Karl Popper
Falsification, not induction
Sir Karl: 1902-1994
Some Bibliography
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Popper, K.R. (1935) Logik der Forschung (The Logic of Research) , Vienna: Springer; trans. The Logic of Scientific
Discovery, London: Hutchinson, 1959. (Crisp and incisive statement of his classic ideas on philosophy of science
and of epistemology. The English translation contains additional material, as explained in §3.)
Popper, K.R. (1945) The Open Society and Its Enemies, London: Routledge. (A critical introduction to the
philosophy of history and of politics that takes Plato, Hegel and Marx to task for their anti-democratic and anticritical tendencies.)
Popper, K.R. (1947a) ‘New Foundations for Logic’, Mind 56: 193–235, with corrections in (1948) 57: 69–70. (A
system of natural deduction.)
Popper, K.R. (1947b) ‘Logic Without Assumptions’, Proceedings of the Aristotelian Society, 48: 251–92.
(Investigates the notion of deducibility and the foundations of logic in language.)
Popper, K.R. (1957) The Poverty of Historicism, London: Routledge. (States and rebuts the arguments for belief in
inexorable laws of human destiny.)
Popper, K.R. (1962) Conjectures and Refutations, London: Routledge. (A wide-ranging collection of essays from the
period 1940–62, including several that have become classics.)
Popper, K.R. (1972) Objective Knowledge, Oxford: Clarendon Press. (A collection of essays from 1948–72, many
being extensions of and alterations to his original philosophy of science and epistemology.)
Popper, K.R. (1983a) Realism and the Aim of Science, London: Hutchinson. (Long commentary on many of the
issues originally discussed in The Logic of Scientific Discovery and their reception; part I of the Postscript.)
Popper, K.R. (1990) A World of Propensities, Bristol: Thoemmes. (The propensity interpretation of probability
statements developed as a full-blown metaphysics.)
Popper, K.R. (1992) In Search of a Better World, London: Routledge. (Speeches and address to general audiences.)
Schilpp, P.A. (ed.) (1974) The Philosophy of Karl Popper, La Salle, IL: Open Court. (Two volumes, containing
Popper’s intellectual autobiography, 33 critical essays and ‘Replies to My Critics’, as well as a complete
bibliography up 1973.)
The Problem of Demarcation
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One central theme of Popper’s work was the problem of demarcation, that is, of
telling the difference between science and non-science.
Unlike the Vienna circle, who sought to reject what wasn’t science as (in some
sense) meaningless, Popper’s position was more modest: Science was a game
which required that in some specifiable circumstances we reject a given scientific
theory; other sorts of theory could perfectly well be meaningful, but they are not
scientific.
At first this was a logical feature of the theory– a scientific theory must have
implications for observation, and when our observations show those implications
are false, we can reject the theory.
Popper objected to inductivist views (according to which a scientific theory had to
be supported by observations) for two reasons:
– First, he argued that it was too easy to find evidence ‘supporting’ a given theory. (Here he
used examples from Freudian psychology, Marxist theory and other places.)
– Second, he argued that Hume’s critique of induction showed that we simply could not actually
support (i.e. give good evidence for) a general claim using evidence from particular cases.
Modus Tollens and Modus Moron
1. If T then O
2. Not O
3. Therefore, not T.
Vs.
1’. If T then O
2’. O
3’. Therefore, T.
• (The polite, official label for the second
argument form is ‘affirming the consequent’…)
Impressive, but…
• Popper’s influence on scientists’ own views of their work has been
considerable– perhaps even greater than T.S. Kuhn’s influence.
• His presentation of science as grounded in the ongoing testing of
our theories and always open to the possibility that they will be
refuted at the next test, resonates strongly with them.
• Philosophically, his rejection of induction allowed him to avoid the
difficulties the logical empiricists struggled with in their attempts to
understand confirmation.
• But the rejection of induction also leaves a major gap in his
understanding of science– we don’t simply regard science as a kind
of intellectual game: we use the results of science to guide our
practice when we are engaged in pursuing our ends. But without
some account of induction, Popper’s position provides no account
of how or why we should prefer to act in ways our present welltested but still-standing theories imply will be successful.
Difficulties
• Popper was extremely persistent and stubborn– as well as
very demanding of the students who worked with him.
• His work certainly did evolve– but he never budged on any
of his fundamental views, including especially the complete
rejection of induction.
• He did attempt to capture some sense of progress for
science in terms of accounts of the truth-content and
question-content of theories, but these notions were
always untenable, and did nothing to resolve the lack of
any commitment to induction.
• He did some influential work on probability theory, in
which conditional probabilities (rather than absolute
probabilities) were taken as primitive.
A puzzle about induction
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There is something odd about Hume’s (and Popper’s) critique of induction and its
relation to the modest attitudes of scientists towards even their most successful
theories.
On one hand, it is clear that (for example) despite its many brilliant successes,
Newtonian physics has turned out to be false.
The history of such failures shows scientists should doubt that their present
theories are true.
But their modesty is not a form of Humean skepticism– where a theory fits a large
collection of well-established facts, we don’t expect similar observations to
undermine the theory later on.
The observations scientists are ‘afraid of’ i.e. the ones they suspect may lead to
trouble for their presently held theories, are observations of new kinds of facts:
observations of events at higher energies, or on smaller scales, or of new
phenomena.
But a Humean has no more reason to expect new phenomena to give trouble than
she has to expect entirely familiar sorts of cases to give trouble–in either case, it’s
only induction that supports our present theory, and Hume and Popper both reject
induction as invalid (though Hume thought, wisely, that it was also indispensable).
Further considerations
• On the other hand, it may well be induction that
leads us to suspect theories are more likely to go
wrong when we apply them to new kinds of
phenomena, or new extremes of parameter
values.
• Still, pure inductive skepticism like Popper’s
draws no line here– induction is invalid, period—
not ‘OK’ when applied to familiar cases and
dubious when extended beyond them.
Fallibilism
• Accepting that our theories may well turn out to
be wrong seems reasonable.
• In fact, scientists can sometimes even predict,
based on their theory, the sorts of circumstances
in which it’s likely to fail (cf. recent reviews of the
LHC and its likely results for the ‘standard model’
in particle physics).
• But fallibilism does not imply that we have no
grounds for accepting our present theories– it
only implies that we suspect they may not turn
out to be completely true.
Truth and other targets
• There is something odd about thinking in terms of truth here, as well.
• The methods of science (as Popper rightly saw) continually expose
theories to further tests– even a theory that is perfectly reliable in the
range of applications we’ve already tried can turn out to be false because
it fails in some new application.
• Perhaps, given the actual record of science, it would be wiser to say that
the goal of science is to achieve greater and greater reliability and
precision in our descriptions and explanations.
• In some sense this actually brings us back to the idea of truth, but in a
more reserved way: Perfect reliability and precision is both something we
don’t ever expect to actually achieve, and extremely close to what we
would expect (even demand) from a theory we would regard as true. Of
course we would also expect maximal power (i.e. we would favour a
theory that would constrain our observations more tightly, assuming equal
pecision and reliability) before we would be tempted to speak of ‘truth,
the whole truth, and…’
Searching for Truth in all the wrong
places….
• Popper’s skepticism about induction leaves us
with little grounds for confidence about making
progress in science—
• Assuming everything works without a hitch as we
falsify theories, we may be right in rejecting one
theory after another, but
• We’re left with no sense of progress towards the
truth (no reason to suspect that our theories are
anything more than falsehoods not yet exposed).
Conjecture and Refutation
• The pattern of scientific work on this account is very simple
and repetitive:
1.
2.
A scientific conjecture is made.
Scientists try to refute it.
• The second step is repeated until it succeeds.
• While Popper acknowledges that we can respond to a failed
test by modifying our theory, or even by modifying some
assumptions that were made about the test, he is
uncomfortable with this sort of defensive strategy– to
Popper this looks ad hoc. Theory construction by means of
empirical guidance doesn’t take the sort of risk that Popper
thinks a good theory should.
Norms and games
• For Popper these are rules that somehow constitute the game of science.
• We can do other things (make ad hoc adjustments to our theories or
auxiliary hypotheses as new evidence accumulates) but that is playing
another game.
• The surviving theories, at any given time, are no more likely to be right
than any other theory not yet refuted by the evidence we have.
• We have no reason even to suppose that the most basic, reliable
phenomena should continue to respect the regularities we have observed
in them (this is Hume’s skepticism about induction, which Popper
endorses).
• But scientists are very confident about their ability to reliably produce
certain phenomena and the success of their current theories at explaining
them–even if they are fallibilists about those theories when applied to
new phenomena and observations.
• Our division of phenomena into kinds is largely a practical matter- the use
of certain kinds of apparatus, making certain kinds of measurements, etc.
Competition and selection
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It’s tempting to draw an analogy between what Popper describes and evolution.
Falsification of a theory would be something like extinction.
Continued work on an unfalsified theory would be like the continuation of a
species over time.
But note how much richer the evolutionary picture is.
Evolution is (as Darwin defined it) descent with modification.
Nothing in such a view rules out modification of a theory in the course of trying to
make it fit the phenomena.
Extinction can still occur, if no suitable modifications arise.
In science, modifications that are purely ad hoc rarely succeed over the longer
term.
Further, in evolution many species that are not optimally adapted to their
environment can still persist for some time.
In particular, in the absence of superior competition work may well continue on a
theory even after scientists are convinced that it is wrong– after all, the work may
well focus further investigation on issues that will help with the development of a
new theory.
From logic to psychology
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For Popper, as for the positivists, the aim of philosophy of science was to lay out how science
should be done, and the criteria for this notion would be fundamentally logical– they would turn on
the logical relations between a scientific theory and observations.
But the Duhem/Quine problem led him to turn this logical line into a psychological one: the
scientist is someone who would not avoid falsification by blaming ‘auxiliaries’– she would place her
theory in the harshest environment possible, and blame her theory if the results don’t fit. Here the
psychology of decision making replaces the logic of theory and evidence…
This means that scientist actually risks rejecting a true theory (if she retains a false auxiliary or even
a false observations and consequently regards the theory as falsified).
The last shred of logical progress (a steady accumulation of falsified theories) in Popper’s game of
science is lost here.
Further, the demarcation Popper had sought becomes problematic too. Popper can still draw a
logical line, if he can specify which theories would be falsified by some possible observations– but
the users of these theories could still defend them, in principle, by shifting the blame elsewhere.
Godfrey-Smith also emphasizes a problem for probabalistic theories (such as classical
thermodynamics)– if all they tell us is that certain observations are very improbable, then strictly
speaking they aren’t really falsifiable! Again, the psychology rather than the logic of science
becomes central here.
The problem of induction comes home
to roost
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Godfrey Smith suggests a question Popper cannot credibly answer:
Suppose we are building a bridge, and we have two theories we might employ to
guide our work.
One theory has been tested in many cases, and passed all the tests.
Another has yet to be tested, but (trivially) has passed every test we’ve ever put it
to.
Which should we use?
There is something odd about this example, though: such a new theory, to be
relevant, has to cover a lot of the same ground as the old successful theory– and in
order not to be falsified, it must succeed on those tests. (But there’s a worse
example in the wings for Popper.)
After all, without induction, success thus far gives us no reason to suppose the
theory will succeed in a further application.
And without induction, failure in some cases gives us no reason to suppose the
theory will not succeed in a further application. (Such a theory could not be true,
but it could still be a reliable guide to future results!)
Of course if we can specify a finite range of theories within which we can show the
truth to lie, then we can proceed by elimination in a quasi-Popperian way.
Taking risks
• Even with all these reservations, Godfrey-Smith thinks
Popper was onto something worthwhile:
• The idea that scientific theories must take risks– they must
have enough ‘empirical content’ (even as qualified by
Duhem and Quine) that they really could be shown (or at
least justifiably accepted to have been shown) false.
• But this focuses more attention on how a theory is
deployed, and how its users respond to difficulties, than on
the ‘logic’ of the theory itself. We can proceed
scientifically, if we treat the theory as subject to empirical
correction and even refutation. Or we can proceed nonscientifically, by isolating and sheltering the theory from
any and all empirical threats. These choices are even open
to Freudians and Marxists.
Precambrian Rabbits
• A standard one-line answer to the question, what would
refute evolution.
• Of course it would take very careful work to establish such
an astounding result.
• But there are many other tests evolution has passed and
continues to pass (the in-principle independent treestructures of taxonomy, biochemistry, development and the
fossil record continue to match up, for example). Applying
new tests to these trees has led to many new results and
much refinement of our views on the course of evolution—
but they have always fit within the narrow range of results
acceptable for evolution.