Tuesday, 21 December 2010

Lipton on Inference to the Best Explanation

I have been having a quick look at (the late) Peter Lipton’s book Inference to the Best Explanation as preparation for revising a philosophy of science module I am due to teach in January. Given that he supervised my PhD rewrite one might have thought that I would have read his book as, at the very least, a courtesy. But it seems that I cannot, or cannot with any care as it seemed quite unfamiliar.

One key respect in which it proved unfamiliar is in a central aim. Lipton aims to use inference to the best explanation (IBE) to shed light on the kind of inference deployed in science. Rather than using inductive inferences to arrive at a theoretical view of the world and then, distinctly and subsequently, using explanation to shed light on particular phenomena, he proposes that the initial inferences can best be understood as being based on explanation. Science works by looking for explanations. The best potential explanation is likely to be true.

That general idea might be backed up / unpacked through any number of different analyses of explanation. Lipton himself – in what seemed to me to be the best aspect of the book – suggests that the best approach to explanation is contrastive. We do not simply explain a fact but rather a fact rather than a foil. Not just ‘why does Smith have paresis?’ but ‘why does Smith rather than Jones have it; or Smith when Jones does not?’. (The answer that Smith had untreated syphilis will be helpful if Jones did not, but not if he too did. The context matters to the answer sought: the relevant bit of the causal history.) Further, he makes a good case for the irreducibility of contrastive explanation to non-contrastive such as a deductive nomological model of explanation. It is not the case, for example, that to explain why P rather than Q is to explain the logical conjunction of P and not Q.

So with contrastive explanation in mind, Lipton argues that scientific inductive inferences can be understood as based on explanations. Elsewhere he gives a short summary of a clinical inference discussed at length in the book:

A good illustration of this is provided by Ignaz Semmelweis’s nineteenth-century investigation into the causes of childbed fever, an often fatal disease contracted by women who gave birth in the hospital where Semmelweis did his research. Semmelweis considered many possible explanations. Perhaps the fever was caused by `epidemic influences’ affecting the districts around the hospital, or perhaps it was caused by some condition in the hospital itself, such as overcrowding, poor diet, or rough treatment. What Semmelweis noticed, however, was that almost all of the women who contracted the fever were in one of the hospital’s two maternity wards, and this led him to ask the obvious contrastive question and then to rule out those hypotheses which, though logically compatible with his evidence, did not mark a difference between the wards. It also lead him to infer an explanation that would explain the contrast between the wards, namely that women were inadvertently being infected by medical students who went directly from performing autopsies to obstetrical examinations, but only examined women in the first ward. This hypothesis was confirmed by a further contrastive procedure, when Semmelweis had the medics disinfect their hands before entering the ward: the infection hypothesis was now seen also to explain not just why women in the first rather than in the second ward contracted childbed fever, but also why women in the first ward contracted the fever before but not after the regime of disinfection was introduced. This general pattern of argument, which seeks explanations that not only would account for a given effect, but also for particular contrasts between cases where the effect occurs and cases where it is absent, is very common in science, for example wherever use is made of controlled experiments. [Lipton 2000]

Still, if explanation is to shed light on our inductive / scientific practices, the notion of best explanation (or best potential explanation) had better not depend too closely on what seems inductively compelling. So best explanation had better not be most likely explanation at the risk of circularity. Instead, Lipton emphasises the potential initial gap between inductive inference and explanation by distinguishing the likeliest and the loveliest explanation. The likeliest explanation is the explanation that is ‘most warranted’. (An example of a likely but unlovely explanation is that opium puts people to sleep through its dormitive powers. Probably true but unhelpful.) The loveliest explanation is the explanation which provides the most understanding, or is the most explanatory. If IBE relies on the latter rather than the former, it can provide a substantial, rather than a merely trivial, analysis of inductive inference. It can come as a surprise that a lovely explanation lies at the core of inductive inference.

But the cost of emphasising the distinction – in order to preserve the substance of the analysis – is that it prompts the following worry, which Lipton calls Voltaire’s Objection. Why assume that loveliness and truth are correlated? Why assume that we live in the loveliest world?

Lipton’s response to this emphasises, inter alia, the fact that to be a potentially lovely explanation requires already meeting a number of constraints on adequacy. (He does not have a knock down reply to the worry. His aim is to suggest that his account is no worse off than any other when it comes to Humean worries about induction in general and he takes Voltaire’s Objection just to amount to that, in the end.) But the problem of this kind of reply, this direction of travel, is that it undermines the contrast which gave the analysis substance. So the question is then whether the two ‘vectors’ can be balanced: to have enough distinction to shed some light on inductive practices whilst maintaining enough connection to avoid Voltaire’s objection. After all if the analysis were to work there would have to be a connection and a gap: the paradox of analysis, again.

Lipton, P. (2000) ‘Inference to the Best Explanation’ in W.H. Newton-Smith (ed) A Companion to the Philosophy of Science, Oxford: Blackwell pp184-193
Lipton, P. (2004) Inference to the Best Explanation, London: Routledge