempner
Subject: Can you know that you are not a brain in a vat?
Date: 20th November 2008 11:26

Dear Alex,

Thank you for your email of 10 November, with your University of London BA Epistemology essay, in response to the question, 'Can you know that you are not a brain in a vat?'

This is a very good essay which raises a number of points which have the potential to at least cause consternation in the philosophical sceptic. However, as an answer to this question (supposing that the question came up in an exam) it would lose some marks because, having recognized that the discussion of the 'brain in a vat' hypothesis originates with Putnam, you haven't made any attempt to reckon with Putnam's arguments: which in my judgement was the main intention of the question.

I won't rehearse Putnam's argument in detail. Putnam seeks to exploit the idea that the meanings of concepts -- like 'brain' and 'vat' -- themselves presuppose an external reference. So that if you WERE a brain in a vat, it would be impossible (semantically speaking) to assert the true statement, 'I am a brain in a vat.' The interesting question, apart from the question whether Putnam is right to make this claim, is whether it follows, purely from semantic considerations, that I know that I am not a brain in a vat.

I don't think it does: but that takes some showing.

The argument of your essay doesn't require a sceptic who goes to the extreme of hypothesizing brains in vats, or a 'conscious cell in the foot of a unicorn' (lovely example). Descartes was raising the question, not merely of scepticism about the things we believe about the world, but the deeper question how we know that there is a 'world' out there ('scepticism about an external world'). Examples like brains in vats, or Neo in the Matrix, by contrast assume a world and merely offer an alterative explanation of the experiences you are having now.

There is no need to go in for science fiction. How do I know that there is such a country as the USA? I've never been there. If I am prepared to go sufficiently down the path of paranoia I can conceive of possible conspiracies (involving, admittedly, thousands or hundreds thousands of of people) designed to get make the inhabitants of the UK believe that there is a country called the USA. Or maybe just me.

Highly improbable, you would say. But that is the nub of the question. The sceptic who goes down this route -- just like the brain in a vat sceptic -- won't accept that you are in a position to make well-founded judgements of probability. Probability is relative to evidence. If the sceptic can construct a sceptical scenario to undermine any particular piece of evidence that is offered, then he is home.

Suppose I say, 'I don't know that it is more probable than not that there is a rabid dog down that alleyway (to take your example) but I believe that it is,' then the question arises of the probability that I would give to my initial judgement of probability. Hume in his 'Treatise of Human Nature' uses this form of argument by vicious regress, against the idea that there can be rationally founded judgements of probability. For Hume, of course, no beliefs are founded in reason. (Like the school of Pyrrho, Hume's ultimate solution to sceptical worry is to 'follow nature'. What Hume adds to ancient scepticism is a 'science' -- the science of human nature -- which purports to explain the psychological processes of belief formation.)

OK, so how is this sceptical challenge to be met?

I don't accept your initial point that the sceptic assumes wrongly from the start that anything that counts as 'knowledge' must be infallible, or come from an infallible source. We have just been talking about probability. The sceptic's case comes down to the observation that judgements of probability are relative to evidence, and any evidence you put forward is up for grabs.

Nor will the sceptic be satisfied, if you agree to remove the term 'know' and its cognates from the vocabulary. (You make this point.) According to the sceptic, no belief is justified, there are no reasons for believing P rather than not-P. What we believe, we believe without reason or justification (and the sceptic accepts that we must have some beliefs; Pyrrhonic suspension of judgement can only go so far).

Now to the major point of our agreement: I fully concur with Pierce's point that the statement or expression of doubt is without content if it does not link, in some manner, with action.

Wittgenstein makes what amounts to the same point in the 'Philosophical Investigations'. To imagine a possible doubt is not the same time as actually doubting (para 84). In another paragraph, in response to the question, 'Are you not shutting your eyes to doubt?' he replies, simply, 'My eyes are shut.' Doubt is fact, or it is nothing.

We can use this as a lever to dismantle the sceptic's case, by showing that the denial of knowledge -- the denial that we have reasons or justification for our beliefs -- is in fact empty. The sceptic is not saying anything we don't already 'know'. However, this only goes part of the way to an answer. What we want to know is why we have a term like 'knowledge'. What use is it?

You ask me when the next bus is coming and I tell you that the buses here are very regular and the next one will be along in ten minutes. Do I know this? I'm pretty sure that I do. Then you ask whether I know that the bus drivers union hasn't called a strike today. I don't know this. In that case, it follows logically that I don't know that the next bus will be along in around ten minutes. How can a knowledge claim be so easily defeated? What's the point of using the term 'know'? (You will come across this issue if you look at the topic of 'contextualism' in epistemology.)

In other words, the defeat of the sceptic is purchased at a price: we owe an account of knowledge which is sufficiently flexible to allow, say, that reliable belief can suffice for 'knowledge' -- or something similar depending on your favourite flavour of epistemological theory. In that case, the sceptic's efforts have not been altogether wasted.

All the best,

Geoffrey