David Deutch - The Fabric of Reality

DAVID: Correct. And even then, the ‘reliability’ that corroboration confers is not absolute but only relative to the other contending theories. That is, we expect the strategy of relying on corroborated theories to select the best theories from those that are proposed. That is a sufficient basis for action. We do not need (and could not validly get) any assurance about how good even the best proposed course of action will be. Furthermore, we may always be mistaken, but so what? We cannot use theories that have yet to be proposed; nor can we correct errors that we cannot yet see.

CRYPTO-INDUCTIVIST: Quite so. I am glad to have learned something about scientific methodology. But now — and I hope you don’t think me impolite — I must draw your attention yet again to the question I have been asking all along. Suppose that a theory has passed through this whole process. Once upon a time it had rivals. Then experiments were performed and all the rivals were refuted. But it itself was not refuted. Thus it was corroborated. What is it about its being corroborated that justifies our relying on it in the future!

DAVID: Since all its rivals have been refuted, they are no longer rationally tenable. The corroborated theory is the only rationally tenable theory remaining.

CRYPTO-INDUCTIVIST: But that only shifts the focus from the future import of past corroboration to the future import of past refutation. The same problem remains. Why, exactly, is an experimentally refuted theory ‘not rationally tenable’? Is it that having even one false consequence implies that it cannot be true?

DAVID: Yes.

CRYPTO-INDUCTIVIST: But surely, as regards the future applicability of the theory, that is not a logically relevant criticism. Admittedly, a refuted theory cannot be true universally [3]— in particular, it cannot have been true in the past, when it was tested. But it could still have many true consequences, and in particular it could be universally true in the future.

DAVID: This ‘true in the past’ and ‘true in the future’ terminology is misleading. Each specific prediction of a theory is either true or false; that cannot change. What you really mean is that though the refuted theory is strictly false, because it makes some false predictions, all its predictions about the future might nevertheless be true. In other words, a different theory, which makes the same predictions about the future but different predictions about the past, might be true.

CRYPTO-INDUCTIVIST: If you like. So instead of asking why a refuted theory is not rationally tenable, I should, strictly speaking, have asked this: why does the refutation of a theory also render untenable every variant of the theory that agrees with it about the future — even a variant that has not been refuted?

DAVID: It is not that refutation renders such theories untenable. It is just that sometimes they already are untenable, by virtue of being bad explanations. And that is when science can make progress. For a theory to win an argument, all its rivals must be untenable, and that includes all the variants of the rivals which anyone has thought of. But remember, it is only the rivals which anyone has thought of that need be untenable. For example, in the case of gravity no one has ever proposed a tenable theory that agrees with the prevailing one in all its tested predictions, but differs in its predictions about future experiments. I am sure that such theories are possible — for instance, the successor to the prevailing theory will presumably be one of them. But if no one has yet thought of such a theory, how can anyone act upon it?

CRYPTO-INDUCTIVIST: What do you mean, ‘no one has yet thought of such a theory’? I could easily think of one right now.

DAVID: I very much doubt that you can.

CRYPTO-INDUCTIVIST: Of course I can. Here it is. ‘Whenever you, David, jump from high places in ways that would, according to the prevailing theory, kill you, you float instead. Apart from that, the prevailing theory holds universally.’ I put it to you that every past test of your theory was also necessarily a test of mine, since all the predictions of your theory and mine regarding past experiments are identical. Therefore your theory’s refuted rivals were also my theory’s refuted rivals. And therefore my new theory is exactly as corroborated as your prevailing theory. How, then, can my theory be ‘untenable’? What faults could it possibly have that are not shared by your theory?

DAVID: Just about every fault in the Popperian book! Your theory is constructed from the prevailing one by appending an unexplained qualification about me floating. That qualification is, in effect, a new theory, but you have given no argument either against the prevailing theory of my gravitational properties, or in favour of the new one. You have subjected your new theory to no criticism (other than what I am giving it now) and no experimental testing. It does not solve — or even purport to solve — any current problem, nor have you suggested a new, interesting problem that it could solve. Worst of all, your qualification explains nothing, but spoils the explanation of gravity that is the basis of the prevailing theory. It is this explanation that justifies our relying on the prevailing theory and not on yours. Thus by all rational criteria your proposed qualification can be summarily rejected.

CRYPTO-INDUCTIVIST: Couldn’t I say exactly the same thing about your theory? Your theory differs from mine only by the same minor qualification, but in reverse. You think I ought to have explained my qualification. But why are our positions not symmetrical?

DAVID: Because your theory does not come with an explanation of its predictions, and mine does.

CRYPTO-INDUCTIVIST: But if my theory had been proposed first, it would have been your theory that appeared to contain an unexplained qualification, and it would be your theory that would be ‘summarily rejected’.

DAVID: That is simply untrue. Any rational person who was comparing your theory with the prevailing one, even if yours had been proposed first, would immediately reject your theory in favour of the prevailing one. For the fact that your theory is an unexplained modification of another theory is manifest in your very statement of it.

CRYPTO-INDUCTIVIST: You mean that my theory has the form ‘such-and-such a theory holds universally, except in such-and-such a situation’, but I don’t explain why the exception holds?

DAVID: Exactly.

CRYPTO-INDUCTIVIST: Aha! Well, I think I can prove you wrong here (with the help of the philosopher Nelson Goodman). Consider a variant of the English language that has no verb ‘to fall’. Instead it has a verb ‘to x-fall’ which means ‘to fall’ except when applied to you, in which case it means ‘to float’. Similarly, ‘to x-float’ means ‘to float’ except when applied to you, in which case it means ‘to fall’. In this new language I could express my theory as the unqualified assertion ‘all objects x-fall if unsupported’. But the prevailing theory (which in English says ‘all objects fall if unsupported’) would, in the new language, have to be qualified: ‘all objects x-fall when unsupported, except David, who x-floats ’. So which of these two theories is qualified depends on the language they are expressed in, doesn’t it?

DAVID: In form, yes. But that is a triviality. Your theory contains, in substance, an unexplained assertion, qualifying the prevailing theory. The prevailing theory is, in substance, your theory stripped of an unexplained qualification. No matter how you slice it, that is an objective fact, independent of language.

CRYPTO-INDUCTIVIST: I don’t see why. You yourself used the form of my theory to spot the ‘unnecessary qualification’. You said that it was ‘manifest’ as an additional clause in my very statement of the theory — in English. But when the theory is translated into my language, no qualification is manifest; and on the contrary, a manifest qualification appears in the very statement of the prevailing theory.