Popper was an Austrian and British philosopher and a professor at
the London School of Economics. He is counted among the most influential
philosophers of science of the 20th century, and also wrote extensively
on social and political philosophy. Popper is perhaps best known
for repudiating the classical observationalist-inductivist account
of scientific method by advancing empirical falsifiability as the
criterion for distinguishing scientific theory from non-science;
and for his vigorous defense of liberal democracy and the principles
of social criticism which he took to make the flourishing of the
"open society" possible.
Born in Vienna (then Austria-Hungary) in 1902 to middle-class
parents of Jewish origins, Karl Popper was educated at the University
of Vienna. He took a Ph.D. in philosophy in 1928, and taught in
secondary school from 1930 to 1936. In 1934 he published his first
book, Logik der Forschung (The Logic of Scientific Discovery),
in which he criticized psychologism, naturalism, inductionism,
and logical positivism, and put forth his theory of potential
falsifiability being the criterion for what should be considered
science. In 1937, the rise of Nazism and the threat of the Anschluss
led Popper to emigrate to New Zealand, where he became lecturer
in philosophy at Canterbury University College New Zealand (at
1946, he moved to England to become reader in logic and scientific
method at the London School of Economics, where he was appointed
professor in 1949. He was knighted by Queen Elizabeth II in 1965,
and was elected a Fellow of the Royal Society in 1976. He retired
from academic life in 1969, though he remained intellectually
active until his death in 1994. He was invested with the Insignia
of a Companion of Honour in 1982.
won many awards and honors in his field, including the Lippincott
Award of the American Political Science Association, the Sonning
Prize, and fellowships in the Royal Society, British Academy,
London School of Economics, King's College London, and Darwin
College Cambridge. Austria awarded him the Grand Decoration of
Honour in Gold.
Popper coined the term critical rationalism to describe his philosophy.
This designation is significant, and indicates his rejection of
classical empiricism, and of the observationalist-inductivist
account of science that had grown out of it. Popper argued strongly
against the latter, holding that scientific theories are universal
in nature, and can be tested only indirectly, by reference to
also held that scientific theory, and human knowledge generally,
is irreducibly conjectural or hypothetical, and is generated by
the creative imagination in order to solve problems that have
arisen in specific historico-cultural settings. Logically, no
number of positive outcomes at the level of experimental testing
can confirm a scientific theory, but a single genuine counter-instance
is logically decisive: it shows the theory, from which the implication
is derived, to be false.
account of the logical asymmetry between verification and falsification
lies at the heart of his philosophy of science. It also inspired
him to take falsifiability as his criterion of demarcation between
what is and is not genuinely scientific: a theory should be considered
scientific if and only if it is falsifiable. This led him to attack
the claims of both psychoanalysis and contemporary Marxism to
scientific status, on the basis that the theories enshrined by
them are not falsifiable. His scientific work was influenced by
his study of quantum mechanics and by Albert Einstein's approach
to scientific theories. Popper's falsificationism resembles Charles
Peirce's fallibilism. In Of Clocks and Clouds (1966), Popper said
he wished he had known of Peirce's work earlier.
All Life is Problem Solving (1999), Popper sought to explain the
apparent progress of scientific knowledge—how it is that
our understanding of the universe seems to improve over time.
This problem arises from his position that the truth content of
our theories, even the best of them, cannot be verified by scientific
testing, but can only be falsified. If so, then how is it that
the growth of science appears to result in a growth in knowledge?
In Popper's view, the advance of scientific knowledge is an evolutionary
process characterized by his formula:
response to a given problem situation (PS1), a number of competing
conjectures, or tentative theories (TT), are systematically subjected
to the most rigorous attempts at falsification possible. This
process, error elimination (EE), performs a similar function for
science that natural selection performs for biological evolution.
Theories that better survive the process of refutation are not
more true, but rather, more "fit"—in other words,
more applicable to the problem situation at hand (PS1). Consequently,
just as a species' "biological fit" does not predict
continued survival, neither does rigorous testing protect a scientific
theory from refutation in the future.
as it appears that the engine of biological evolution has produced,
over time, adaptive traits equipped to deal with more and more
complex problems of survival, likewise, the evolution of theories
through the scientific method may, in Popper's view, reflect a
certain type of progress: toward more and more interesting problems
(PS2). For Popper, it is in the interplay between the tentative
theories (conjectures) and error elimination (refutation) that
scientific knowledge advances toward greater and greater problems;
in a process very much akin to the interplay between genetic variation
and natural selection.
his earlier work Conjectures and Refutations (1963), Popper abandoned
his previous rejection of the idea that truth is not a particularly
relevant goal for science to pursue. He adopted the semantic theory
of truth formulated by the logician Alfred Tarski in 1948. According
to this theory, the conditions for the truth of a sentence as
well as the sentences themselves are part of a metalanguage. So,
for example, the sentence "Snow is white" is true if
and only if snow is white. Although many philosophers have interpreted,
and continue to interpret, Tarski's theory as a deflationary theory,
Popper explicitly refers to it as a theory in which "truth"
must be replaced with "corresponds to the facts."
bases this interpretation on the fact that examples such as the
one described above refer to two things: assertions and the facts
to which they refer. He identifies Tarski's formulation of the
truth conditions of sentences as the introduction of a "metalinguistic
predicate" and distinguishes the following cases:
"John called" is true.
2) "It is true that John called."
first case belongs to the metalanguage whereas the second is more
likely to belong to the object language. Hence, "it is true
that" possesses the logical status of a redundancy. "Is
true", on the other hand, is a predicate necessary for making
general observations such as "John was telling the truth
this basis, along with that of the logical content of assertions
(where logical content is inversely proportional to probability),
Popper went on to develop his important notion of verisimilitude.
In words, the intuitive idea behind verisimilitude is that the
assertions or hypotheses of scientific theories can be objectively
measured with respect to the amount of truth and falsity that
they imply. And, in this way, one theory can be evaluated as more
or less true than another on a quantitative basis which, Popper
emphasizes forcefully, has nothing to do with "subjective
probabilities" or other merely "epistemic" considerations.
simplest mathemical formulation that Popper gives of this concept
can be found in the tenth chapter of Conjectures and Refutations..
Here he defines it as:
Vs(a) is the verisimilitude (or truthlikeness) of a, Ctv(a) is
a measure of the content of truth of a, and CTf(a) is a measure
of the content of the falsity of a.
for Popper, was objective, both in the sense that it is objectively
true (or truthlike), and also in the sense that knowledge has
an ontological status (i.e.–knowledge as object) independent
of the knowing subject (Objective Knowledge: An Evolutionary Approach,
1972). He proposed three:
World One , being the phenomenal world, or the world of direct
2. World Two , being the world of mind, or mental states, ideas,
and perceptions; and
3. World Three, being the body of human knowledge expressed in
its manifold forms, or the products of the second world made manifest
in the materials of the first world (i.e.–books, papers,
paintings, symphonies, and all the products of the human mind).
Three, he argued, was the product of individual human beings in
exactly the same sense that an animal path is the product of individual
animals, and that, as such, has an existence and evolution independent
of any individual knowing subjects. The influence of World Three,
in his view, on the individual human mind (World Two) is at least
as strong as the influence of World One.
other words, the knowledge held by a given individual mind owes
at least as much to the total accumulated wealth of human knowledge,
made manifest, than to the world of direct experience. As such,
the growth of human knowledge could be said to be a function of
the independent evolution of World Three. Compare with Memetics.
Contemporary philosophers have not embraced Popper's Three World
conjecture, due mostly, it seems, to its resemblance to Cartesian
In The Open Society and Its Enemies and The Poverty of Historicism,
Popper developed a critique of historicism and a defence of the
'Open Society' and liberal democracy. Historicism is the theory
that history develops inexorably and necessarily according to
knowable general laws towards a determinate end. Popper argued
that this view is the principal theoretical presupposition underpinning
most forms of authoritarianism and totalitarianism.
argued that historicism is founded upon mistaken assumptions regarding
the nature of scientific law and prediction. Since the growth
of human knowledge is a causal factor in the evolution of human
history, and since "no society can predict, scientifically,
its own future states of knowledge", it follows, he argued,
that there can be no predictive science of human history. For
Popper, metaphysical and historical indeterminism go hand in hand.
Problem of Induction
Among his contributions to philosophy is his answer to David Hume's
Problem of Induction. Hume stated that just because the sun has
risen every day for as long as anyone can remember, doesn't mean
that there is any rational reason to believe it will come up tomorrow.
There is no rational way to prove that a pattern will continue
on just because it has before.
reply is characteristic, and ties in with his criterion of falsifiability.
He states that while there is no way to prove that the sun will
come up, we can theorize that it will. If it does not come up,
then it will be disproven, but since right now it seems to be
consistent with our theory, the theory is not disproven. Thus,
Popper's demarcation between science and non-science serves as
an answer to an old logical problem as well. This approach was
criticised by Peter Singer for masking the role induction plays
in empirical discovery.
By all accounts, Popper has played a vital role in establishing
the philosophy of science as a vigorous, autonomous discipline
within analytic philosophy, through his own prolific and influential
works, and also through his influence on his own contemporaries
and students -- chief among them, Imre Lakatos and Paul Feyerabend,
two of the foremost philosophers of science in the next generation
of analytic philosophy. (Lakatos's work drastically modifies Popper's
position, and Feyerabend's repudiates it entirely, but the work
of both is deeply influenced by Popper and engaged with many of
the problems that Popper set.)
is also widely credited as one of the leading voices in the revolt
against the logical positivism of the Vienna Circle in the middle
of the 20th century; his revival of Humean skepticism toward induction
and his introduction of the principle of falsifiability as a non-inductive
solution to the puzzle of scientific knowledge, played a major
role in the criticism of the view of science that was central
to the positivist programme. Popper entitled a chapter of his
memoirs "Who Killed Logical Positivism?" — a question
which he answered by saying "I fear that I must admit responsibility".
Popper may have overstated his own influence somewhat vis-a-vis
fellow critics of positivism, such as W.V.O. Quine and Popper's
philosophical nemesis Ludwig Wittgenstein.
there is some dispute as to the matter of influence, Popper had
a long standing and close friendship with economist Friedrich
Hayek, also from Vienna. Each found support and similarities in
each other's work, citing each other often, though not without
qualification. In a letter to Hayek in 1944, Popper stated, "I
think I have learnt more from you than from any other living thinker,
except perhaps Alfred Tarski." (See Hacohen, 2000). Popper
dedicated his Conjectures and Refutations to Hayek. For his part,
Hayek dedicated a collection of papers, Studies in Philosophy,
Politics, and Economics, to Popper, and in 1982 said, "...ever
since his Logik der Forschung first came out in 1934, I have been
a complete adherent to his general theory of methodology."
(See Weimer and Palermo, 1982).
influence, both through his work in philosophy of science and
through his political philosophy, has also extended beyond the
academy. Among Popper's students and advocates is the multibillionaire
investor George Soros, who says his investment strategies are
modelled on Popper's understanding of the advancement of knowledge
through falsification. Among Soros's philanthropic foundations
is the Open Society Institute, a think-tank named in honor of
Popper's The Open Society and Its Enemies, which Soros founded
to advance the Popperian defense of the open society against authoritarianism
The Quine-Duhem thesis argues that it is impossible to test a
single hypothesis on its own, since each one comes as part of
an environment of theories. Thus we can only say that the whole
package of relevant theories has been collectively falsified,
but cannot conclusively say which element of the package must
be replaced. An example of this is given by the discovery of the
planet Neptune: when the motion of Uranus was found not to match
the predictions of Newton's laws, the theory "There are seven
planets in the solar system" was rejected, and not Newton's
discussed this critique of naïve falsificationism in Chapters
3 & 4 of The Logic of Scientific Discovery. For Popper, theories
are accepted or rejected via a sort of 'natural selection'. Theories
that say more about the way things appear are to be preferred
over those that do not; the more generally applicable a theory
is, the greater its value. Thus Newton’s laws, with their
wide general application, are to be preferred over the much more
specific “the solar system has seven planets”.
Kuhn’s influential book The Structure of Scientific Revolutions
argued that scientists work in a series of paradigms, and found
little evidence of scientists actually following a falsificationist
methodology. Popper's student Imre Lakatos attempted to reconcile
Kuhn’s work with falsificationism by arguing that science
progresses by the falsification of research programs rather than
the more specific universal statements of naïve falsificationism.
Another of Popper’s students Paul Feyerabend ultimately
rejected any prescriptive methodology, and argued that the only
universal method characterizing scientific progress was anything
objection is that it is not always possible to demonstrate falsehood
definitively, especially if one is using statistical criteria
to evaluate a null hypothesis. More generally, it is not always
clear that evidence contradicting a hypothesis is a sign of flaws
in the hypothesis rather than of flaws in the procedures used
to test it.
critics seek to vindicate the claims of historicism or holism
to intellectual respectability, or psychoanalysis or Marxism to
scientific status. It has been argued that Popper's student Imre
Lakatos transformed Popper's philosophy using historicist and
updated Hegelian historiographic ideas.
Taylor accuses Popper of exploiting his fame as an epistemologist
to diminish the importance of philosophers of the continental
tradition. According to Taylor, Popper's criticims are completely
baseless, but they are received with an attention and respect
that Popper's "intrinsic worth hardly merits".
2004 philosopher and psychologist Michel ter Hark (Groningen,
The Netherlands) published a book, called Popper, Otto Selz and
the rise of evolutionary epistemology, in which he claims that
Popper got a part of his ideas from his tutor, the German-Jewish
psychologist Otto Selz. Selz himself never published his ideas,
partly because of the rise of Nazism which ordered him to quit
his work in 1933, and the prohibition of referencing to Selz'