Karl Popper is generally regarded as one of the greatest philosophers
of science of the 20th century. He was also a social and political philosopher
considerable stature, a self-professed "critical-rationalist", a dedicated opponent of all forms of scepticism, conventionalism, and relativism
in science and in human affairs generally, a committed advocate and staunch defender of the "Open Society", and an implacable critic of
totalitarianism in all of its forms. One of the many remarkable features of Popper's thought is the scope of his intellectual influence. In the modern
technological and highly-specialised world scientists are rarely aware of the work of philosophers; it is virtually unprecedented to find them queuing up, as
they have done in Popper's case, to testify to the enormously practical beneficial impact which that philosophical work has had upon their own. But
notwithstanding the fact that he wrote on even the most technical matters with consummate clarity, the scope of Popper's work is such that it is
commonplace by now to find that commentators tend to deal with the epistemological, scientific and social elements of his thought as if they were quite
disparate and unconnected, and thus the fundamental unity of his philosophical vision and method has to a large degree been dissipated. Here we will try
to trace the threads which interconnect the various elements of his philosophy, and which give it its fundamental unity.
Backdrop to his Thought
The Problem of Demarcation
The Growth of Human Knowledge
Probability, Knowledge and Verisimilitude
Social and Political Thought -- The Critique of Historicism and Holism
Scientific Knowledge, History, and Prediction
Immutable Laws and Contingent Trends
Other Internet Resources
Karl Raimund Popper was born on 28 July 1902 in Vienna, which at that
time could make some claim to be the cultural epicentre of the western
His parents, who were of Jewish origin, brought him up in an atmosphere which he was later to describe as "decidedly bookish";. His father
was a lawyer by profession, but he also took a keen interest in the classics and in philosophy, and communicated to his son an interest in social and
political issues which he was to never lose. His mother inculcated in him such a passion for music that for a time he seriously contemplated taking it up as a
career, and indeed he initially chose the history of music as a second subject for his Ph.D examination. Subsequently, his love for music became one of the
inspirational forces in the development of his thought, and manifested itself in his highly original interpretation of the relationship between dogmatic and
critical thinking, in his account of the distinction between objectivity and subjectivity, and, most importantly, in the growth of his hostility towards all forms
of historicism, including historicist ideas about the nature of the "progressive" in music. The young Karl attended the local Realgymnasium,
where he was unhappy with the standards of the teaching, and, after an illness which kept him at home for a number of months, he left to attend the
University of Vienna in 1918. However, he did not formally enrol at the University by taking the matriculation examination for another four years. 1919
was in many respects the most important formative year of his intellectual life. In that year he became heavily involved in left-wing politics, joined the
Association of Socialist School Students, and became for a time a Marxist. However, he was quickly disillusioned with the doctrinaire character of the
latter, and soon abandoned it entirely. He also discovered the psychoanalytic theories of Freud and Adler (under whose aegis he engaged briefly in social
work with deprived children), and listened entranced to a lecture which Einstein gave in Vienna on relativity theory. The dominance of the critical spirit in
Einstein, and its total absence in Marx, Freud and Adler, struck Popper as being of fundamental importance: the latter, he came to think, couched their
theories in terms which made them amenable only to confirmation, while Einstein's theory, crucially, had testable implications which, if false, would have
falsified the theory itself.
Popper obtained a primary school teaching diploma in 1925, took a Ph.D.
in philosophy in 1928, and qualified to teach mathematics and physics in
secondary school in 1929. The dominant philosophical group in Vienna from its inception in 1928 was the Wiener Kreis, the circle of
"scientifically-minded" intellectuals who gathered around the figure of Moritz Schlick. This included Rudolf Carnap, Otto Neurath, Viktor
Kraft, Hans Hahn, and Herbert Feigl. The principal objective of the members of the Circle was to unify the sciences, which carried with it, in their view,
the need to eliminate metaphysics once and for all by showing that metaphysical propositions are meaningless. Thus was born the movement in philosophy
known as logical positivism, and its chief tool became the verification principle. Although he was friendly with some of the Circle's members - especially
Feigl, who encouraged him to write his first book - and shared their esteem for science, Popper was heavily critical of the main tenets of logical positivism,
especially of what he considered to be its misplaced focus on the theory of meaning in philosophy and upon verification in scientific methodology. He
articulated his own view of science, and his criticisms of the positivists, in his first work, published under the title Logik der Forschung in 1934. The book
- which he was later to claim rang the death knell for logical positivism - attracted more attention than Popper had anticipated, and he was invited to
lecture in England in 1935. He spent the next few years working productively on science and philosophy, but storm clouds were gathering - the growth of
Nazism in Germany and Austria compelled him, like many other intellectuals who shared his Jewish origins, to leave his native country.
In 1937 Popper took up a position teaching philosophy at the University
of Canterbury in New Zealand, where he was to remain for the duration of
Second World War. The annexation of Austria in 1938 became the catalyst which prompted him to refocus his writings on social and political philosophy.
In 1946 he moved to England to teach at the London School of Economics, and became professor of logic and scientific method at the University of
London in 1949. From this point on Popper's reputation and stature as a philosopher of science and social thinker grew enormously, and he continued to
write prolifically - a number of his works, particularly The Logic of Scientific Discovery (1959), are now universally recognised as classics in the field.
He was knighted in 1965, and retired from the University of London in 1969, though he remained active as a writer, broadcaster and lecturer until his
death in 1994. (For more detail on Popper's life, cf. his Unended Quest).
Backdrop to his Thought
A number of biographical features may be identified as having a particular
influence upon Popper's thought. In the first place, his teenage flirtation
Marxism left him thoroughly familiar with the Marxist view of economics, class-war, and history. Secondly, he was appalled by the failure of the
democratic parties to stem the rising tide of fascism in his native Austria in the 1920s and 1930s, and the effective welcome extended to it by the Marxists.
The latter acted on the ideological grounds that it constituted what they believed to be a necessary dialectical step towards the implosion of capitalism and
the ultimate revolutionary victory of communism. This was one factor which led to the much feared Anschluss, the annexation of Austria by the German
Reich, the anticipation of which forced Popper into permanent exile from his native country. The Poverty of Historicism (1944) and The Open Society
and Its Enemies (1945), his most impassioned and brilliant social works, are as a consequence a powerful defence of democratic liberalism as a social
and political philosophy, and a devastating critique of the principal philosophical presuppositions underpinning all forms of totalitarianism. Thirdly, as we
have seen, Popper was profoundly impressed by the differences between the allegedly "scientific" theories of Freud and Adler and the
revolution effected by Einstein's theory of relativity in physics in the first two decades of this century. The main difference between them, as Popper saw it,
was that while Einstein's theory was highly "risky", in the sense that it was possible to deduce consequences from it which were, in the light
of the then dominant Newtonian physics, highly improbable (e.g. that light is deflected towards solid bodies - confirmed by Eddington's experiments in
1919), and which would, if they turned out to be false, falsify the whole theory, nothing could, even in principle, falsify psychoanalytic theories. These
latter, Popper came to feel, have more in common with primitive myths than with genuine science. That is to say, he saw that what is apparently the chief
source of strength of psychoanalysis, and the principal basis on which its claim to scientific status is grounded, viz. its capability to accommodate, and
explain, every possible form of human behaviour, is in fact a critical weakness, for it entails that it is not, and could not be, genuinely predictive.
Psychoanalytic theories by their nature are insufficiently precise to have negative implications, and so are immunised from experiential falsification.
The Marxist account of history too, Popper held, is not scientific,
although it differs in certain crucial respects from psychoanalysis. For
believed, had been initially scientific, in that Marx had postulated a theory which was genuinely predictive. However, when these predictions were not in
fact borne out, the theory was saved from falsification by the addition of ad hoc hypotheses which made it compatible with the facts. By this means,
Popper asserted, a theory which was initially genuinely scientific degenerated into pseudo-scientific dogma.
These factors combined to make Popper take falsifiability as his criterion
for demarcating science from non-science: if a theory is incompatible with
possible empirical observations it is scientific; conversely, a theory which is compatible with all such observations, either because, as in the case of
Marxism, it has been modified solely to accommodate such observations, or because, as in the case of psychoanalytic theories, it is consistent with all
possible observations, is unscientific. For Popper, however, to assert that a theory is unscientific, is not necessarily to hold that it is unenlightening, still less
that it is meaningless, for it sometimes happens that a theory which is unscientific (because it is unfalsifiable) at a given time may become falsifiable, and
thus scientific, with the development of technology, or with the further articulation and refinement of the theory. Further, even purely mythogenic
explanations have performed a valuable function in the past in expediting our understanding of the nature of reality.
[Return to Section Headings]
The Problem of Demarcation
As Popper represents it, the central problem in the philosophy of science
is that of demarcation, i.e. of distinguishing between science and
what he terms
"non-science", under which heading he ranks, amongst others, logic, metaphysics, psychoanalysis, and Adler's individual psychology.
Popper is unusual amongst contemporary philosophers in that he accepts the validity of the Humean critique of induction, and indeed, goes beyond it in
arguing that induction is never actually used by the scientist. However, he does not concede that this entails the scepticism which is associated with Hume,
and argues that the Baconian/Newtonian insistence on the primacy of "pure"; observation, as the initial step in the formation of theories, is
completely misguided: all observation is selective and theory-laden - there are no pure or theory-free observations. In this way he destabilises the
traditional view that science can be distinguished from non-science on the basis of its inductive methodology; in contradistinction to this, Popper holds that
there is no unique methodology specific to science. Science, like virtually every other human, and indeed organic, activity, Popper believes, consists largely
Popper, then, repudiates induction, and rejects the view that it is
the characteristic method of scientific investigation and inference, and
falsifiability in its place. It is easy, he argues, to obtain evidence in favour of virtually any theory, and he consequently holds that such
"corroboration", as he terms it, should count scientifically only if it is the positive result of a genuinely "risky" prediction, which
might conceivably have been false. For Popper, a theory is scientific only if it is refutable by a conceivable event. Every genuine test of a scientific theory,
then, is logically an attempt to refute or to falsify it, and one genuine counter-instance falsifies the whole theory. In a critical sense, Popper's theory of
demarcation is based upon his perception of the logical asymmetry which holds between verification and falsification: it is logically impossible to
conclusively verify a universal proposition by reference to experience (as Hume saw clearly), but a single counter-instance conclusively falsifies the
corresponding universal law. In a word, an exception, far from "proving" a rule, conclusively refutes it.
Every genuine scientific theory then, in Popper's view, is prohibitive,
in the sense that it forbids, by implication, particular events or occurrences.
it can be tested and falsified, but never logically verified. Thus Popper stresses that it should not be inferred from the fact that a theory has withstood the
most rigorous testing, for however long a period of time, that it has been verified; rather we should recognise that such a theory has received a high
measure of corroboration. and may be provisionally retained as the best available theory until it is finally falsified (if indeed it is ever falsified), and/or is
superseded by a better theory.
Popper has always drawn a clear distinction between the logic of falsifiability
and its applied methodology. The logic of his theory is utterly simple:
single ferrous metal is unaffected by a magnetic field it cannot be the case that all ferrous metals are affected by magnetic fields. Logically speaking, a
scientific law is conclusively falsifiable although it is not conclusively verifiable. Methodologically, however, the situation is much more complex: no
observation is free from the possibility of error - consequently we may question whether our experimental result was what it appeared to be.
Thus, while advocating falsifiability as the criterion of demarcation
for science, Popper explicitly allows for the fact that in practice a single
counter-instance is never sufficient methodologically to falsify a theory, and that scientific theories are often retained even though much of the available
evidence conflicts with them, or is anomalous with respect to them. Scientific theories may, and do, arise genetically in many different ways, and the
manner in which a particular scientist comes to formulate a particular theory may be of biographical interest, but it is of no consequence as far as the
philosophy of science is concerned. Popper stresses in particular that there is no unique way, no single method such as induction, which functions as the
route to scientific theory, a view which Einstein personally endorsed with his affirmation that "There is no logical path leading to [the highly universal
laws of science]. They can only be reached by intuition, based upon something like an intellectual love of the objects of experience". Science, in
Popper's view, starts with problems rather than with observations - it is, indeed, precisely in the context of grappling with a problem that the scientist
makes observations in the first instance: his observations are selectively designed to test the extent to which a given theory functions as a satisfactory
solution to a given problem.
On this criterion of demarcation physics, chemistry, and (non-introspective)
psychology, amongst others, are sciences, psychoanalysis is a pre-science
(i.e. it undoubtedly contains useful and informative truths, but until such time as psychoanalytical theories can be formulated in such a manner as to be
falsifiable, they will not attain the status of scientific theories), and astrology and phrenology are pseudo-sciences. Formally, then, Popper's theory of
demarcation may be articulated as follows: where a "basic statement" is to be understood as a particular observation-report, then we may
say that a theory is scientific if and only if it divides the class of basic statements into the following two non-empty sub-classes: (a) the class of all those
basic statements with which it is inconsistent, or which it prohibits - this is the class of its potential falsifiers (i.e. those statements which, if true, falsify the
whole theory), and (b) the class of those basic statements with which it is consistent, or which it permits (i.e. those statements which, if true, corroborate it,
or bear it out).
[Return to Section Headings]
The Growth of Human Knowledge
For Popper accordingly, the growth of human knowledge proceeds from
our problems and from our attempts to solve them. These attempts involve
formulation of theories which, if they are to explain anomalies which exist with respect to earlier theories, must go beyond existing knowledge and
therefore require a leap of the imagination. For this reason, Popper places special emphasis on the role played by the independent creative imagination in
the formulation of theory. The centrality and priority of problems in Popper's account of science is paramount, and it is this which leads him to
characterise scientists as "problem-solvers". Further, since the scientist begins with problems rather than with observations or "bare
facts", Popper argues that the only logical technique which is an integral part of scientific method is that of the deductive testing of theories which
are not themselves the product of any logical operation. In this deductive procedure conclusions are inferred from a tentative hypothesis. These
conclusions are then compared with one another and with other relevant statements to determine whether they falsify or corroborate the hypothesis. Such
conclusions are not directly compared with the facts, Popper stresses, simply because there are no "pure" facts available; all
observation-statements are theory-laden, and are as much a function of purely subjective factors (interests, expectations, wishes, etc.) as they are a
function of what is objectively real.
How then does the deductive procedure work? Popper specifies four steps:
(a) The first is formal, a testing of the internal consistency of the theoretical system to see if it involves any contradictions.
(b) The second step is semi-formal, the axiomatising
of the theory to distinguish between its empirical and its logical elements.
this step the scientist makes the logical form of the theory explicit. Failure to do this can lead to category-mistakes - the scientist ends up
asking the wrong questions, and searches for empirical data where none are available. Most scientific theories contain analytic (i.e. a priori)
and synthetic elements, and it is necessary to axiomatise them in order to distinguish the two clearly.
(c) The third step is the comparing of the
new theory with existing ones to determine whether it constitutes an advance
upon them. If it does
not constitute such an advance, it will not be adopted. If, on the other hand, its explanatory success matches that of the existing theories, and
additionally, it explains some hitherto anomalous phenomenon, or solves some hitherto unsolvable problems, it will be deemed to constitute
an advance upon the existing theories, and will be adopted. Thus science involves theoretical progress. However, Popper stresses that we
ascertain whether one theory is better than another by deductively testing both theories, rather than by induction. For this reason, he argues
that a theory is deemed to be better than another if (while unfalsified) it has greater empirical content, and therefore greater predictive power
than its rival. The classic illustration of this in physics was the replacement of Newton's theory of universal gravitation by Einstein's theory of
relativity. This elucidates the nature of science as Popper sees it: at any given time there will be a number of conflicting theories or
conjectures, some of which will explain more than others. The latter will consequently be provisionally adopted. In short, for Popper any
theory X is better than a "rival" theory Y if X has greater empirical content, and hence greater predictive power, than Y.
(d) The fourth and final step is the testing
of a theory by the empirical application of the conclusions derived from
it. If such conclusions are
shown to be true, the theory is corroborated (but never verified). If the conclusion is shown to be false, then this is taken as a signal that the
theory cannot be completely correct (logically the theory is falsified), and the scientist begins his quest for a better theory. He does not,
however, abandon the present theory until such time as he has a better one to substitute for it. More precisely, the method of theory-testing
is as follows: certain singular propositions are deduced from the new theory - these are predictions, and of special interest are those
predictions which are "risky" (in the sense of being intuitively implausible or of being startlingly novel) and experimentally
testable. From amongst the latter the scientist next selects those which are not derivable from the current or existing theory - of particular
importance are those which contradict the current or existing theory. He then seeks a decision as regards these and other derived statements
by comparing them with the results of practical applications and experimentation. If the new predictions are borne out, then the new theory is
corroborated (and the old one falsified), and is adopted as a working hypothesis. If the predictions are not borne out, then they falsify the
theory from which they are derived. Thus Popper retains an element of empiricism: for him scientific method does involve making an appeal
to experience. But unlike traditional empiricists, Popper holds that experience cannot determine theory (i.e. we do not argue or infer from
observation to theory), it rather delimits it: it shows which theories are false, not which theories are true. Moreover, Popper also rejects the
empiricist doctrine that empirical observations are, or can be, infallible, in view of the fact that they are themselves theory-laden.
The general picture of Popper's philosophy of science, then is this:
Hume's philosophy demonstrates that there is a contradiction implicit in
empiricism, which holds both that all knowledge is derived from experience and that universal propositions (including scientific laws) are verifiable by
reference to experience. The contradiction, which Hume himself saw clearly, derives from the attempt to show that, notwithstanding the open-ended
nature of experience, scientific laws may be construed as empirical generalisations which are in some way finally confirmable by a "positive"
experience. Popper eliminates the contradiction by rejecting the first of these principles and removing the demand for empirical verification in favour of
empirical falsification in the second. Scientific theories, for him, are not inductively inferred from experience, nor is scientific experimentation carried out
with a view to verifying or finally establishing the truth of theories; rather, all knowledge is provisional, conjectural, hypothetical - we can never finally
prove our scientific theories, we can merely (provisionally) confirm or (conclusively) refute them; hence at any given time we have to choose between the
potentially infinite number of theories which will explain the set of phenomena under investigation. Faced with this choice, we can only eliminate those
theories which are demonstrably false, and rationally choose between the remaining, unfalsified theories. Hence Popper's emphasis on the importance of
the critical spirit to science - for him critical thinking is the very essence of rationality. For it is only by critical thought that we can eliminate false theories,
and determine which of the remaining theories is the best available one, in the sense of possessing the highest level of explanatory force and predictive
power. It is precisely this kind of critical thinking which is conspicuous by its absence in contemporary Marxism and in psychoanalysis.
[Return to Section Headings]
Probability, Knowledge and Verisimilitude
In the view of many social scientists, the more probable a theory is,
the better it is, and if we have to choose between two theories which are
strong in terms of their explanatory power, and differ only in that one is probable and the other is improbable, then we should choose the former. Popper
rejects this. Science, or to be precise, the working scientist, is interested, in Popper's view, in theories with a high informative content, because such
theories possess a high predictive power and are consequently highly testable. But if this is true, Popper argues, then, paradoxical as it may sound, the
more improbable a theory is the better it is scientifically, because the probability and informative content of a theory vary inversely - the higher the
informative content of a theory the lower will be its probability, for the more information a statement contains, the greater will be the number of ways in
which it may turn out to be false. Thus the statements which are of special interest to the scientist are those with a high informative content and
(consequentially) a low probability, which nevertheless come close to the truth. Informative content, which is in inverse proportion to probability, is in
direct proportion to testability. Consequently the severity of the test to which a theory can be subjected, and by means of which it is falsified or
corroborated, is all-important.
For Popper, all scientific criticism must be piecemeal, i.e. he holds
that it is not possible to question every aspect of a theory at once. More
while attempting to resolve a particular problem a scientist of necessity accepts all kinds of things as unproblematic. These things constitute what Popper
terms the "background knowledge". However, he stresses that the background knowledge is not knowledge in the sense of being
conclusively established; it may be challenged at any time, especially if it is suspected that its uncritical acceptance may be responsible for difficulties which
are subsequently encountered. Nevertheless, it is clearly not possible to question both the theory and the background knowledge at the same time (e.g. in
conducting an experiment the scientist of necessity assumes that the apparatus used is in working order).
How then can one be certain that one is questioning the right thing?
The Popperian answer is that we cannot have absolute certainty here, but
tests usually show where the trouble lies. Even observation statements, Popper maintains, are fallible, and science in his view is not a quest for certain
knowledge, but an evolutionary process in which hypotheses or conjectures are imaginatively proposed and tested in order to explain facts or to solve
problems. Popper emphasises both the importance of questioning the background knowledge when the need arises, and the significance of the fact that
observation-statements are theory-laden, and hence fallible. For while falsifiability is simple as a logical principle, in practice it is exceedingly complicated -
no single observation can ever be taken to falsify a theory, for there is always the possibility (a) that the observation itself is mistaken, or (b) that the
assumed background knowledge is faulty or defective.
Popper was initially uneasy with the concept of truth, and in his earliest
writings he avoided asserting that a theory which is corroborated is true
clearly if every theory is an open-ended hypothesis, as he maintains, then ipso facto it has to be at least potentially false. For this reason Popper restricted
himself to the contention that a theory which is falsified is false and is known to be such, and that a theory which replaces a falsified theory (because it has
a higher empirical content than the latter, and explains what has falsified it) is a "better theory" than its predecessor. However, he came to
accept Tarski's reformulation of the correspondence theory of truth, and in Conjectures and Refutations (1963) he integrated the concepts of truth and
content to frame the metalogical concept of "truthlikeness" or "verisimilitude". A "good" scientific theory,
Popper thus argued, has a higher level of verisimilitude than its rivals, and he explicated this concept by reference to the logical consequences of theories.
A theory's content is the totality of its logical consequences, which can be divided into two classes: there is the "truth-content" of a theory,
which is the class of true propositions which may be derived from it, on the one hand, and the "falsity-content" of a theory, on the other
hand, which is the class of the theory's false consequences (this latter class may of course be empty, and in the case of a theory which is true is necessarily
Popper offered two methods of comparing theories in terms of verisimilitude,
the qualitative and quantitative definitions. On the qualitative account,
Assuming that the truth-content and the falsity-content
of two theories t1 and t2 are comparable, we can say that t2 is more closely
the truth, or corresponds better to the facts, than t1, if and only if either:
(a) the truth-content but not the falsity-content of t2 exceeds that of t1, or
(b) the falsity-content of t1, but not its truth-content, exceeds that of t2. (Conjectures and Refutations, 233).
Here, verisimilitude is defined in terms of subclass relationships:
t2 has a higher level of verisimilitude than t1 if and only if their truth-
are comparable through subclass relationships, and either (a) t2's truth-content includes t1's and t2's falsity-content, if it exists, is included in, or is the same
as, t1's, or (b) t2's truth-content includes or is the same as t1's and t2's falsity-content, if it exists, is included in t1's.
On the quantitative account, verisimilitude is defined by assigning
quantities to contents, where the index of the content of a given theory
is its logical
improbability (given again that content and probability vary inversely). Formally, then, Popper defines the quantitative verisimilitude which a statement
possesses by means of a formula:
Vs(a)=CtT(a) - CtF(a),
where Vs(a) represents the verisimilitude of "a", CtT(a) is a measure
of the truth-content of "a", and CtF(a) is a measure of its
The utilisation of either method of computing verisimilitude shows,
Popper held, that even if a theory t2 with a higher content than a rival
theory t1 is
subsequently falsified, it can still legitimately be regarded as a better theory than t1, and "better" is here now understood to mean t2 is closer
to the truth than t1. Thus scientific progress involves, on this view, the abandonment of partially true, but falsified, theories, for theories with a higher level
of verisimilitude, i.e., which approach more closely to the truth. In this way, verisimilitude allowed Popper to mitigate what many saw as the pessimism of
an anti-inductivist philosophy of science which held that most, if not all scientific theories are false, and that a true theory, even if discovered, could not be
known to be such. With the introduction of the new concept, Popper was able to represent this as an essentially optimistic position in terms of which we
can legitimately be said to have reason to believe that science makes progress towards the truth through the falsification and corroboration of theories.
Scientific progress, in other words, could now be represented as progress towards the truth, and experimental corroboration could be seen an indicator
However, in the 1970's a series of papers published by researchers such
as Miller, Tichý, and Grünbaum in particular revealed fundamental
Popper's formal definitions of verisimilitude. The significance of this work was that verisimilitude is largely important in Popper's system because of its
application to theories which are known to be false. In this connection, Popper had written:
Ultimately, the idea of verisimilitude is most
important in cases where we know that we have to work with theories which
are at best
approximations—that is to say, theories of which we know that they cannot be true. (This is often the case in the social sciences). In
these cases we can still speak of better or worse approximations to the truth (and we therefore do not need to interpret these cases in an
instrumentalist sense). (Conjectures and Refutations, 235).
For these reasons, the deficiencies discovered by the critics in Popper's
formal definitions were seen by many as devastating, precisely because
significant of these related to the levels of verisimilitude of false theories. In 1974, Miller and Tichý, working independently of each other, demonstrated
that the conditions specified by Popper in his accounts of both qualitative and quantitative verisimilitude for comparing the truth- and falsity-contents of
theories can be satisfied only when the theories are true. In the crucially important case of false theories, however, Popper's definitions are formally
defective. For while Popper had believed that verisimilitude intersected positively with his account of corroboration, in the sense that he viewed an
improbable theory which had withstood critical testing as one the truth-content of which is great relative to rival theories, while its falsity-content (if it
exists) would be relatively low, Miller and Tichý proved, on the contrary, that in the case of a false theory t2which has excess content over a rival theory
false t1both the truth-content and the falsity-content of t2will exceed that of t1. With respect to theories which are false, therefore, Popper's conditions for
comparing levels of verisimilitude, whether in quantitative and qualitative terms, can never be met.
Commentators on Popper, with few exceptions, had initially attached
little importance to his theory of verisimilitude. However, after the failure
definitions in 1974, some critics came to see it as central to his philosophy of science, and consequentially held that the whole edifice of the latter had been
subverted. For his part, Popper's response was two-fold. In the first place, while acknowledging the deficiencies in his own formal account ("my main
mistake was my failure to see at once that … if the content of a false statement a exceeds that of a statement b, then the truth-content of a exceeds
the truth-content of b, and the same holds of their falsity-contents", Objective Knowledge, 371), Popper argued that "I do think that we should not
conclude from the failure of my attempts to solve the problem [of defining verisimilitude] that the problem cannot be solved" (Objective Knowledge,
372), a point of view which was to precipitate more than two decades of important technical research in this field. At another, more fundamental level, he
moved the task of formally defining the concept from centre-stage in his philosophy of science, by protesting that he had never intended to imply "that
degrees of verisimilitude ... can ever be numerically determined, except in certain limiting cases" (Objective Knowledge, 59), and arguing instead that the
chief value of the concept is heuristic and intuitive, in which the absence of an adequate formal definition is not an insuperable impediment to its utilisation in
the actual appraisal of theories relativised to problems in which we have an interest. The thrust of the latter strategy seems to many to genuinely reflect the
significance of the concept of verisimilitude in Popper's system, but it has not satisfied all of his critics.
[Return to Section Headings]
Social and Political Thought -- The Critique of Historicism and Holism
Given Popper's personal history and background, it is hardly surprising
that he developed a deep and abiding interest in social and political philosophy.
However, it is worth emphasising that his angle of approach to these fields is through a consideration of the nature of the social sciences which seek to
describe and explicate them systematically, particularly history. It is in this context that he offers an account of the nature of scientific prediction, which in
turn allows him a point of departure for his attack upon totalitarianism and all its intellectual supports, especially holism and historicism. In this context
holism is to be understood as the view that human social groupings are greater than the sum of their members, that such groupings are
"organic" entities in their own right, that they act on their human members and shape their destinies, and that they are subject to their own
independent laws of development. Historicism, which is closely associated with holism, is the belief that history develops inexorably and necessarily
according to certain principles or rules towards a determinate end (as for example in the dialectic of Hegel, which was adopted and implemented by
Marx). The link between holism and historicism is that the holist believes that individuals are essentially formed by the social groupings to which they
belong, while the historicist - who is usually also a holist - holds that we can understand such a social grouping only in terms of the internal principles which
determine its development.
These beliefs lead to what Popper calls "The Historicist Doctrine of
the Social Sciences", the views (a) that the principal task of the social
sciences is to make predictions about the social and political development of man, and (b) that the task of politics, once the key predictions have been
made, is, in Marx's words, to lessen the "birth pangs" of future social and political developments. Popper thinks that this view of the social
sciences is both theoretically misconceived (in the sense of being based upon a view of natural science and its methodology which is totally wrong), and
socially dangerous, as it leads inevitably to totalitarianism and authoritarianism - to centralised governmental control of the individual and the attempted
imposition of large-scale social planning. Against this Popper strongly advances the view that any human social grouping is no more (or less) than the sum
of its individual members, that what happens in history is the (largely unplanned and unforeseeable) result of the actions of such individuals, and that large
scale social planning to an antecedently conceived blueprint is inherently misconceived - and inevitably disastrous - precisely because human actions have
consequences which cannot be foreseen. Popper, then, is an historical indeterminist, insofar as he holds that history does not evolve in accordance with
intrinsic laws or principles, that in the absence of such laws and principles unconditional prediction in the social sciences is an impossibility, and that there is
no such thing as historical necessity.
The link between Popper's theory of knowledge and his social philosophy
is his fallibilism - just as we make theoretical progress in science by
subjecting our theories to critical scrutiny, and abandoning those which have been falsified, so too, Popper holds, the critical spirit can and should be
sustained at the social level. More specifically, the open society can be brought about only if it is possible for the individual citizen to evaluate critically the
consequences of the implementation of government policies, which can then be abandoned or modified in the light of such critical scrutiny - in such a
society, the rights of the individual to criticise administrative policies will be formally safeguarded and upheld, undesirable policies will be eliminated in a
manner analogous to the elimination of falsified scientific theories, and differences between people on social policy will be resolved by critical discussion
and argument rather than by force. The open society as thus conceived of by Popper may be defined as "an association of free individuals
respecting each other's rights within the framework of mutual protection supplied by the state, and achieving, through the making of responsible, rational
decisions, a growing measure of humane and enlightened life" (Levinson, R.B. In Defense of Plato, 17). As such, Popper holds, it is not a utopian
ideal, but an empirically realised form of social organisation which, he argues, is in every respect superior to its (real or potential) totalitarian rivals. But he
does not engage in a moral defence of the ideology of liberalism; rather his strategy is the much deeper one of showing that totalitarianism is typically
based upon historicist and holist presuppositions, and of demonstrating that these presuppositions are fundamentally incoherent.
[Return to Section Headings]
Scientific Knowledge, History, and Prediction
At a very general level, Popper argues that historicism and holism have
their origins in what he terms "one of the oldest dreams of mankind - the
dream of prophecy, the idea that we can know what the future has in store for us, and that we can profit from such knowledge by adjusting our policy to
it." (Conjectures and Refutations, 338). This dream was given further impetus, he speculates, by the emergence of a genuine predictive capability
regarding such events as solar and lunar eclipses at an early stage in human civilisation, which has of course become increasingly refined with the
development of the natural sciences and their concomitant technologies. The kind of reasoning which has made, and continues to make, historicism
plausible may, on this account, be reconstructed as follows: if the application of the laws of the natural sciences can lead to the successful prediction of
such future events as eclipses, then surely it is reasonable to infer that knowledge of the laws of history as yielded by a social science or sciences
(assuming that such laws exist) would lead to the successful prediction of such future social phenomena as revolutions? Why should it be possible to
predict an eclipse, but not a revolution? Why can we not conceive of a social science which could and would function as the theoretical natural sciences
function, and yield precise unconditional predictions in the appropriate sphere of application? These are amongst the questions which Popper seeks to
answer, and in doing so, to show that they are based upon a series of misconceptions about the nature of science, and about the relationship between
scientific laws and scientific prediction.
His first argument may be summarised as follows: in relation to the
critically important concept of prediction, Popper makes a distinction
between what he
terms "conditional scientific predictions", which have the form "If X takes place, then Y will take place", and
"unconditional scientific prophecies", which have the form "Y will take place". Contrary to popular belief, it is the former
rather than the latter which are typical of the natural sciences, which means that typically prediction in natural science is conditional and limited in scope - it
takes the form of hypothetical assertions stating that certain specified changes will come about if particular specified events antecedently take place. This is
not to deny that "unconditional scientific prophecies", such as the prediction of eclipses, for example, do take place in science, and that the
theoretical natural sciences make them possible. However, Popper argues that (a) these unconditional prophecies are not characteristic of the natural
sciences, and (b) that the mechanism whereby they occur, in the very limited way in which they do, is not understood by the historicist.
What is the mechanism which makes unconditional scientific prophecies
possible? The answer is that such prophecies can sometimes be derived from
combination of conditional predictions (themselves derived from scientific laws) and existential statements specifying that the conditions in relation to the
system being investigated are fulfilled. Schematically, this can be represented as follows:
[C.P. + E.S.]=U.P.
where C.P.=Conditional Prediction; E.S.=Existential Statement; U.P.=Unconditional
Prophecy. The most common examples of unconditional scientific
prophecies in science relate to the prediction of such phenomena as lunar and solar eclipses and comets.
Given, then, that this is the mechanism which generates unconditional
scientific prophecies, Popper makes two related claims about historicism:
the historicist does not in fact derive his unconditional scientific prophecies in this manner from conditional predictions, and (b) the historicist cannot do so
because long-term unconditional scientific prophecies can be derived from conditional predictions only if they apply to systems which are well-isolated,
stationary, and recurrent (like our solar system). Such systems are quite rare in nature, and human society is most emphatically not one of them.
This, then, Popper argues, is the reason why it is a fundamental mistake
for the historicist to take the unconditional scientific prophecies of
being typical and characteristic of the predictions of natural science - in fact such predictions are possible only because our solar system is a stationary and
repetitive system which is isolated from other such systems by immense expanses of empty space. The solar system aside, there are very few such systems
around for scientific investigation - most of the others are confined to the field of biology, where unconditional prophecies about the life-cycles of
organisms are made possible by the existence of precisely the same factors. Thus one of the fallacies committed by the historicist is to take the (relatively
rare) instances of unconditional prophecies in the natural science as constituting the essence of what scientific prediction is, to fail to see that such
prophecies apply only to systems which are isolated, stationary, and repetitive, and to seek to apply the method of scientific prophecy to human society
and human history. The latter, of course, is not an isolated system (in fact it's not a system at all), it is constantly changing, and it continually undergoes
rapid, non-repetitive development. In the most fundamental sense possible, every event in human history is discrete, novel, quite unique, and ontologically
distinct from every other historical event. For this reason, it is impossible in principle that unconditional scientific prophecies could be made in relation to
human history - the idea that the successful unconditional prediction of eclipses provides us with reasonable grounds for the hope of successful
unconditional prediction regarding the evolution of human history turns out to be based upon a gross misconception, and is quite false. As Popper himself
concludes, "The fact that we predict eclipses does not, therefore, provide a valid reason for expecting that we can predict revolutions." (Conjectures and
[Return to Section Headings]
Immutable Laws and Contingent Trends
This argument is one of the strongest that has ever been brought against
historicism, cutting, as it does, right to the heart of one of its main
presuppositions. However, it is not Popper's only argument against it. An additional mistake which he detects in historicism is the failure of the historicist to
distinguish between scientific laws and trends, which is also frequently accompanied by a simple logical fallacy. The fallacy is that of inferring from the fact
that our understanding of any (past) historical event - such as, for example, the French Revolution - is in direct proportion to our knowledge of the
antecedent conditions which led to that event, that knowledge of all the antecedent conditions of some future event is possible, and that such knowledge
would make that future event precisely predictable. For the truth is that the number of factors which predate and lead to the occurrence of any event, past,
present, or future, is indefinitely large, and therefore knowledge of all of these factors is impossible, even in principle. What gives rise to the fallacy is the
manner in which the historian (necessarily) selectively isolates a finite number of the antecedent conditions of some past event as being of particular
importance, which are then somewhat misleadingly termed "the causes" of that event, when in fact what this means is that they are the
specific conditions which a particular historian or group of historians take to be more relevant than any other of the indefinitely large number of such
conditions (for this reason, most historical debates range over the question as to whether the conditions thus specified are the right ones). While this kind
of selectivity may be justifiable in relation to the treatment of any past event, it has no basis whatsoever in relation to the future - if we now select, as Marx
did, the "relevant" antecedent conditions for some future event, the likelihood is that we will select wrongly.
The historicist's failure to distinguish between scientific laws and
trends is equally destructive of his cause. This failure makes him think
it possible to explain
change by discovering trends running through past history, and to anticipate and predict future occurrences on the basis of such observations. Here
Popper points out that there is a critical difference between a trend and a scientific law, the failure to observe which is fatal. For a scientific law is universal
in form, while a trend can be expressed only as a singular existential statement. This logical difference is crucial because unconditional predictions, as we
have already seen, can be based only upon conditional ones, which themselves must be derived from scientific laws. Neither conditional nor unconditional
predictions can be based upon trends, because these may change or be reversed with a change in the conditions which gave rise to them in the first
instance. As Popper puts it, there can be no doubt that "the habit of confusing trends with laws, together with the intuitive observation of trends such as
technical progress, inspired the central doctrines of ... historicism." (The Poverty of Historicism, 116). Popper does not, of course, dispute the existence
of trends, nor does he deny that the observation of trends can be of practical utility value - but the essential point is that a trend is something which itself
ultimately stands in need of scientific explanation, and it cannot therefore function as the frame of reference in terms of which anything else can be
scientifically explained or predicted.
A point which connects with this has to do with the role which the evolution
of human knowledge has played in the historical development of human
society. It is incontestable that, as Marx himself observed, there has been a causal link between the two, in the sense that advances in scientific and
technological knowledge have given rise to widespread global changes in patterns of human social organisation and social interaction, which in turn have
led to social structures (e.g. educational systems) which further growth in human knowledge. In short, the evolution of human history has been strongly
influenced by the growth of human knowledge, and it is extremely likely that this will continue to be the case - all the empirical evidence suggests that the
link between the two is progressively consolidating. However, this gives rise to further problems for the historicist. In the first place, the statement that
"if there is such a thing as growing human knowledge, then we cannot anticipate today what we shall know only tomorrow" is, Popper
holds, intuitively highly plausible. Moreover, he argues, it is logically demonstrable by a consideration of the implications of the fact that no scientific
predictor, human or otherwise, can possibly predict, by scientific methods, its own future results. From this it follows, he holds, that "no society can
predict, scientifically, its own future states of knowledge". (The Poverty of Historicism, vii). Thus, while the future evolution of human history is
extremely likely to be influenced by new developments in human knowledge, as it always has in the past, we cannot now scientifically determine what such
knowledge will be. From this it follows that if the future holds any new discoveries or any new developments in the growth of our knowledge (and given
the fallible nature of the latter, it is inconceivable that it does not), then it is impossible for us to predict them now, and it is therefore impossible for us to
predict the future development of human history now, given that the latter will, at least in part, be determined by the future growth of our knowledge. Thus
once again historicism collapses - the dream of a theoretical, predictive science of history is unrealisable, because it is an impossible dream.
Popper's arguments against holism, and in particular his arguments against
the propriety of large-scale planning of social structures, are interconnected
his demonstration of the logical shortcomings of the presuppositions of historicism. Such planning (which actually took place, of course, in the USSR, in
China, and in Cambodia, for example, under totalitarian regimes which accepted forms of historicism and holism), Popper points out, is necessarily
structured in the light of the predictions which have been made about future history on the basis of the so-called "laws" which historicists
such as Marx and Mao claimed to have discovered in relation to human history. Accordingly, recognition that there are no such laws, and that
unconditional predictions about future history are based, at best, upon nothing more substantial than the observation of contingent trends, shows that, from
a purely theoretical as well as a practical point of view, large-scale social planning is indeed a recipe for disaster. In summary, unconditional large-scale
planning for the future is theoretically as well as practically misguided, because, again, part of what we are planning for is our future knowledge, and our
future knowledge is not something which we can in principle now possess - we cannot adequately plan for unexpected advances in our future knowledge,
or for the effects which such advances will have upon society as a whole. The acceptance of historical indeterminism, then, as the only philosophy of
history which is commensurate with a proper understanding of the nature of scientific knowledge, fatally undermines both historicism and holism.
Popper's critique of both historicism and holism is balanced, on the
positive side, by his strong defence of the open society, the view, again,
that a society
is equivalent to the sum of its members, that the actions of the members of society serve to fashion and to shape it, not conversely, and that the social
consequences of intentional actions are very often, and very largely, unintentional. This is why Popper himself advocates what he (rather unfortunately)
terms "piecemeal social engineering" as the central mechanism for social planning - for in utilising this mechanism intentional actions are
directed to the achievement of one specific goal at a time, which makes it possible to monitor the situation to determine whether adverse unintended
effects of intentional actions occur, in order to correct and readjust when this proves necessary. This, of course, parallels precisely the critical testing of
theories in scientific investigation. This approach to social planning (which is explicitly based upon the premise that we do not, because we cannot, know
what the future will be like) encourages attempts to put right what is problematic in society - generally-acknowledged social ills - rather than attempts to
impose some preconceived idea of the "good" upon society as a whole. For this reason, in a genuinely open society piecemeal social
engineering goes hand-in-hand for Popper with negative utilitarianism (the attempt to minimise the amount of misery, rather than, as with positive
utilitarianism, the attempt to maximise the amount of happiness). The state, he holds, should concern itself with the task of progressively formulating and
implementing policies designed to deal with the social problems which actually confront it, with the goal of eliminating human misery and suffering to the
highest possible degree. The positive task of increasing social and personal happiness, by contrast, can and should be should be left to individual citizens
(who may, of course, act collectively to this end), who, unlike the state, have at least a chance of achieving this goal, but who in a free society are rarely in
a position to systematically subvert the rights of others in the pursuit of idealised objectives. Thus in the final analysis for Popper the activity of
problem-solving is as definitive of our humanity at the level of social and political organisation as it is at the level of science, and it is this key insight which
unifies and integrates the broad spectrum of his thought.
[Return to Section Headings]
While it cannot be said that Popper was a modest man, he took criticism
of his theories very seriously, and spent much of his time in his later
endeavouring to show that such criticisms were either based upon misunderstandings, or that his theories could, without loss of integrity, be made
compatible with new and important insights (such as Kuhn's distinction between normal and revolutionary science). The following is a summary of some of
the main criticisms which he has had to address.
1. Popper professes to be anti-conventionalist, and his commitment to
the correspondence theory of truth places him firmly within the realist's
following Kant, he strongly repudiates the positivist/empiricist view that basic statements (i.e. present-tense observation statements about sense-data) are
infallible, and argues convincingly that such basic statements are not mere "reports" of passively registered sensations. Rather they are
descriptions of what is observed as interpreted by the observer with reference to a determinate theoretical framework. This is why Popper repeatedly
emphasises that basic statements are not infallible, and it indicates what he means when he says that they are "theory laden" - perception
itself is an active process, in which the mind assimilates data by reference to an assumed theoretical backdrop. He accordingly asserts that basic
statements themselves are open-ended hypotheses: they have a certain causal relationship with experience, but they are not determined by experience,
and they cannot be verified or confirmed by experience. However, this poses a difficulty regarding the consistency of Popper's theory: if a theory X is to
be genuinely testable (and so scientific) it must be possible to determine whether or not the basic propositions which would, if true, falsify it, are actually
true or false (i.e. whether its potential falsifiers are actual falsifiers). But how can this be known, if such basic statements cannot be verified by experience?
Popper's answer is that "basic statements are not justifiable by our immediate experiences, but are .... accepted by an act, a free decision".
(Logic of Scientific Discovery, 109). However, and notwithstanding Popper's claims to the contrary, this itself seems to be a refined form of
conventionalism - it implies that it is almost entirely an arbitrary matter whether it is accepted that a potential falsifier is an actual one, and consequently that
the falsification of a theory is itself the function of a "free" and arbitrary act. It also seems very difficult to reconcile this with Popper's view
that science progressively moves closer to the truth, conceived of in terms of the correspondence theory, for this kind of conventionalism is inimical to this
(classical) conception of truth.
2. As Lakatos has pointed out, Popper's theory of demarcation hinges
quite fundamentally on the assumption that there are such things as critical
which either conclusively falsify a theory, or give it a strong measure of corroboration. Popper himself is fond of citing, as an example of such a critical test,
the resolution, by Adams and Leverrier, of the problem which the anomalous orbit of Uranus posed for nineteenth century astronomers. Both men
independently came to the conclusion that, assuming Newtonian mechanics to be precisely correct, the observed divergence in the elliptical orbit of
Uranus could be explained if the existence of a seventh, as yet unobserved outer planet was posited. Further, they were able, again within the framework
of Newtonian mechanics, to calculate the precise position of the "new" planet. Thus when subsequent research by Galle at the Berlin
observatory revealed that such a planet (Neptune) did in fact exist, and was situated precisely where Adams and Leverrier had calculated, this was hailed
as by all and sundry as a magnificent triumph for Newtonian physics: in Popperian terms, Newton's theory had been subjected to a critical test, and had
passed with flying colours. Popper himself refers to this strong corroboration of Newtonian physics as "the most startling and convincing success of
any human intellectual achievement". Yet Lakatos flatly denies that there are critical tests, in the Popperian sense, in science, and argues the point
convincingly by turning the above example of an alleged critical test on its head. What, he asks, would have happened if Galle had not found the planet
Neptune? Would Newtonian physics have been abandoned, or would Newton's theory have been falsified? The answer is clearly not, for Galle's failure
could have been attributed to any number of causes other than the falsity of Newtonian physics (e.g. the interference of the earth's atmosphere with the
telescope, the existence of an asteroid belt which hides the new planet from the earth, etc). The point here is that the
"falsification/corroboration" disjunction offered by Popper is far too logically neat: non-corroboration is not necessarily falsification, and
falsification of a high-level scientific theory is never brought about by an isolated observation or set of observations. Such theories are, it is now generally
accepted, highly resistant to falsification. They are falsified, if at all, Lakatos argues, not by Popperian critical tests, but rather within the elaborate context
of the research programmes associated with them gradually grinding to a halt, with the result that an ever-widening gap opens up between the facts to be
explained, and the research programmes themselves. (Lakatos, I. The Methodology of Scientific Research Programmes, passim). Popper's distinction
between the logic of falsifiability and its applied methodology does not in the end do full justice to the fact that all high-level theories grow and live despite
the existence of anomalies (i.e. events/phenomena which are incompatible with the theories). The existence of such anomalies is not usually taken by the
working scientist as an indication that the theory in question is false; on the contrary, he will usually, and necessarily, assume that the auxiliary hypotheses
which are associated with the theory can be modified to incorporate, and explain, existing anomalies.
3. Scientific laws are expressed by universal statements (i.e. they
take the logical form "All A's are X", or some equivalent) which are
therefore concealed conditionals - they have to be understood as hypothetical statements asserting what would be the case under certain ideal conditions.
In themselves they are not existential in nature. Thus "All A's are X" means "If anything is an A, then it is X". Since scientific
laws are non-existential in nature, they logically cannot imply any basic statements, since the latter are explicitly existential. The question arises, then, as to
how any basic statement can falsify a scientific law, given that basic statements are not deducible from scientific laws in themselves? Popper answers that
scientific laws are always taken in conjunction with statements outlining the "initial conditions" of the system under investigation; these
latter, which are singular existential statements, do, when combined with the scientific law, yield hard and fast implications. Thus, the law "All A's
are X", together with the initial condition statement "There is an A at Y", yields the implication "The A at Y is X",
which, if false, falsifies the original law.
This reply is adequate only if it is true, as Popper assumes, that singular
existential statements will always do the work of bridging the gap between
universal theory and a prediction. Hilary Putnam in particular has argued that this assumption is false, in that in some cases at least the statements required
to bridge this gap (which he calls "auxiliary hypotheses") are general rather than particular, and consequently that when the prediction turns
out to be false we have no way of knowing whether this is due to the falsity of the scientific law or the falsity of the auxiliary hypotheses. The working
scientist, Putnam argues, always initially assumes that it is the latter, which shows not only that scientific laws are, contra Popper, highly resistant to
falsification, but also why they are so highly resistant to falsification.
Popper's final position is that he acknowledges that it is impossible
to discriminate science from non-science on the basis of the falsifiability
of the scientific
statements alone; he recognizes that scientific theories are predictive, and consequently prohibitive, only when taken in conjunction with auxiliary
hypotheses, and he also recognizes that readjustment or modification of the latter is an integral part of scientific practice. Hence his final concern is to
outline conditions which indicate when such modification is genuinely scientific, and when it is merely ad hoc. This is itself clearly a major alteration in his
position, and arguably represents a substantial retraction on his part: Marxism can no longer be dismissed as "unscientific" simply because
its advocates preserved the theory from falsification by modifying it (for in general terms, such a procedure, it now transpires, is perfectly respectable
scientific practice). It is now condemned as unscientific by Popper because the only rationale for the modifications which were made to the original
theory was to ensure that it evaded falsification, and so such modifications were ad hoc, rather than scientific. This contention - though not at all
implausible - has, to hostile eyes, a somewhat contrived air about it, and is unlikely to worry the convinced Marxist. On the other hand, the shift in
Popper's own basic position is taken by some critics as an indicator that falsificationism, for all its apparent merits, fares no better in the final analysis than
[Return to Section Headings]
Works By Popper
Logik der Forschung. Julius Springer Verlag,
The Open Society and Its Enemies. (2 Vols). Routledge, London, 1945.
The Logic of Scientific Discovery. (translation of Logik der Forschung). Hutchinson, London, 1959.
Conjectures and Refutations: The Growth of Scientific Knowledge. Routledge, London, 1963.
The Poverty of Historicism (2nd. ed). Routledge, London, 1961.
Objective Knowledge: An Evolutionary Approach. Clarendon Press, Oxford, 1972.
Unended Quest; An Intellectual Autobiography. Fontana, London, 1976.
"A Note on Verisimilitude", The British Journal for the Philosophy of Science 27, 1976, 147-159.
The Self and Its Brain: An Argument for Interactionism (with J.C. Eccles). Springer International, London, 1977.
The Open Universe: An Argument for Indeterminism. (ed. W.W. Bartley 111). Hutchinson, London, 1982.
Realism and the Aim of Science, Hutchinson, London, 1982.
The Myth of the Framework: In Defence of Science and Rationality. Routledge, London, 1994.
Knowledge and the Mind-Body Problem: In Defence of Interactionism. (ed. M.A. Notturno). Routledge, London, 1994.
Works by Other Authors
Ackermann, R. The Philosophy of Karl Popper.
University of Massachusetts Press, Amherst, 1976.
Bambrough, R. (ed). Plato, Popper, and Politics: Some Contributions to a Modern Controversy. Barnes and Noble, New York, 1967.
Baudoin, J. Karl Popper. PUF, Paris, 1989.
Brink, C. & Heidema, J. "A Verisimilar Ordering of Theories Phrased in a Propositional Language", British Journal for the
Philosophy of Science 38, 1987, 533-549.
Brink, C. "Verisimilitude: Views and Reviews", History and Philosophy of Logic 10, 1989, 181-201.
Brink, C. & Britz, K. "Computing Verisimilitude", Notre Dame Journal of Formal Logic 36, 1, 1995, 31-43.
Bunge, M. (ed). The Critical Approach to Science and Philosophy. The Free Press, London & New York, 1964.
Burke, T.E. The Philosophy of Popper. Manchester University Press, Manchester, 1983.
Carr, E.H. What is History? Macmillan, London, 1962.
Cornforth, M. The Open Philosophy and the Open Society: A Reply to Dr. Popper's Refutations of Marxism. Lawrence & Wishart, London,
Corvi, R. An Introduction to the Thought of Karl Popper. (trans. P. Camiller). Routledge, London & New York, 1997.
Currie, G. & Musgrave, A. (eds). Popper and the Human Sciences. Nijhoff, Dordrecht, 1985.
Feyerabend, P. Against Method. New Left Books, London, 1975.
Grünbaum, A. "Is the Method of Bold Conjectures and Attempted Refutations Justifiably the Method of Science?", British Journal
for the Philosophy of Science 27, 1976, 105-136.
Hume, D. A Treatise of Human Nature, in The Philosophical Works (ed. T.H. Green & T.H. Grose), 4 vols (reprint of 1886 edition). Scientia
Verlag Aalen, Darmstadt, 1964.
Jacobs, S. Science and British Liberalism: Locke, Bentham, Mill and Popper. Avebury, Aldershot, 1991.
James, R. Return to Reason: Popper's Thought in Public Life. Open Books, Shepton Mallet, 1980.
Johannson, I. A Critique of Karl Popper's Methodology. Scandinavian University Books, Stockholm, 1975.
Kekes, J. "Popper in Perspective", Metaphilosophy 8 (1977), pp. 36-61.
Keuth, H. "Verisimilitude or the Approach to the Whole Truth", Philosophy of Science 1976, 311-336.
Kuipers, T. A. F. "Approaching Descriptive and Theoretical Truth", Erkenntnis 18, 1982, 343-378.
Kuipers, T. A. F., (ed). What is Closer-to-the-Truth?, Rodopi, Amsterdam, 1987.
Kuhn, T.S. The Structure of Scientific Revolutions. University of Chicago Press, Chicago and London, 1962.
Lakatos, I. "Falsification and the Methodology of Scientific Research Programmes", in Lakatos, I & Musgrove, A. (eds). Criticism
and the Growth of Knowledge. Cambridge University Press, Cambridge, 1970.
Lakatos, I. The Methodology of Scientific Research Programmes, (ed. J. Worrall & G. Currie). Cambridge University Press, 1978.
Lakatos, I & Musgrove, A. (eds). Criticism and the Growth of Knowledge. Cambridge University Press, Cambridge, 1970.
Laudan, L. Progress and Its Problems: Towards a Theory of Scientific Growth. Routledge, London, 1977.
Levinson, P. (ed.). In Pursuit of Truth. Essays in Honour of Karl Popper on the Occasion of his 80th Birthday. Humanities Press, Atlantic
Levinson, R.B. In Defense of Plato. Cambridge University Press, Cambridge, 1957.
Magee, B. Popper. Fontana, London, 1977.
Mellor, D.H. "The Popper Phenomenon", Philosophy 52 (1977), pp. 195-202.
Miller, D. "On the Comparison of False Theories by their Bases", The British Journal for the Philosophy of Science 25, 1974,
Miller, D. "Popper's Qualitative Theory of Verisimilitude", The British Journal for the Philosophy of Science 25, 1974, 166-177.
Miller, D. Critical Rationalism: A Restatement and Defence, Open Court, Chicago, 1994.
Munz, P. Our Knowledge of the Growth of Knowledge: Popper or Wittgenstein? Routledge, London, 1985.
Naydler, J. "The Poverty of Popperism", Thomist 46 (1982), pp. 92-107.
Niiniluoto, I. Truthlikeness, Reidel, Dordrecht, 1987.
Oddie, G. Likeness to Truth, Reidel, Dordrecht, 1986.
O'Hear, A. Karl Popper. Routledge, London, 1980.
Putnam, H. "The Corroboration of Theories", in The Philosophy of Karl Popper (ed. P.A. Schilpp). Open Court Press, La Salle,
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Other Internet Resources
Karl Popper Web
Open Universe of the Japan Popper Society
Institut Wiener Kreis (Society for the Advancement of the Scientific World Conception)
Abstracts of Papers Presented at the Karl Popper 2002 Centenary Congress in Vienna
"Popper, Karl Raimund", by Peter Munz in the Dictionary of New Zealand Biography
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