Realists’ Retreat on Inference to the Best Explanation
As I understand it, what is at issue in the scientific realism debates is not a question about rational management of opinion and belief—the main topic of epistemology today—but the question what science is.
However, it is not surprising that so much in those debates has focused on issues in epistemology. For the debate to start at all, it has to make sense to think that in response to the empirical evidence one could have a choice between believing that the theory is true or believing only that the theory is empirically adequate. Any position in epistemology which entails that, given the evidence, there is a unique rationally compelled conclusion, would leave nothing more to be said.
The main entry of epistemology into the scientific realism debates was the rule of Inference to the Best Explanation (IBE). There were various formulations of course, but they had in common precisely that under suitable conditions, the question of what to believe on the basis of available evidence would have a unique answer. That answer would be ‘the best explanation of the facts adduced in that evidence’. I place this in scare quotes, because each of the main terms in this phrase beg for explication (and thereby hangs a tale...).
Not only in philosophy of science but elsewhere this rule received much attention. Is it a good and rational policy to apply this rule when managing your beliefs and opinions? David Armstrong gave short shrift to this question, offering the following truly irrefutable argument:
If making such an inference is not rational, what is? (Armstrong 1983, 53).To infer to the best explanation is part of what it is to be rational. If that is not rational, what is? (ibid. p. 59).
This rule, IBE, was the subject of much criticism, and here I think it is appropriate to defer to Alan Musgrave, and his acute, insightful critique of what various scientific realists have written about the ‘rule’ of IBE.
He agrees with the empiricist critique that it cannot be a matter of inferring truth from explanatory success, but he offers in effect the following moderate form:It is reasonable to believe that the best available explanation of any fact is true. F is a fact.
Hypothesis H explains F.
No available competing hypothesis explains F as well as H does.
Therefore, it is reasonable to believe that H is true.[27]
So let us call this the Musgrave version of IBE, “MIBE” for short. While Musgrave feels that this version of IBE needs a defense, namely a defense of its first premise, I have no such worry. The premise seems to me eminently reasonable. (Since “reasonable” is a normative term, the question of truth does not arise.)
But this form of argument has no bite unless it can be accompanied by another conclusion:
(*) Therefore, it is unreasonable not to believe that H is true.
To mention just the application that Musgrave obviously has in mind, there is no empiricist objection to be made to a full belief that our best scientific theories are true. (At least in principle, perhaps not currently in practice: arguably, we are currently in a situation where our best theories in fundamental physics do not form a coherent whole.) But for the constructive empiricist position, such a belief is reasonable enough, but supererogatory. Against this, MIBE without (*) is powerless.
That Musgrave did not notice the lacuna is clear from the argument in which he ostensibly applies his rule. I’ll quote the relevant part:
Suppose that H is the best explanation we have of some phenomena. [,..]So what is it to accept that H is empirically adequate? It is... to accept a meta-claim about H, namely the meta-claim “H is empirically adequate” or equivalently “The observable phenomena are as if H were true”. Call this meta-claim H*. Now, and crucially, H* is no explanation at all of the phenomena. The hypothesis H that it is raining explains why the streets are wet-but “The phenomena are as if it were raining” does not.
Ergo, H* is not the best explanation-H is, or so we assumed. (Actually, all we need assume here is that H is a better explanation than H*.) So given IBE, H* should not be accepted as true. That is, given IBE, H should not be accepted as empirically adequate.[28]Why conclude that H should be accepted as true?1 Because it is the best explanation, and H* does not dislodge it from that status. But MIBE allows only to conclude that it is reasonable to believe that H is true. Without the strengthening by (*), MIBE does not rule out that it is also reasonable to suspend belief about H, and to believe only the logically weaker H*.
Is there an argument, perhaps along lines pointed to by Musgrave, to strengthen MIBE with (*)? I see no reason to think so: at this point the only sort of consideration that could help would have to be ones to establish Lipton's (2004) contention that the lovelier is likelier to be true. But Lipton's admirable efforts were ultimately unsuccessful (cf. van Fraassen 2005).
5