KARL POPPER AND FALSIFICATIONIST CRITICISM
BOOK V - Page 1
Karl Popper (1902-1995) was born in Vienna, Austria. He enrolled in the University of Vienna in 1918, where he studied physics, mathematics, and philosophy. In 1928 he received his Ph.D. for a dissertation titled On the Problem of Method in the Psychology of Thinking. He never returned to the subject of psychology again during his professional career, because he became convinced that methodology of science is exclusively a matter of logic and objective knowledge instead of psychology. Popper was personally acquainted with Rudolf Carnap and other members of the Vienna Circle, and although he had been invited to address the group at a meeting in which he set forth his philosophy of science, he was never a member of the Circle. In 1937 he was appointed a senior lecturer to Canterbury University College in Christchurch, New Zealand, and then in 1945 he was appointed to a readership at the London School of Economics, University of London. In 1949 he was made professor of logic and scientific method at the London School. He was knighted in 1964.
Einstein's Influence and the Falsificationist Thesis of Criticism
In his intellectual autobiography in
Philosophy of Karl Popper (1974) Popper states
that Einstein was the most important influence on
The influence was not a personal one, since
Popper and Einstein did not actually meet until
1950; the influence was through Einstein's published
The year 1919 was the fateful year in
Popper's intellectual life.
At that time he was interested in the views
of several thinkers including Marx's theory of
history, Freud's theory of psychoanalysis, and
Alfred Adler's theory called "individual
Popper relates in his "Science:
Conjectures and Refutations" (1957) in Conjectures and Refutations (1963), that he had come into personal
contact with Alfred Adler and cooperated with Adler
in the latter's social work with children and young
people in the working class districts of Vienna
during the last years of the Austrian Empire and the
In the summer of 1919 Popper became dissatisfied
with the views of Marx, Freud and Adler, because the
persons who accepted and advocated these theories
were strongly impressed by the theories' purported
explanatory power, and because study of these
theories had the effect of an intellectual
conversion or revelation.
Most objectionable to Popper was the fact
that once the reader's eyes were opened to the
theory, he found that the theory was verified
everywhere one might think of applying it.
Unbelievers were dismissed as persons who
could not see the verifications.
In Popper's view the apparent strength of
these theories' purported “explanatory” power is
their principal weakness.
Popper saw in Einstein's theory a striking contrast to the situation he found in the views of Marx, Freud and Adler. Eddington's solar eclipse observations in 1919 brought the first important test to bear upon Einstein's relativity theory of gravitation. This test was distinctive, because in the test there was a risk involved in the theory's prediction. Had Eddington's observations showed that the predicted effect is definitely absent, then Einstein's theory would simply have been refuted. And the risk in Einstein's case was very great, since the predicted effect was different from what was expected from Newton's theory, which had long demonstrated great success culminating with the discovery of the planet Neptune. In his autobiography Popper said that what impressed him most was Einstein's own clear statement that he should regard his theory of relativity as untenable, if it should fail certain tests. This was an attitude that was very different from the dogmatic attitude of the Marxians, Freudians, and Adlerians. Einstein was looking for crucial experiments where agreement with his predictions would by no means establish his theory, but where disagreement with his predictions, as Einstein was the first to say, would show his theory to be untenable. Thus in 1919 Popper concluded that the critical attitude, which does not look for verifications but rather looks for crucial tests that can refute the tested theory, is the correct attitude for science, even though the crucial tests can never establish the theory. This is Popper's falsificationist philosophy of scientific criticism, the central thesis of his philosophy of science.
Explanation, Information, and the Growth of Science
Popper's philosophy recognizes the dynamic
character of science that is not recognized in the
philosophy of the Positivists.
His statements on the dynamics of science are
found in appendices to the 1968 edition of his Logic
of Scientific Discovery, in his "Truth,
Rationality, and the Growth of Scientific
Knowledge" in his Conjectures
and Refutations, and in "The Rationality of
Scientific Revolutions" in Problems of Scientific Revolutions (ed. Harre, 1975), as well as
elsewhere in his literary corpus.
His falsificationist thesis is not only a
philosophy of scientific criticism but also a
philosophy of scientific explanation and of the
growth of scientific knowledge.
As a philosophy of scientific criticism, it
says that the empirical test outcome can never
establish or "verify" a scientific theory,
but can only refute or "falsify" the
And even before a theory's claims are
considered for testing, it is possible to determine
whether or not it is a scientific explanation: it is
not a scientific explanation if it is not
Another way that Popper describes this
condition is that what makes a theory scientific is
its power to exclude the occurrence of some possible
events, and he calls the singular statements that
describe these excluded events "potential
This way of speaking introduces his idea of
various degrees of explanatory power: the more
that a theory forbids or excludes and therefore the
larger the class of potential falsifiers, then the
more the theory tells us about the world.
Popper calls the variability of degree of
explanatory power the “amount of information
content” of a theory or explanation.
The idea of the amount of information content
may be illustrated by reflection on the logical
conjunction of two statements a and ß.
It is intuitively evident that the
conjunction aß has no lesser amount of information
content than do the component statements taken
separately, and it usually has more information
content than its components.
This is because there are more potential
falsifiers for the conjunction than for the
component statements taken separately; the conjunction
is false if either component is false.
In some contexts Popper calls information
content "empirical content", and he calls
the falsifiability of the theory its
All of these terms refer to a logical
relation between a theory or a hypothesis and its
class of potential falsifiers.
Popper relates the idea of information content to probability theory. He says that the amount of information content is inversely related to the degree of probability that may be associated with a hypothesis. This view can be illustrated also by the logical conjunction: if the probability value P(a) is associated with the statement a and the probability value P(ß) is associated with the statement ß, then by the probability calculus the probability P(aß) associated with the conjunction aß must be less than the probability values P(a) and P(ß). Therefore as the information content of a theory increases, the associated probability must decrease. Popper maintains that the whole problem of the probability of hypotheses as viewed by Carnap is misconceived, because on Carnap's idea of degree of confirmation, scientists should prefer statements having higher associated probabilities, while on Popper's view scientists should prefer theories with higher information content. Therefore in contrast to Carnap's idea of degree of confirmation Popper advances the idea of “degree of corroboration", although in some contexts Popper also uses the phrase "degree of confirmation" in a sense that is synonymous with his idea of degree of corroboration. On the corroboration thesis a scientific theory that has greater information content (because it is more universal, or because it is more accurate than an alternative theory) also has a higher degree of corroboration, if when it is tested it is not falsified. Like the idea of information content, the idea of corroboration is based on the idea of falsifiability, but a theory would not be said to have been corroborated until it had been tested and found to have no falsifying test outcome; the degree of corroboration actually attained does not depend only on the degree of falsifiability. A statement may be falsifiable to a high degree yet it may be only slightly corroborated or it may be falsified.
The measures for corroboration, C(h,e), and probability, P(h,e), for hypothesis h and for basic statement e of evidence describing a test outcome, are related by certain equations. The inverse relation between the measures of corroboration and probability is related as follows:
C(h,e) = 1- P(h,e)
and he is willing to admit a proposal by Kemery in The Journal of Symbolic Logic (1954) that the relation may also be expressed in terms of information science concepts as:
C(h,e) = 1- log P(h,e).
Popper states that the measure of the degree of corroboration, C(h,e), may be interpreted as a measure of the rationality of belief in the statistical hypothesis, h, in the light of test outcomes, e, only if e consists of reports of the outcome of sincere attempts to refute the hypothesis by the severest test that can be devised, rather than attempts to verify h. But the degree of corroboration does not measure the degree of rationality in our belief in the truth of h, since C(h,e) = 0 whenever h is logically true. Rather, it is the measure of accepting tentatively a problematic guess. On the other hand the measure of explanatory power, E(h,e), may be interpreted as the measure of the explanatory power of h with respect to e, even though e is not a report of any genuine and sincere attempts to refute h. The measure E(h,e) is a purely logical relation to the infinite class of potential falsifiers, and in an appendix to his Logic of Scientific Discovery (1959) Popper relates E(h,e) positively to C(h,e) as follows:
E(h,e) = C(h,e)/(1+ P(h) P(h,e)).
The concepts of relatively greater or lesser
degrees of information content and falsifiability
provide the basis for Popper's ideas on scientific
progress, the growth of scientific knowledge, and
the aim of science.
He advances a "metascientific"
criterion of progress, that enables the scientist
and methodologist to know in advance of any
empirical test, whether or not a new theory would be
an improvement over existing theories, were the new
theory able to pass crucial tests, in which its
performance is compared to older existing alternatives.
He calls this criterion the "potential
satisfactoriness" of the theory, and it is
measured by the degree or amount of information
Simply stated, his thesis is that the theory
that tells us more is preferable to one that tells
us less, and the theory that tells us more is also
one which is most falsifiable.
From this it follows that the aim of science
is high empirical information content as well as
successful performance in tests.
It is the criterion of high information
content that makes the growth of science rational.
The aim of science is not high probability,
and the rationality of science does not consist of
constructing deductive axiomatic systems, since
there is little merit in formalizing a theory beyond
the requirements for testing it.
Nor does the growth of science consist of the
accumulation of observations.
Rather it consists of the repeated overthrow
of scientific theories and their replacement by
more satisfactory theories.
The continued growth and progress of science
is essential to the rational and empirical character
of scientific knowledge.
The growth is continuous, because criticism
of theories, which are proposed solutions, in turn
generates new problems.
Scientific problems occur when expectations
Science starts from problems, not from
observations, although unexpected observations give
rise to new problems.
Popper views science as progressing from
old problems to new problems, to new problems having
increased depth as it progresses from old theories
to new theories having increased information
He also views progress in science as
approaching more and more closely to the truth,
where truth is understood as a correspondence with
the facts and as a regulative idea.
Just as there are degrees of information
content, so too there are degrees of approach to the
truth that he calls "verisimilitude.”
In his "Rationality of Scientific Revolutions" Popper therefore sets forth two criteria for the rationality of scientific revolutions, which are also two logical properties that enable the scientist to evaluate any new theory even before it is tested. The first criterion may be called a criterion of discontinuity: the new theory must conflict with the old one in the sense that it leads to conflicting results. Popper says that in this sense scientific progress is always revolutionary, and that the Marxian refrain "revolution in permanence" is applicable to science. The second criterion may be called a criterion of continuity: the new theory must be able to explain fully the success of its predecessor in the sense that either there are applications in which the old theory must appear to be a good approximation to the results of the new theory, or there are cases where the new theory yields different and better results than the old one. Scientific revolutions are rational because unlike ideological revolutions, which are sociological, the former cannot simply break with tradition.
Against Psychologism, Induction, and Naturalistic Semantics
Popper's philosophy of knowledge is a
critique of psychologism and a defense of the
objectivity of knowledge.
In the opening chapter of Logic
of Scientific Discovery, which is titled "A
Survey of Some Fundamental Problems", he
devotes a section to the elimination of psychologism.
This section follows the opening section on
the problem of induction, which he views as a
fallacy resulting from the psychologistic
philosophy of knowledge.
He sets forth his own theory of knowledge in
the fifth chapter titled "The Problem of the
Empirical Basis" and the opening section is a
critique of the psychologistic view that perceptual
experiences are the empirical basis for science.
In his "Demarcation Between Science and
Metaphysics" (1955) in Conjectures
and Refutations he criticizes Carnap's theory of
meaninglessness, which he describes as a
"naturalistic theory of meaningfulness" of
The linguistic expressions of particular
relevance are those singular statements that are
used for describing observations in science.
All of these ideas are interrelated according
to Popper: induction as the logic for making
generalizations and hypotheses, psychologism which
proposes perception as the empirical basis of
observation in science, and the naturalistic theory
of the semantics of language.
Popper rejects all of them together.
In his "Epistemology Without a Knowing
Subject" (1967) and his "On the Theory of
the Objective Mind" (1968) published as
chapters three and four in his Objective
Knowledge (1972), in Part I of The
Self and Its Brain (1977), and also in an
appendix to The
Open Universe (1982), Popper sets forth his
own philosophy of the three "worlds" of
reality which locates subjective psychology and
objective knowledge in different worlds.
The development of Popper's own philosophy of science began with the objective of demarcating empirical science and pseudoscience (e.g. Astrology, Marxism, Freudianism, and Adlerian psychology). His solution to the problem of demarcation is the criterion of empirical falsifiability, which he also uses to demarcate empirical science from metaphysics, and he contrasts this criterion with the criterion of meaningfulness that Carnap and other Logical Positivists used for distinguishing science from metaphysics. Carnap's criterion of meaningfulness is based on the naturalistic philosophy of language. Popper argues that the Positivists have never succeeded in distinguishing science from metaphysics or in distinguishing theory from observation, that metaphysics need not be meaningless even though it is not a science, and that Positivism excludes scientific theories as meaningless while failing to exclude metaphysics as meaningless. Popper maintains that there is no observation without theory, and that the observation terms occurring in observation language are "theory impregnated", such that observation terms are a type of theoretical term that Carnap calls disposition terms. The reason that the Positivists have not succeeded in distinguishing science from metaphysics, is that they cannot define meaningfulness, and they cannot define meaningfulness because they interpret the problem in a naturalistic way, as though it were a problem in natural science or in psychology. Popper maintains that the Positivists have confused the psychology of knowledge with the logic of knowledge, which is to say that they have adopted a psychologistic philosophy of knowledge. Popper rejects both behaviorism and psychologism, and maintains that the content of thought, the meanings of words, the semantics of language, are not determined either by the natural laws of the physical world or by the natural laws of psychology. The world of objective knowledge, which is governed by the laws of logic, is a third world that is autonomous from the world of objective physical nature and also from the world of subjective psychology. In The Self and Its Brain he argues against behaviorism and physicalist reductionism by the display of ambiguous drawings that he emphasizes may be interpreted in different ways by voluntary action, in order to demonstrate the existence of world 2, the world of the mind and of subjective mental experiences. He argues against the psychologistic view by stating that the objects of world 3 are intersubjectively testable. Hence there are the three separate worlds which cannot be reduced to one another: world 1 is the world of objective physical nature, world 2 is the subjective world of psychological experience, and world 3 is the objective world of human artifacts or creations including knowledge. Popper emphasizes that while the three worlds interact through world 2, nevertheless the world of objective knowledge is autonomous of the world of subjective psychological experience including perceptual experiences. Advocates of psychologism and the naturalistic theory of the semantics of language fail to recognize the autonomy of world 3 from the other two worlds. More recently in his "The Foundations of Information Science: Philosophical Aspects" in The Journal of Information Science (1980) the information scientist Bertram C. Brookes proposed that the task of information science as a discipline can be defined as the exploration of the world of objective knowledge understood as Popper's world 3, and that this discipline is distinct from documentation and library science.
Popper's rejection of inductive logic is based on his thesis that world 3 is autonomous from worlds 1 and 2. He references Einstein's stated view that there is no logical path leading to the universal laws that scientists search for, and that these laws can only be reached by intuition. Popper accepts Hume's thesis that universal statements cannot be justified by the singular statements describing observations, and he rejects the early Wittgenstein's verifiability criterion of meaningfulness adopted by Carnap and the other Logical Positivists of the Vienna Circle. He also rejects the probabilistic inductive logic developed by Carnap and set forth in the latter's The Logical Foundations of Probability, and he wonders why anyone would ever write such a book. In Popper's view there is no logic of scientific discovery; there is only a psychology of scientific discovery. He explains that the title of his own book, The Logic of Scientific Discovery in not about the psychological processes involved in inventing new scientific theories, but rather is about the growth of scientific knowledge by conjectures and refutations, the proposal and criticism of new theories.