DAVID: Not quite. Theories postulating anomalies without explaining them are less likely than their rivals to make true predictions. More generally, it is a principle of rationality that theories are postulated in order to solve problems. Therefore any postulate which solves no problem is to be rejected. That is because a good explanation qualified by such a postulate becomes a bad explanation.
CRYPTO-INDUCTIVIST: Now that I understand that there really is an objective difference between theories which make unexplained predictions and theories which don’t, I must admit that this does look promising as a solution of the problem of induction. You seem to have discovered a way of justifying your future reliance on the theory of gravity, given only the past problem-situation (including past observational evidence) and the distinction between a good explanation and a bad one. You do not have to make any assumption such as ‘the future is likely to resemble the past’.
DAVID: It was not I who discovered this.
CRYPTO-INDUCTIVIST: Well, I don’t think Popper did either. For one thing, Popper did not think that scientific theories could be justified at all. You make a careful distinction between theories being justified by observations (as inductivists think) and being justified by argument. But Popper made no such distinction. And in regard to the problem of induction, he actually said that although future predictions of a theory cannot be justified, we should act as though they were!
DAVID: I don’t think he said that, exactly. If he did, he didn’t really mean it.
CRYPTO-INDUCTIVIST: What?
DAVID: Or if he did mean it, he was mistaken. Why are you so upset? It is perfectly possible for a person to discover a new theory (in this case Popperian epistemology) but nevertheless to continue to hold beliefs that contradict it. The more profound the theory is, the more likely this is to happen.
CRYPTO-INDUCTIVIST: Are you claiming to understand Popper’s theory better than he did himself?
DAVID: I neither know nor care. The reverence that philosophers show for the historical sources of ideas is very perverse, you know. In science we do not consider the discoverer of a theory to have any special insight into it. On the contrary, we hardly ever consult original sources. They invariably become obsolete, as the problem-situations that prompted them are transformed by the discoveries themselves. For example, most relativity theorists today understand Einstein’s theory better than he did. The founders of quantum theory made a complete mess of understanding their own theory. Such shaky beginnings are to be expected; and when we stand upon the shoulders of giants, it may not be all that hard to see further than they did. But in any case, surely it is more interesting to argue about what the truth is, than about what some particular thinker, however great, did or did not think.
CRYPTO-INDUCTIVIST: All right, I agree. But wait a moment, I think I spoke too soon when I said that you were not postulating any sort of principle of induction. Look: you have justified a theory about the future (the prevailing theory of gravity) as being more reliable than another theory (the one I proposed), even though they are both consistent with all currently known observations. Since the prevailing theory applies both to the future and to the past, you have justified the proposition that, as regards gravity, the future resembles the past. And the same would hold whenever you justify a theory as reliable on the grounds that it is corroborated. Now, in order to go from ‘corroborated’ to ‘reliable’, you examined the theories’ explanatory power. So what you have shown is that what we might call the ‘principle of seeking better explanations’, together with some observations — yes, and arguments — imply that the future will, in many respects, resemble the past. And that is a principle of induction! If your ‘explanation principle’ implies a principle of induction, then, logically, it is a principle of induction. So inductivism is true after all, and a principle of induction does indeed have to be postulated, explicitly or implicitly, before we can predict the future.
DAVID: Oh dear! This inductivism really is a virulent disease. Having gone into remission for only a few seconds, it now returns more violently than before.
CRYPTO-INDUCTIVIST: Does Popperian rationalism justify ad hominem arguments as well? I ask for information only.
DAVID: I apologize. Let me go straight to the substance of what you said. Yes, I have justified an assertion about the future. You say this implies that ‘the future resembles the past’. Well, vacuously, yes, inasmuch as any theory about the future would assert that it resembled the past in some sense. But this inference that the future resembles the past is not the sought-for principle of induction, for we could neither derive nor justify any theory or prediction about the future from it. For example, we could not use it to distinguish your theory of gravity from the prevailing one, for they both say, in their own way, that the future resembles the past.
CRYPTO-INDUCTIVIST: Couldn’t we derive, from the ‘explanation principle’, a form of the principle of induction that could be used to select theories? What about: ‘if an unexplained anomaly does not happen in the past, then it is unlikely in the future’?
DAVID: No. Our justification does not depend on whether a particular anomaly happens in the past. It has to do with whether there is an explanation for the existence of that anomaly.
CRYPTO-INDUCTIVIST: All right then, let me formulate it more carefully: ‘if, in the present, there is no explanatory theory predicting that a particular anomaly will happen in the future, then that anomaly is unlikely to happen in the future’.
DAVID: That may well be true. I, for one, believe that it is. However, it is not of the form ‘the future is likely to resemble the past’. Moreover, in trying to make it look as much like that as possible, you have specialized it to cases ‘in the present’, ‘in the future’, and to the case of an ‘anomaly’. But it is just as true without these specializations. It is just a general statement about the efficacy of argument. In short, if there is no argument in favour of a postulate, then it is not reliable. Past, present or future. Anomaly or no anomaly. Period.
CRYPTO-INDUCTIVIST: Yes, I see.
DAVID: Nothing in the concepts of ‘rational argument’ or ‘explanation’ relates the future to the past in any special way. Nothing is postulated about anything ‘resembling’ anything. Nothing of that sort would help if it were postulated. In the vacuous sense in which the very concept of ‘explanation’ implies that the future ‘resembles the past’, it nevertheless implies nothing specific about the future, so it is not a principle of induction. There is no principle of induction. There is no process of induction. No one ever uses them or anything like them. And there is no longer a problem of induction. Is that clear now?
CRYPTO-INDUCTIVIST: Yes. Please excuse me for a few moments while I adjust my entire world-view.
DAVID: To assist you in that exercise, I think you should consider your alternative ‘theory of gravity’ more closely.
CRYPTO-INDUCTIVIST: …
DAVID: As we have agreed, your theory consists objectively of a theory of gravity (the prevailing theory), qualified by an unexplained prediction about me. It says that I would float, unsupported. ‘Unsupported’ means ‘without any upward force acting’ on me, so the suggestion is that I would be immune to the ‘force’ of gravity which would otherwise pull me down. But according to the general theory of relativity, gravity is not a force but a manifestation of the curvature of spacetime. This curvature explains why unsupported objects, like myself and the Earth, move closer together with time. Therefore, in the light of modern physics your theory is presumably saying that there is an upward force on me, as required to hold me at a constant distance from the Earth. But where does that force come from, and how does it behave? For example, what is a ‘constant distance’? If the Earth were to move downwards, would I respond instantaneously to maintain the same height (which would allow communication faster than the speed of light, contrary to another principle of relativity), or would the information about where the Earth is have to reach me at the speed of light first? If so, what carries this information? Is it a new sort of wave emitted by the Earth — in which case what equations does it obey? Does it carry energy? What is its quantum-mechanical behaviour? Or is it that I respond in a special way to existing waves, such as light? In that case, would the anomaly disappear if an opaque barrier were placed between me and the Earth? Isn’t the Earth mostly opaque anyway? Where does ‘the Earth’ begin: what defines the surface above which I am supposed to ‘float’?