Hey all of you! Post your comments to the first summer assignment. That is how you turn in your work and get credit. As I write this, only three have responded to the first assignment.
This week we wrap-up our look at the nature of science
with another classic in the philosophy of science cannon,
“Science: Conjectures and Refutations” by Karl Popper. This essay is
meaty so take your time to digest what it has to offer.
But first, some vocabulary -
Induction: inferring a general conclusion or principle from particular
instances (from specific to general). The theory of plate tectonics
(general principle) is inferred from many individual evidences
2. Deduction: inference in which the conclusion
about particulars follows from general principle (from general to
specific). Sherlock Holms used deduction. Fingerprints are unique to the
individual (general principle) so if the suspect’s fingerprints are on
the knife, he held the knife (specific).
3. Demarcation: to set apart, distinguish
Distinguishing science from pseudo-science:
truth through science is an empirical process based on inductive
reasoning. It is different from other “truth” finding activities such
as astrology and tarot card reading, which are based on other
principles. While induction does result in new knowledge it comes at a
cost. The cost is that science is tentative and subject to revision. Due
to the nature of science, two philosophical problems arise. The first
problem is distinguishing science from pseudo-science also known as the
problem of demarcation. For example, what makes astronomy science yet
astrology not? The second problem is the logical problem of induction.
In other words how can science find truth if it is always tentative and
subject to revision? You will find that being tentative - subject to revision is the strength of science!
The philosopher, Sir Karl Popper
(1902-1994) addressed these problems in the essay you are about to read.
In the essay he first distinguishes science from pseudo-science by
solving the problem of demarcation. He then explores the logical problem
of induction and concludes that it and the problem of demarcation are
the same. The ideas presented in this essay likely are new to you. Your
understanding will require careful attention to the essay, thoughtful
responses to the questions, and careful discussion with your instructor.
There are two primary objectives for this activity. First, you will
learn what separates science from pseudo-science. Second, will be to
understand how we can know a theory is true even if it is tentative and
subject to revision.
My hope is that after reading these
two essays you begin to appreciate the dimensions of science. As you
have already discovered, science is not just doing experiments. It is
much more – a grand achievement of the human mind that is capable of
understanding the universe in which we live.
So sit back, relax, and enjoy Popper’s essay you will find the questions at the end of the essay.
See you in class. I am looking forward to meeting you.
Karl Popper - Science: Conjectures and Refutations
Turnbull had predicted evil consequences, . . . and was now doing the
best in his power to bring about the verification of his own prophecies.
When I received the list of
participants in this course and realized that I had been asked to speak
to philosophical colleagues I thought, after some hesitation and
consultation that you would probably prefer me to speak about those
problems which interest me most, and about those developments with which
I am most intimately acquainted. I therefore decided to do what I have
never done before: to give you a report on my own work in the philosophy
of science, since the autumn of 1919 when I first began to grapple with
the problem, "When should a theory be ranked as scientific?" or "Is
there a criterion for the scientific character or status of a theory?"
problem which troubled me at the time was neither, "When is a theory
true?"nor, "When is a theory acceptable?" My problem was different. I
wished to distinguish between science and pseudo-science; knowing very
well that science often errs, and that pseudo-science may happen to
stumble on the truth.
I knew, of course, the most
widely accepted answer to my problem: that science is distinguished from
pseudo-science or from "metaphysics" by its empirical method, which is
essentially inductive, proceeding from observation or experiment. But
this did not satisfy me. On the contrary, I often formulated my problem
as one of distinguishing between a genuinely empirical method and a
non-empirical or even a pseudo-empirica1 method-that is to say, a method
which, although it appeals to observation and experiment, nevertheless
does not come up to scientific standards. The latter method may be
exemplified by astrology with its stupendous mass of empirical evidence
based on observation-on horoscopes and on biographies.
as it was not the example of astrology which led me to my problem I
should perhaps briefly describe the atmosphere in which my problem arose
and the examples by which it was stimulated. After the collapse of the
Austrian Empire there had been a revolution in Austria: the air was full
of revolutionary slogans and ideas, and new and often wild theories.
Among the theories which interested me Einstein's theory of relativity
was no doubt by far the most important. Three others were Marx's theory
of history, Freud's psycho-analysis, and Alfred Adler's so-called
There was a lot of popular
nonsense talked about these theories, and especially about relativity
(as still happens even today), but I was fortunate in those who
introduced me to the study of this theory. We all-the small circle of
students to which I belonged-were thrilled with the result of
Eddington's eclipse observations which in 1919 brought the first
important confirmation of Einstein's theory of gravitation. It was a
great experience for us, and one which had a lasting influence on my
The three other theories I
have mentioned were also widely discussed among students at that time. I
myself happened to come into personal contact with Alfred Adler, and
even to co-operate with him in his social work among the children and
young people in the working-class districts of Vienna where he had
established social guidance clinics.
It was during the
summer of 1919 that I began to feel more and more dissatisfied with
these three theories-the Marxist theory of history, psychoanalysis, and
individual psychology; and I began to feel dubious about their claims to
scientific status. My problem perhaps first took the simple form, "What
is wrong with Marxism, psycho-analysis, and individual psychology? Why
are they so different from physical theories, from Newton's theory, and
especially from the theory of relativity?"
this contrast clear I should explain that few of us at the time would
have said that we believed in the truth of Einstein's theory of
gravitation. This shows that it was not my doubting the truth of those
other three theories which bothered me, but something else. Yet neither
was it that I merely felt mathematical physics to be more exact than the
sociological or psychological type of theory. Thus what worried me was
neither the problem of truth, at that stage at least, nor the problem of
exactness or measurability. It was rather that I felt that these other
three theories, though posing as sciences, had in fact more in common
with primitive myths than with science; that they resembled astrology
rather than astronomy.
I found that those of my
friends who were admirers of Marx, Freud, and Adler, were impressed by a
number of points common to these theories, and especially by their
apparent explanatory power. These theories appeared to be able to
explain practically everything that happened within the fields to which
they referred. The study of any of them seemed to have the effect of an
intellectual conversion or revelation, opening your eyes to a new truth
hidden from those not yet initiated. Once your eyes were thus opened you
saw confirming instances everywhere: the world was full of
verifications of the theory. Whatever happened always confirmed it. Thus
its truth appeared manifest; and unbelievers were clearly people who
did not want to see the manifest truth; who refused to see it, either
because it was against their class interest, or because of their
repressions which were still "un-analysed" and crying aloud for
The most characteristic element in this
situation seemed to me the incessant stream of confirmations, of
observations which "verified" the theories in question; and this point
was constantly emphasized by their adherents. A Marxist could not open a
newspaper without finding on every page confirming evidence for his
interpretation of history; not only in the news, but also in its
presentation-which revealed the class bias of the paper-and especially
of course in what the paper did not say. The Freudian analysts
emphasized that their theories were constantly verified by their
"clinical observations." As for Adler, I was much impressed by a
personal experience. Once, in 1919, 1 reported to him a case which to me
did not seem particularly Adlerian, but which he found no difficulty in
analysing in terms of his theory of inferiority feelings, although he
had not even seen the child. Slightly shocked, I asked him how he could
be so sure. "Because of my thousandfold experience," he replied;
whereupon I could not help saying: "And with this new case, I suppose,
your experience has become thousand-and-one-fold."
I had in mind was that his previous observations may not have been much
sounder than this new one; that each in its turn had been interpreted
in the light of "previous experience," and at the same time counted as
additional confirmation. What, I asked myself, did it confirm? No more
than that a case could be interpreted in the light of the theory. But
this meant very little, I reflected, since every conceivable case could
be interpreted in the light of Adler's theory, or equally of Freud's. I
may illustrate this by two very different examples of human behaviour:
that of a man who pushes a child into the water with the intention of
drowning it; and that of a man who sacrifices his life in an attempt to
save the child. Each of these two cases can be explained with equal ease
in Freudian and in Adlerian terms. According to Freud the first man
suffered from repression (say, of some component of his Oedipus
complex), while the second man had achieved sublimation. According to
Adler the first man suffered from feelings of inferiority (producing
perhaps the need to prove to himself that he dared to commit some
crime), and so did the second man (whose need was to prove to himself
that he dared to rescue the child). I could not think of any human
behaviour which could not be interpreted in terms of either theory. It
was precisely this fact-that they always fitted, that they were always
confirmed-which in the eyes of their admirers constituted the strongest
argument in favour of these theories. It began to dawn on me that this
apparent strength was in fact their weakness.
Einstein's theory the situation was strikingly different. Take one
typical instance-Einstein's prediction, just then confirmed by the
findings of Eddington's expedition. Einstein's gravitational theory had
led to the result that light must be attracted by heavy bodies (such as
the sun), precisely as material bodies were attracted. As a consequence
it could be calculated that light from a distant fixed star whose
apparent position was close to the sun would reach the earth from such a
direction that the star would seem to be slightly shifted away from the
sun; or, in other words, that stars close to the sun would look as if
they had moved a little away from the sun, and from one another. This is
a thing which cannot normally be observed since such stars are rendered
invisible in daytime by the sun's overwhelming brightness; but during
an eclipse it is possible to take photographs of them. If the same
constellation is photographed at night one can measure the distances on
the two photographs, and check the predicted effect.
the impressive thing about this case is the risk involved in a
prediction of this kind. If observation shows that the predicted effect
is definitely absent, then the theory is simply refuted. The theory is
incompatible with certain possible results of observation-in fact with
results which everybody before Einstein would have expected. This is
quite different from the situation I have previously described, when it
turned out that the theories in question were compatible with the most
divergent human behaviour, so that it was practically impossible to
describe any human behaviour that might not be claimed to be a
verification of these theories.
These considerations led me in the winter of 1919-20 to conclusions which I may now reformulate as follows.
(1) It is easy to obtain confirmations, or verifications, for nearly every theory-if we look for confirmations.
Confirmations should count only if they are the result of risky
predictions; that is to say, if, unenlightened by the theory in
question, we should have expected an event which was incompatible with
the theory-an event which would have refuted the theory.
Every "good" scientific theory is a prohibition: it forbids certain
things to happen. The more a theory forbids, the better it is.
A theory which is not refutable by any conceivable event is
nonscientific. Irrefutability is not a virtue of theory (as people often
think) but a vice.
(5) Every genuine test of a theory
is an attempt to falsify it, or to refute it. Testability is
falsifiability; but there are degrees of testability; some theories are
more testable, more exposed to refutation, than others; they take, as it
were, greater risks.
(6) Confirming evidence should
not count except when it is the result of a genuine test of the theory;
and this means that it can be presented as a serious but unsuccessful
attempt to falsify the theory. (I now speak in such cases of
(7) Some genuinely testable
theories, when found to be false, are still upheld by their
admirers-for example by introducing ad hoc some auxiliary assumption, or
by re-interpreting theory ad hoc in such a way that it escapes
refutation. Such a procedure is always possible, but it rescues the
theory from refutation only at the price of destroying, or at least
lowering, its scientific status. (I later described such a rescuing
operation as a "conventionalist twist" or a "conventionalist
One can sum up all this by saying that
the criterion of the scientific status of a theory is its
falsifiability, or refutability, or testability.
may perhaps exemplify this with the help of the various theories so far
mentioned. Einstein's theory of gravitation clearly satisfied the
criterion of falsifiability. Even if our measuring instruments at the
time did not allow us to pronounce on the results of the tests with
complete assurance, there was clearly a possibility of refuting the
Astrology did not pass the test. Astrologers
were greatly impressed, and misled, by what they believed to be
confirming evidence_so much so that they were quite unimpressed by any
unfavourable evidence. Moreover, by making their interpretations and
prophecies sufficiently vague they were able to explain away anything
that might have been a refutation of the theory had the theory and the
prophecies been more precise. In order to escape falsification they
destroyed the testability of their theory. It is a typical soothsayer's
trick to predict things so vaguely that the predictions can hardly fail:
that they become irrefutable.
The Marxist theory of
history, in spite of the serious efforts of some of its founders and
followers, ultimately adopted this soothsaying practice. In some of its
earlier formulations (for example in Marx's analysis of the character of
the "coming social evolution') their predictions were testable, and in
fact falsified.2 Yet instead of accepting the refutations the followers
of Marx reinterpreted both the theory and the evidence in order to make
them agree. In this way they rescued the theory from refutation; but
they did so at the price of adopting a device which made it irrefutable.
They thus gave a "conventionalist twist" to the theory; and by this
stratagem they destroyed its much advertised claim to scientific status.
The two psycho-analytic theories were in a different
class. They were simply non-testable, irrefutable. There was no
conceivable human behaviour which could contradict them. This does not
mean that Freud and Adler were not seeing certain things correctly: I
personally do not doubt that much of what they say is of considerable
importance, and may well play its part one day in a psychological
science which is testable. But it does mean that those "clinical
observations" which analysts naively believe confirm their theory cannot
do this any more than the daily confirmations which astrologers find in
their practice.3 And as for Freud's epic of the Ego, the Super-ego, and
the Id, no substantially stronger claim to scientific status can be
made for it than for Homer's collected stories from Olympus. These
theories describe some facts, but in the manner of myths. They contain
most interesting psychological suggestions, but not in a testable form.
the same time I realized that such myths may be developed, and become
testable; that historically speaking all-or very nearly all-scientific
theories originate from myths, and that a myth may contain important
anticipations of scientific theories. Examples are Empedocles' theory of
evolution by trial and error, or Parmenides' myth of the unchanging
block universe in which nothing ever happens and which, if we add
another dimension, becomes Einstein's block universe (in which, too,
nothing ever happens, since everything is, four dimensionally speaking,
determined and laid down from the beginning). I thus felt that if a
theory is found to be non-scientific, or "metaphysical" (as we might
say), it is not thereby found to be unimportant, or insignificant, or
"meaningless," or "nonsensical." it cannot claim to be backed by
empirical evidence in the scientific sense-although it may easily be, in
some genetic sense, the "result of observation."
were a great many other theories of this pre-scientific or
pseudoscientific character, some of them, unfortunately, as influential
as the Marxist interpretation of history; for example, the racialist
interpretation of history-another of those impressive and
all-explanatory theories which act upon weak minds like revelations.)
the problem which I tried to solve by proposing the criterion of
falsifiability was neither a problem of meaningfulness or significance,
nor a problem of truth or acceptability. It was the problem of drawing a
line (as well as this can be done) between the statements, or systems
of statements, of the empirical sciences, and all other
statements-whether they are of a religious or of a metaphysical
character, or simply pseudo-scientific. Years later-it must have been in
1928 or 1929-I called this first problem of mine the "problem of
demarcation. " The criterion of falsifiability is a solution to this
problem of demarcation, for it says that statements or systems of
statements, in order to be ranked as scientific, must be capable of
conflicting with possible, or conceivable, observations....
us now turn from our logical criticism of the psychology of experience
to our real problem-the problem of the logic of science. Although some
of the things I have said may help us here, in so far as they may have
eliminated certain psychological prejudices in favour of induction, my
treatment of the logical problem of induction is completely independent
of this criticism, and of all psychological considerations. Provided you
do not dogmatically believe in the alleged psychological fact that we
make inductions, you may now forget my whole story with the exception of
two logical points: my logical remarks on testability or falsifiability
as the criterion of demarcation; and Hume's logical criticism of
From what I have said it is obvious that
there was a close link between the two problems which interested me at
that time: demarcation, and induction or scientific method. It was easy
to see that the method of science is criticism, i.e. attempted
falsifications. Yet it took me a few years to notice that the two
problems-of demarcation and of induction-were in a sense one....
recently came across an interesting formulation of this belief in a
remarkable philosophical book by a great physicist-Max Born's Natural
Philosophy of Cause and Chance.5 He writes: "Induction allows us to
generalize a number of observations into a general rule: that night
follows day and day follows night . . . But while everyday life has no
definite criterion for the validity of an induction, . . .science has
worked out a code, or rule of craft, for its application." Born nowhere
reveals the contents of this inductive code (which, as his wording
shows, contains a "definite criterion for the validity of an
induction"); but he stresses that "there is no logical argument" for its
acceptance: "it is a question of faith"; and he is therefore "willing
to call induction a metaphysical principle." But why does he believe
that such a code of valid inductive rules must exist? This becomes clear
when he speaks of the "vast communities of people ignorant of, or
rejecting, the rule of science, among them the members of
anti-vaccination societies and believers in astrology. It is useless to
argue with them; I cannot compel them to accept the same criteria of
valid induction in which I believe: the code of scientific rules." This
makes it quite clear that "valid induction" was here meant to serve as a
criterion of demarcation between science and pseudo-science.
it is obvious that this rule or craft of "valid induction" is not even
metaphysical: it simply does not exist. No rule can ever guarantee that a
generalization inferred from true observations, however often repeated,
is true.(Born himself does not believe in the truth of Newtonian
physics, in spite of its success, although he believes that it is based
on induction.) And the success of science is not based upon rules of
induction, but depends upon luck, ingenuity, and the purely deductive
rules of critical argument.
I may summarize some of my conclusions as follows:
Induction, i.e. inference based on many observations, is a myth. It is
neither a psychological fact, nor a fact of ordinary life, nor one of
(2) The actual procedure of
science is to operate with conjectures: to jump to conclusions-often
after one single observation (as noticed for example by Hume and Born).
(3) Repeated observations and experiments function in science as tests of our
conjectures or hypotheses, i.e. as attempted refutations.
The mistaken belief in induction is fortified by the need for a
criterion of demarcation which, it is traditionally but wrongly
believed, only the inductive method can provide.
(5) The conception of such an inductive method, like the criterion of verifiability, implies a faulty demarcation.
(6) None of this is altered in the least if we say that induction makes theories only probable rather than certain.
as I have suggested, the problem of induction is only an instance or
facet of the problem of demarcation, then the solution to the problem of
demarcation must provide us with a solution to the problem of
induction. This is indeed the case, I believe, although it is perhaps
not immediately obvious.
For a brief formulation of
the problem of induction we can turn again to Born, who writes: ". . .
no observation or experiment, however extended can give more than a
finite number of repetitions"; therefore, "the statement of a law-B
depends on A-always transcends experience. Yet this kind of statement is
made everywhere and all the time, and sometimes from scanty material.'
other words, the logical problem of induction arises from (a) Hume's
discovery (so well expressed by Born) that it is impossible to justify a
law by observation or experiment, since it "transcends experience"; (b)
the fact that science proposes and uses laws "everywhere and all the
time." (Like Hume, Born is struck by the "scanty material," i.e. the few
observed instances upon which the law may be based.) To this we have to
add (c) the principle of empiricism which asserts that in science, only
observation and experiment may decide upon the acceptance or rejection
of scientific statements, including laws and theories.
three principles, (a), (b), and (c), appear at first sight to clash;
and this apparent clash constitutes the logical problem of induction.
with this clash, Born gives up (c), the principle of empiricism (as
Kant and may others, including Bertrand Russell, have done before him),
in favour of what he calls a "metaphysical principle"; a metaphysical
principle which he does not even attempt to formulate; which he vaguely
describes as a "code or rule of craft"; and of which I have never seen
any formulation which even looked promising and was not clearly
But in fact the principles (a) to (c) do
not clash. We can see this the moment we realize that the acceptance by
science of a law or of a theory is tentative only; which is to say that
all laws and theories are conjectures, or tentative hypotheses(a
position which I have sometimes called "hypotheticism") and that we may
reject a law or theory on the basis of new evidence, without necessarily
discarding the old evidence which originally led us to accept it.7
principles of empiricism (c) can be fully preserved, since the fate of a
theory, its acceptance or rejection, is decided by observation and
experiment_ by the result of tests. So long as a theory stands up to the
severest tests we can design, it is accepted; if it does not, it is
rejected. But it is never inferred, in any sense, from the empirical
evidence. There is neither a psychological nor a logical induction. Only
the falsity of the theory can be inferred from empirical evidence, and
this inference is a purely deductive one.
that it is not possible to infer a theory from observation statements;
but this does not affect the possibility of refuting a theory by
observation statements. The full appreciation of the possibility makes
the relation between theories and observations perfectly clear. This
solves the problem of the alleged clash between the principles (a), (b),
and(c), and with it Hume's problem of induction....
This is a slight oversimplification, for about half of the Einstein
effect may be derived from the classical theory, provided we assume a
ballistic theory of light.
2. See for example, my Open Society and Its Enemies, ch. 15, section iii, and notes 13-14.
"Clinical observations," like all other observations, are
interpretations in the light of theories; and for this reason alone they
are apt to seem to support those theories in the light of which they
were interpreted. But real support can be obtained only from
observations undertaken as tests (by "attempted refutations"); and for
this purpose criteria of refutation have to be laid down beforehand; it
must be agreed which observable situations, if actually observed, mean
that the theory is refuted. But what kind of clinical responses would
refute to the satisfaction of the analyst not merely a particular
analytic diagnosis but psycho-analysis itself? And have such criteria
ever been discussed or agreed upon by analysts? Is there not, on the
contrary, a whole family of analytic concepts, such as "ambivalence" (l
do not suggest that there is no such thing as ambivalence), which would
make it difficult, if not impossible, to agree upon such criteria?
Moreover, how much headway has been made in investigating the question
of the extent to which the (conscious or unconscious) expectations and
theories held by the analyst influence the "clinical responses" of the
patient? To say nothing about the conscious attempts to influence the
patient by proposing interpretations to him, etc.) Years ago Iintroduced
the term "Oedipus effect" to describe the influence of a theory or
expectation or prediction upon the event which it predicts or describes:
it will be remembered that the causal chain leading to Oedpus'
parricide was started by the oracle's prediction of this event. This is a
characteristic and recurrent theme of such myths, but one which seems
to have failed to attract the interest of the analysts, perhaps not
accidentally. (The problem of confirmatory dreams suggested by the
analyst is discussed by Freud, for example in Gesammelte Schriften,i
111, 1925, where he says on p. 314: "If anybody asserts that most of the
dreams which can be utilized in an analysis . . . owe their origin to
[the analyst's] suggestion, then no objection can be made from the point
of view of analytic theory. Yet there is nothing in this fact,"he
surprisingly adds, "which would detract from the reliability of our
4. The case of astrology, nowadays a typical
pseudo-science, may illustrate this point. It was attacked, by
Aristotelians and other rationalists, down to Newton's day, for the
wrong reason-for its now an accepted assertion that the planets had an
"influence" upon terrestrial ("sublunar") events. In fact Newton's
theory of gravity, and especially the lunar theory of the tides, was
historically speaking an offpsring of astrological lore. Newton, it
seems, was most reluctant to adopt a theory which came from the same
stable as for example the theory that "influenza"epidemics are due to an
astral "influence." And Galileo, no doubt for the same reason, actually
rejected the lunar theory of the tides; and his misgivings about Kepler
may easily be explained by his misgivings about astrology.
5. Max Born, Natural Philosophy of Cause and Chance, Oxford, 1949, p. 7.
6. Natural Philosophy of Cause and Chance, p. 6.
I do not doubt that Born and many others would agree that theories are
accepted only tentatively. But the widespread belief in induction shows
that the far-reaching implications of this view are rarely seen.
Karl Popper, Conjectures and Refutations: The Growth of Scientific
Knowledge (New York: Harper Torchbooks, 1963), pp. 33-39, 52-55.
1. What makes a theory scientific?
Explain why in Popper’s view Marx’s theory of history, Freud’s
psycho-analysis and Alfred Adler’s “individual psychology” are not
science. Please provide at least one example from the reading in your
3. Explain why Einstein’s theory of relativity is science.
4. What is the danger of ad hoc theories?
5. In your words, explain how the problem of demarcation solved?
6. What is the logical problem of induction?
7. How does Popper solve the problem of induction?
8. Why is being tentative and subject to revision the strength of science?
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