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Hempel's deductive-nomological model of explanation


To: Francis M.
From: Geoffrey Klempner
Subject: Hempel's deductive-nomological model of explanation
Date: 20 March 2006 12:39

Dear Frank,

Thank you for your email of 9 March, with your University of London Methodology essay in response to the question, 'Outline Hempel's theory of explanation. Is it an acceptable theory?'

This is another excellent essay. I liked the way you introduced the topic with a homely example. Your investigation of the topic is well argued and thoroughgoing.

One question which is always worth asking is why a particular theory is attractive. Why should we go to considerable lengths to defend a theory, e.g. Hempel's, which appears not to work in some cases?

Causation is the crucial question. Pre-reflectively, one would think that the explanation, e.g., of an event is one which gives the cause of that event. Why did E happen? Because F caused E. If you want more explanation, you just follow the causes back until you reach the point where you are satisfied or don't feel the need to ask any more.

But causation is a difficult concept in itself. There is considerable controversy over how 'A caused B' should be analysed. The attractiveness of Hempel's theory is that we don't have to go into that. So it is a bit of a climb down if we then have to modify the theory to include consideration of causal chains.

One thing you might have mentioned concerns the family of examples used to argue for the alleged the insufficiency of the DN model. (In connection with this I remember from long ago something called 'Valberg's Bomb', but I couldn't find anything on Google. However, you could probably work it out for yourself.) This is where we have an explanation of an event in DN terms which turns out to be irrelevant because at the last moment something else happened which was the actual cause of the event in question.

Here's an example I found (being lazy): 'Why did Jones die when he did? He ate a pound of arsenic five minutes earlier, and anyone who eats a pound of arsenic dies within 24 hours. However, what the explanation does not mention is that Jones was run over by a bus immediately before his death. The arsenic would have caused his death if the bus hadn't, but in fact the arsenic never had a chance; the bus got him first.'

Let's see if we can could use Salmon's relevance criterion.

The probability of Jones dying at t, given that he took arsenic and was run over by a bus at t minus ten seconds is the same as the probability of Jones dying at t, given that he was run over by a bus at t minus ten seconds. So his taking arsenic is irrelevant.

Suppose that the dose of arsenic was such that Jones would have died at t even if he had not been run over by the bus at t minus ten seconds (he was staggering across the road, in his last death throes). Then by the same criterion his being run over by the bus is irrelevant.

But notice that we have to make the times exactly the same (t). If he would have died from arsenic at t plus ten seconds, then the arsenic doesn't explain why he died at t.

This looks like a general way to deal with cases of 'causal pre-emption'. However, it leaves us with a range of cases which cannot be tackled in this way, where there is strict 'over-determination'. The arsenic and the bus, as it were, 'hit' Jones simultaneously. The answer is to distinguish two events of 'death' which happened at the same time, say, massive organ failure caused by the arsenic, and massive physical damage caused by the bus. By distinguishing Jones' 'poison death' and 'crash death' you can apply the Hempel DN model to both.

However, in order to do this it looks as though we have to apply our ordinary (philosophically unanalysed) understanding of a 'cause' in order to unravel the two causal chains of 'arsenic poison death' and 'bus crash death'.

You asked about how you would 'cut the essay down' for exam purposes.

This is a very important issue and I am thinking about writing a page for the Pathways site on this. Apologies if I am repeating something I have said before.

I would strongly advise against the general strategy of memorising your best essays for the exam. It does work sometimes, when you get lucky with a question that 'fits'. More often than not, however, the question will be phrased in a slightly different way, or have dink in it, which means that you have to 'think on your feet'. This is really what the examiners want you to do, and if you should be mentally prepared for it.

There is no way to save yourself the pain of having to 'perform on the night'. But it can be fun too, if you have a positive attitude.

Suppose you wanted to explain Hempel's theory and the problems it faces quickly (say, you meet someone who has just started a course of philosophy). If you understand the issues, you would just *know* how to do it without too much waffle. It's something human beings are amazingly good at. What I am saying is, trust yourself to be able to stick to the essentials and keep to the time limit. Concentrate on the wording of the question and what is needed to answer the question as posed. You might find paragraphs from essays you have written just flowing naturally, or not. Don't wait for them to come because you don't have time for that.

All the best,