To: Alistair L.
From: Geoffrey Klempner
Subject: Hallucination and disjunctive theories of perception
Date: 24th June 2009 11:06

Dear Alistair,

Thank you for your email of 18 June, with your essay for the University of London Epistemology module, in response to the question, 'All that you can know about your current experience is that either you are seeing an exam paper or you are undergoing a perfectly matching hallucination as of seeing an exam paper. Hence your experience must be consistent with either alternative.' Discuss.

You expressed concern that there might be 'too much science' in your answer. My feeling is that the science is always important because philosophers need a 'reality check' on discussions of illusions and hallucinations. The 'brain in a vat' thought experiment is another case where philosophers blithely create a scenario which is nonsense from a scientific standpoint.

Also, I am glad that you did take the opportunity to demonstrate your knowledge, because it gives me something I can take away and use, at least some very pertinent questions to raise about hypothetical hallucination scenarios.

However, having taken the reality check, we are still in the business of conceptual inquiry and (as difficult as this might be for you to accept) philosophers can and do consider all sorts of 'what ifs' which go well beyond any shred of scientific credibility.

I think it is a valid question to ask, if Scotty 'beams down' AL from the Starship Enterprise, and two identical ALs materialize on the planet Zorg, what would be the consequences for our view of personal identity? Never mind how this is supposed to happen.

It is in this spirit that you are being asked to consider the possibility that, e.g. the questions you are currently 'reading' in your 'exam paper' are not the questions in the exam paper on the desk in front of you, but are in fact (e.g.) the questions that you were hoping would appear. You pick up the sheet of paper, feel it crinkle in your hands, there is not the least evidence that your mind is playing tricks. (Never mind how this is supposed to happen.)

But what if...?

You give a decent answer to this question. After considering the alternatives of the sense datum theory and intentional theories of perception, you plump for the disjunctive view. But what does this mean exactly for the concept of 'having an experience'?

The question talks about 'your experience' as if it goes without question that in both cases, veridical perception or hallucination, you are 'experiencing something', as if the two logical possibilities were conceptually parallel (never mind the scientific implausibility of the second alternative). This is a point you could have made.

If we go for the disjunctive theory, there is a cost. I would have liked to have seen a fuller appreciation of the cost (which partly explains the continued attractiveness of alternative, non-disjunctive views).

However, another point occurs to me: we can ditch the notion that there is something, 'my experience' which is 'consistent with either alternative' and still find the thought experiment challenging. Let's use a less contested form of words: the given evidential basis on which I make my knowledge claims. Maybe I'm 'having experience' and maybe I'm not. If you ask me, right now, am I prepared to reject, absolutely and without question, the sceptical doubt that the questions are the ones I was hoping for because I'm merely imagining them -- however we describe the total situation in terms of a favoured theory of perception -- then the remaining only answer is the one Wittgenstein gives (in response to an interlocutor who asks, 'Aren't you shutting your eyes to doubt?')

'They are shut.'

Whatever you think of this as the ultimate response to scepticism (or a cop-out) the point is that the disjunctive theory of perception does not (as it might have been touted to be) rescue us from the threat of scepticism about the external world, because there are ways of formulating the sceptical hypothesis which do not assume the validity of any particular theory of perception.

Maybe this is what gets you. So little seems to turn on whether or not we have the ultimately valid theory of percepion. It doesn't resolve the big questions. It merely gives us a more accurate map of the conceptual landscape, than any of any of the alternative theories do. But why is that so important? -- I am bound to say that I think it is, but won't try to argue that here.

All the best,

Geoffrey