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Is there an experiential difference between seeing and hallucinating?


To: Scott B.
From: Geoffrey Klempner
Subject: Is there an experiential difference between seeing and hallucinating?
Date: 5th November 2009 12:28

Dear Scott,

Thank you for your email of 27 October, with your essay for the University of London Epistemology module, in response to the question, ''You cannot distinguish between a case in which you see the exam paper before you and a case in which you undergo a perfectly matching hallucination. Therefore the difference between seeing and hallucinating is not an experiential difference.' Discuss'.

You start off by giving an eloquent account of how one might be led to the conclusion that there is no 'experiential difference' between veridical perception and hallucination. This is the best part of the essay.

You then draw the conclusion that this view -- the argument from illusion -- leads to the conclusion that 'knowledge I have of the world which is gained through sight, is no better than the knowledge I have when I am told something'. In other words, in both cases, we have data -- perceptual experiences, or what a person is saying to you -- for which you then look for the best explanation. Are you in a situation where your perceptual experiences are likely to be reliable or unreliable? Is your would be informant honest and knowledgeable, or not?

This is undoubtedly a serious problem for the indirect realist, calling up the spectre of scepticism.

It is always a good move to show difficulties with a view by drawing consequences which for one reason or another we do not like. However, this is not satisfactory in itself as a way of formulating a valid objection in the absence of a challenge to the argument which was originally put forward for that view. Sometimes, in philosophy, we find ourselves between a rock and a hard place. If you don't like the consequences of a theory which you are unable to refute, that's tough. In this case, the key assumption which needs to be challenged is the claim of 'no experiential difference'.

That is really the meat of the essay, and what the essay question was in fact looking for. The term which has become common for the view that rejects the assumption of no experiential difference is 'disjunctivism'. (Look up the combination '"Scott Sturgeon" perception disjunctivism' in Google.)

Here is a quote from the Stanford Encyclopaedia which neatly summarizes the disjunctivist position:

'The disjunctivist theory of perception holds that the objects of genuine perception are mind-independent; and that the phenomenal character of a genuinely perceptual experience depends upon these objects. It also accepts that illusion and hallucination are possible. But the conjunction of all these views is not inconsistent, according to disjunctivism, because it also denies that genuine perception and a subjectively indistinguishable hallucination are mental states of the same fundamental psychological kind. The disjunctivist therefore rejects what Martin (2004) calls the 'common kind assumption' about perception: whatever fundamental kind of mental event occurs when one is veridically perceiving some scene can occur whether or not one is perceiving.'

You go on to give a summary of Locke's view of primary and secondary qualities, followed by accounts of Berkeley and Kant.

In principle, it would be possible to work these into an essay in response to this question. However, an examiner will conclude that you are not sufficiently knowledgeable about the issue. You are merely repeating the case for, and difficulties with indirect realism.

This is compounded by your account of Berkeleian idealism and Kant. Kant is actually a bad example of an 'indirect realist'. His 'Refutation of Idealism' (from the 2nd edition of the 'Critique of Pure Reason') argues that the objects of perception are necessarily spatio-temporal particulars. While Kant distinguishes the 'given' in perception -- 'anschauung' translated as 'intuition' -- from the spatially located objects we perceive, he does not regard 'intuition' as describable except in terms of concepts which describe the external world. In other words, the Refutation of Idealism in effect rejects the 'sense datum' view of perception.

As for Berkeley, the view that the objects of perception are 'ideas' in the mind of God raises exactly the same problem for perception as direct realism. When you are looking at your exam paper, are you looking at the 'idea in the mind of God' which IS the exam paper, or are you merely enjoying a subjective experience (idea) which does not correspond to an idea in the mind of God?

After this, the problems with indirect realism get rehashed again. The only thing which you add to what has been said before is reference to 'sense data'. It is certainly a question which can be raised, whether the 'no perceptual difference' view entails some form of 'sense datum' theory. Contemporary philosophers who resist the disjunctivist view of perception would say, no.

I am not going to give you a lecture on disjunctivism. This is obviously something you will have to research for yourself. In an exam, if you found that you were struggling to find questions that you could answer, then I would say, use whatever knowledge you have. There's nothing wrong in principle with e.g. citing the modern philosophers you have studied. But be aware that the examiner will be quick to spot whether you are papering over the cracks.

All the best,