The Fabric of Reality David Deutch

participants — the ‘Floater’ — decides to descend by jumping over the side
instead of using the lift in the usual way. The others try to persuade the
Floater that jumping off means certain death. They use the best available
scientific and philosophical arguments. But the infuriating Floater still expects
to float down safely, and keeps pointing out that no rival expectation can
logically be proved to be preferable on the basis of past experience.
I believe that we can justify our expectation that the Floater would be killed.
The justification (always tentative, of course) comes from the explanations
provided by the relevant scientific theories. To the extent that those

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explanations are good, it is rationally justified to rely on the predictions of
corresponding theories. So, in reply to Worrall, I now present a dialogue of
my own, set in the same place.
DAVID: Since I read what Popper has to say about induction, I have
believed that he did indeed, as he claimed, solve the problem of induction.
But few philosophers agree. Why?
CRYPTO-INDUCTIVIST: Because Popper never addressed the problem of
induction as we understand it. What he did was present a critique of
inductivism. Inductivism said that there is an ‘inductive’ form of reasoning
which can derive, and justify the use of, general theories about the future,
given evidence in the form of individual observations made in the past. It
held that there was a principle of nature, the 
principle of induction, which
said something like ‘observations made in the future are likely to resemble
observations made under similar circumstances in the past’. Attempts were
made to formulate this in such a way that it would indeed allow one to
derive, or justify, general theories from individual observations. They all
failed. Popper’s critique, though influential among scientists (especially in
conjunction with his other work, elucidating the methodology of science),
was hardly original. The unsoundness of inductivism had been known almost
since it was invented, and certainly since David Hume’s critique of it in the
early eighteenth century. The problem of induction is not how to justify or
refute the principle of induction, but rather, taking for granted that it is invalid,
how to justify any conclusion about the future from past evidence. And
before you say that one doesn’t need to …
DAVID: One doesn’t need to.
CRYPTO-INDUCTIVIST: But one does. This is what is so irritating about you
Popperians: you deny the obvious. Obviously the reason why you are not
even now leaping over this railing is, in part, that you consider it 
justified to
rely on our best theory of gravity and 
unjustified to rely on certain other
theories. (Of course, by ‘our best theory of gravity’ in this case I mean more
than just general relativity. I am also referring to a complex set of theories
about such things as air resistance, human physiology, the elasticity of
concrete and the availability of mid-air rescue devices.)
DAVID: Yes, I would consider it justified to rely on that theory. According to
Popperian methodology, one should in these cases rely on the 
best-
corroborated theory — that is, the one that has been subjected to the most
stringent tests and has survived them while its rivals have been refuted.
CRYPTO-INDUCTIVIST: You say ‘one 
should’ rely on the best-corroborated
theory, but why, exactly? Presumably because, according to Popper, the
process of corroboration has justified the theory, in the sense that its
predictions are more likely to be true than the predictions of other theories.
DAVID: Well, not more likely than 
all other theories, because no doubt one
day we’ll have even better theories of gravity …
CRYPTO-INDUCTIVIST: Now look. Please let’s agree not to trip each other
up with quibbles that do not bear on the substance of what we are
discussing. 
Of course there may be a better theory of gravity one day, but
you have to decide whether to jump now, 
now. And given the evidence
available to you now, you have chosen a certain theory to act upon. And you

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have chosen it according to Popperian criteria because you believe that
those criteria are the ones most likely to select theories which make true
predictions.
DAVID: Yes.
CRYPTO-INDUCTIVIST: So to summarize, you believe that the evidence
currently available to you justifies the prediction that you would be killed if
you leapt over the railing.
DAVID: No, it doesn’t.
CRYPTO-INDUCTIVIST: But dammit, you are contradicting yourself. Just
now you said that that prediction 
is justified.
DAVID: It is justified. But it was not justified by the evidence, if by ‘the
evidence’ you mean all the experiments whose outcomes the theory
correctly predicted in the past. As we all know, that evidence is consistent
with an infinity of theories, including theories predicting every logically
possible outcome of my jumping over the railing.
CRYPTO-INDUCTIVIST: So in view of that, I repeat, the whole problem is to
find what does justify the prediction. That is the problem of induction.
DAVID: Well, that is the problem that Popper solved.
CRYPTO-INDUCTIVIST: That’s news to me, and I’ve studied Popper
extensively. But anyway, what is the solution? I’m eager to hear it. What
justifies the prediction, if it isn’t the evidence?
DAVID: Argument.
CRYPTO-INDUCTIVIST: Argument?
DAVID: Only argument ever justifies anything — tentatively, of course. All
theorizing is subject to error, and all that. But still, argument can sometimes
justify theories. That is what argument is for.
CRYPTO-INDUCTIVIST: I think this is another of your quibbles. You can’t
mean that the theory was justified by 
pure argument, like a mathematical
theorem.[2] The evidence played some role, surely.
DAVID: Of course. This is an empirical theory, so, according to Popperian
scientific methodology, crucial experiments play a pivotal role in deciding
between it and its rivals. The rivals were refuted; it survived.
CRYPTO-INDUCTIVIST: And in consequence of that refuting and surviving,
all of which happened in the past, the practical use of the theory to predict
the future is now justified.
DAVID: I suppose so, though it seems misleading to say ‘in consequence of’
when we are not talking about a logical deduction.
CRYPTO-INDUCTIVIST: Well that’s the whole point again: 
what sort of
consequence was it? Let me try to pin you down here. You admit that it was
both argument 
and the outcomes of experiments that justified the theory. If
the experiments had gone differently, the argument would have justified a
different theory. So do you accept that in that sense — yes, via the
argument, but I don’t want to keep repeating that proviso — the outcomes of
past experiments did justify the prediction?
DAVID: Yes.

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CRYPTO-INDUCTIVIST: So what exactly was it about those actual past
outcomes that justified the prediction, as opposed to other possible past
outcomes which might well have justified the contrary prediction?
DAVID: It was that the actual outcomes refuted all the rival theories, and
corroborated the theory that now prevails.
CRYPTO-INDUCTIVIST: Good. Now listen carefully, because you have just
said something which is not only provably untrue, but which you yourself
conceded was untrue only moments ago. You say that the outcomes of
experiments ‘refuted all the rival theories’. But you know very well that no set
of outcomes of experiments can refute all possible rivals to a general theory.
You said yourself that any set of past outcomes is (I quote) ‘consistent with
an infinity of theories, including theories predicting every logically possible
outcome of my jumping over the railing’. It follows inexorably that the
prediction you favour 
was not justified by the experimental outcomes,
because there are infinitely many other rivals to your theory, also unrefuted
as yet, which make the opposite prediction.
DAVID: I’m glad I listened carefully, as you asked, for now I see that at least