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Hume on ideas and impressions, and brains in vats


To: Vincent L.
From: Geoffrey Klempner
Subject: Hume on ideas and impressions, and brains in vats
Date: 7 April 2006 11:14

Dear Vincent,

Thank you for your email of 31 March, with your University of London essay in response to the question, 'How does Hume distinguish between impressions and ideas? Does he make a convincing case for his claim that every simple idea is a copy of a preceding impression?' and your email of 3 April, with your UoL essay in response to the questions, 'Do you know that you are reading an exam question?', and 'If I were a brain in a vat, I would be unable to think of brains in vats. So I am not a brain in a vat. Discuss.'

As a general rule, you will gain the best marks if you can show that you have thought about the question for yourself and are not merely repeating what you have read somewhere.

In other words, the best kind of answer combines knowledge of what other philosophers have said with evidence of your own thinking about the topic.


There are two main issues raised by this question. The first concerns what it means to say that impressions differ only from ideas by their 'liveliness'. The second issue is how convincing a case Hume makes. Here the 'missing shade of blue' plays a major part.

What does it mean to say that impressions are 'livelier' than ideas? You quote Bennett as contrasting two criteria, force and vivacity, or the 'unacknowledged criterion of objectivity'.

I can only assume that it is your interpretation (or misinterpretation) of the objectivity criterion which leads you to make the astounding claim that Hume 'did not accept the skeptical arguments that our belief in a material world... is not susceptible to rational justification'. Hume, in the section 'On Scepticism With Regard to the Senses' arguably goes one step further than Descartes in questioning the very meaning of the notion that objects have a 'continued' or 'distinct' existence from acts of perception.

Hume emphasizes again and again that beliefs about the external world cannot be justified by reason (another ground for saying this is that there is no rational justification for our belief in induction).

There is, however, an important distinction to make between the liveliness criterion conceived literally in 'phenomenal' terms (my image of a tree which I perceive outside my window is 'brighter', 'clearer' than a remembered or imagined tree) and conceived in terms of the effect upon belief. Let's say as I am crossing the road, I imagine a bus bearing down on me. This does not have the same effect on my actions as the impression of a bus bearing down on me. In other words, 'liveliness' is a matter of our response, or what we are prepared to act on, or form judgements about.

Taking either interpretation, is Hume right? This is not part of the question, as asked, but there is scope to observe briefly that something more might need to be said, e.g. about the difference between imagining a bus and perceiving (having the impression of) a bus.

The missing shade of blue is notorious because Hume dismisses the objection so quickly, yet it seems to go to the heart of his theory. How can I have an idea of a shade of blue of which I have never had an impression?

This discussion should have taken up much more space in your essay. Here is one plausible defence for Hume (you might find, or think of others). There are not millions (or billions) of 'ideas' of blue, each corresponding to a slightly different shade. There is just blue, which can be very light blue or very dark blue or anything in between. This is THE 'idea of blue' (things get more complicated when we combine colours, blue-green for example but let's keep it simple).

The idea of blue just is the idea of something which can have different degrees. From impressions of different shades of blue we construct an idea, which is 'simple' in one sense (because it is not a combination of two or more ideas) but complex in another sense, in that it is a variable quality, not just one single thing.

Brains in vats

In an exam situation you only have limited time, so you should not waste words on preliminaries. It is not part of the answer to this question to give a long preamble on different kinds of scepticism, or how, in particular, Descartes argued for scepticism.

One point that could be made, however, is that there is a sharp difference between Evil Demon scepticism and BIV or Matrix scepticism. Evil Demon scepticism questions the very idea of a world of physical objects in space and time, whereas BIV scepticism assumes a physical world in constructing its sceptical hypothesis.

Just to prod your intuitions a little bit, I am going to explain why I am totally unconvinced by Putnam's argument.

I fully accept that, on the basis of externalist semantic principles - but also more than this, on Wittgenstein's private language argument which arguably forms the ultimate basis for an externalist approach - that if I am a brain in a vat then any statement I make is meaningless.

How is that supposed to defeat scepticism? I say to myself, 'I don't know whether or not I am a brain in a vat. Hmm. But if I were a brain in a vat I couldn't meaningfully say, "I don't know whether or not I am a brain in a vat." Therefore, I can't be a brain in a vat!' Does that argument convince you? honestly?

It may have occurred to you that there is a suppressed premiss in this argument, namely, that anything I 'say to myself' must be meaningful. I know what I mean when I mean it, don't I? But it would be more in the true spirit of scepticism to question this assumption. How do I know that the very thought that I am expressing now makes sense?

So it would (in my view) be a more accurate portrayal of Putnam's argument to see it as raising the price of scepticism. The true global sceptic sees words and meanings as, in the words of Wittgenstein in the Tractatus, 'A ladder which you throw away after you have climbed up it.' Not only do I not know whether or not I am a brain in a vat, I don't even know whether my words make any sense at all!

All the best,