Karl Popper is generally regarded as one of the greatest philosophersof science of the 20th century. He was also a social and politicalphilosopher of considerable stature, a self-professedcritical-rationalist, a dedicated opponent of all forms of scepticism,conventionalism, and relativism in science and in human affairsgenerally and a committed advocate and staunch defender of the‘Open Society’. One of the many remarkable features ofPopper’s thought is the scope of his intellectual influence: hewas lauded by Bertrand Russell, taught Imre Lakatos, Paul Feyerabendand the future billionaire investor and philanthropist George Soros atthe London School of Economics, numbered David Miller, Joseph Agassi,Alan Musgrave and Jeremy Shearmur amongst his research assistantsthere and had reciprocally beneficial friendships with the economistFriedrich Hayek and the art historian Ernst Gombrich. Additionally,Peter Medawar, John Eccles and Hermann Bondi are amongst thedistinguished scientists who have acknowledged their intellectualindebtedness to his work, the latter declaring that “There is nomore to science than its method, and there is no more to its methodthan Popper has said.”

• 1. Life
• 2. Backdrop to his Thought
• 3. The Problem of Demarcation
• 4. The Growth of Human Knowledge
• 5. Probability, Knowledge and Verisimilitude
• 6. Social and Political Thought—The Critique of Historicism and Holism
• 7. Scientific Knowledge, History, and Prediction
• 8. Immutable Laws and Contingent Trends
• 9. Critical Evaluation
• Bibliography
  • Primary Literature: Works By Popper
  • Secondary Literature
• Academic Tools
• Other Internet Resources
• Related Entries

1. Life

Karl Raimund Popper was born on 28 July 1902 in Vienna, which at thattime could make some claim to be the cultural epicentre of the westernworld. His parents, who were of Jewish origin, brought him up in anatmosphere which he was later to describe as ‘decidedlybookish’. His father was a lawyer by profession, but he alsotook a keen interest in the classics and in philosophy, andcommunicated to his son an interest in social and political issueswhich he was to never lose. His mother inculcated in him such apassion for music that for a time he seriously contemplated taking itup as a career, and indeed he initially chose the history of music asa second subject for his Ph.D. examination. Subsequently, his love formusic became one of the inspirational forces in the development of histhought, and manifested itself in his highly original interpretationof the relationship between dogmatic and critical thinking, in hisaccount of the distinction between objectivity and subjectivity, and,most importantly, in the growth of his hostility towards all forms ofhistoricism, including historicist ideas about the nature of the‘progressive’ in music. The young Karl attended the localRealgymnasium, where he was unhappy with the standards of theteaching, and, after an illness which kept him at home for a number ofmonths, he left to attend the University of Vienna in 1918. However,he did not formally enroll at the University by taking thematriculation examination for another four years. 1919 was in manyrespects the most important formative year of his intellectual life.In that year he became heavily involved in left-wing politics, joinedthe Association of Socialist School Students, and became for a time aMarxist. However, he was quickly disillusioned with the doctrinairecharacter of the latter, and soon abandoned it entirely. He alsodiscovered the psychoanalytic theories of Freud and Adler (he servedbriefly as a voluntary social worker with deprived children in one ofthe latter’s clinics in the 1920s), and listened entranced to alecture which Einstein gave in Vienna on relativity theory. Thedominance of the critical spirit in Einstein, and its total absence inMarx, Freud and Adler, struck Popper as being of fundamentalimportance: the pioneers of psychoanalysis, he came to think, couchedtheir theories in terms which made them amenable only to confirmation,while Einstein’s theory, crucially, had testable implicationswhich, if false, would have falsified the theory itself.

Popper had a rather melancholic personality and took some time tosettle on a career; he trained as a cabinetmaker, obtained a primaryschool teaching diploma in 1925 and qualified to teach mathematics andphysics in secondary school in 1929. He undertook a doctoral programmewith the department of psychology at the University of Vienna theunder the supervision of Karl Bühler, who, with Otto Külpe, was oneof the founder members of the Würzburg school of experimentalpsychology. Popper’s project was initially designed as apsychological investigation of human memory, on which he had conductedinitial research. However, the subject matter of a plannedintroductory chapter on methodology assumed a position of increasingpre-eminence and this resonated with Bühler, who, as a distinguishedKantian scholar, a professor of philosophy as well as psychology, hadfamously addressed the issue of the contemporary ‘crisis inpsychology’. This ‘crisis’, for Bühler, related to the questionof the unity of psychology and had been generated by the proliferationof then competing paradigms within psychology which had undermined thehitherto dominant associationist one and problematized the question ofmethod. Accordingly, under Bühler’s direction, Popper switched histopic to the methodological problem of cognitive psychology andreceived his doctorate in 1928 for his dissertation “Die Methodenfrage derDenkpsychologie”. In extending Bühler’s Kantian approach tothe crisis in the dissertation, Popper critiqued Moritz Schlick’sphysicalist programme for a scientific psychology based ultimatelyupon the transformation of psychology into a science of brainprocesses. This latter ideal, Popper argued, was misconceived, but theissues raised by it ultimately had the effect of refocusing hisattention from Bühler’s question of the unity of psychology to thatof its scientificity, and this philosophical focus on questions ofmethod, objectivity and claims to scientific status was to become aprincipal life-long concern for him. This also brought the orientationof his thought into line with that of such contemporary ‘analytic’philosophers as Frege and Russell as well as that of many members ofthe Vienna Circle and led him to effectively abandon psychology forphilosophy of science.

Popper married Josephine Anna Henninger (‘Hennie’) in1930, and she oversaw his welfare with unflagging support anddevotion, serving additionally as his amanuensis until her death in1985. At an early stage of their marriage they decided that they wouldnever have children, a decision which Popper was able to look back onin later life with apparent equanimity. In 1937 Popper took up aposition teaching philosophy at the University of Canterbury in NewZealand, where he was to remain for the duration of the Second WorldWar, though he had a rather tense relationship with his head ofdepartment. Additionally, Hennie had difficulty adapting to life awayfrom her native Vienna and homesickness made her increasingly unhappy;this was exacerbated by the sheer relentlessness of Popper’s personalwork ethic, which they both found exhausting.

The annexation of Austria in 1938 became the catalyst which promptedPopper to refocus his writings on social and political philosophy andhe publishedThe Open Society and Its Enemies, his critiqueof totalitarianism, in 1945. In 1946 he moved to England to teach atthe London School of Economics, and became professor of logic andscientific method at the University of London in 1949. From this pointon his reputation and stature as a philosopher of science and socialthinker grew enormously, and he continued to write prolifically—a number of his works, particularlyThe Logic of ScientificDiscovery (1959), are now widely seen as pioneering classics inthe field. However, he combined a combative personality with a zealfor self-aggrandisement that did little to endear him to professionalcolleagues at a personal level. He was ill-at-ease in thephilosophical milieu of post-war Britain which was, as he saw it,fixated with trivial linguistic concerns dictated by Wittgenstein,whom he considered to be his nemesis. Popper was a somewhatparadoxical man, whose theoretic commitment to the primacy of rationalcriticism was counterpointed by hostility towards anything thatamounted to less than total acceptance of his own thought, and inBritain—as had been the case in Vienna—he becameincreasingly an isolated figure, though his ideas continued to inspireadmiration.

In later years Popper came under philosophical criticism for hisprescriptive approach to science and his emphasis on the logic offalsification. This was superseded in the eyes of many by thesocio-historical approach taken by Thomas Kuhn inThe Structure ofScientific Revolutions (1962), who—in arguing for theincommensurability of rival scientific paradigms—reintroducedthe idea that change in science is essentially dialectical and isdependent upon the establishment of consensus within communities ofresearchers.

Popper was knighted in 1965, and retired from the University of Londonin 1969, though he remained active as a writer, broadcaster andlecturer until his death in 1994. (For more detail on Popper’s life,cf. hisUnended Quest).

2. Backdrop to his Thought

A number of biographical features may be identified as having aparticular influence upon Popper’s thought. In the first place, histeenage flirtation with Marxism left him thoroughly familiar with theMarxist view of economics, class-war, and history. Secondly, he wasappalled by the failure of the democratic parties to stem the risingtide of fascism in his native Austria in the 1920s and 1930s, and theeffective welcome extended to it by the Marxists. The latter acted onthe ideological grounds that it constituted what they believed to be anecessary dialectical step towards the implosion of capitalism and theultimate revolutionary victory of communism. This was one factor whichled to the much fearedAnschluss, the annexation of Austriaby the German Reich, the anticipation of which forced Popper intopermanent exile from his native country.The Poverty ofHistoricism (1944) andThe Open Society and Its Enemies(1945), his most impassioned and brilliant social works, are as aconsequence a powerful defence of democratic liberalism as a socialand political philosophy, and a devastating critique of the principalphilosophical presuppositions underpinning all forms oftotalitarianism. Thirdly, as we have seen, Popper was profoundlyimpressed by the differences between the allegedly‘scientific’ theories of Freud and Adler and therevolution effected by Einstein’s theory of relativity in physics inthe first two decades of this century. The main difference betweenthem, as Popper saw it, was that while Einstein’s theory was highly‘risky’, in the sense that it was possible to deduceconsequences from it which were, in the light of the then dominantNewtonian physics, highly improbable (e.g., that light is deflectedtowards solid bodies—confirmed by Eddington’s experiments in1919), and which would, if they turned out to be false, falsify thewhole theory, nothing could, evenin principle, falsifypsychoanalytic theories. These latter, Popper came to feel, have morein common with primitive myths than with genuine science. That is tosay, he saw that what is apparently the chief source of strength ofpsychoanalysis, and the principal basis on which its claim toscientific status is grounded, viz. its capability to accommodate, andexplain, every possible form of human behaviour, is in fact a criticalweakness, for it entails that it is not, and could not be, genuinelypredictive. Psychoanalytic theories by their nature are insufficientlyprecise to have negative implications, and so are immunised fromexperiential falsification.

The Marxist account of history too, Popper held, is not scientific,although it differs in certain crucial respects from psychoanalysis.For Marxism, Popper believed, had been initially scientific, in thatMarx had postulated a theory which was genuinely predictive. However,when these predictions were not in fact borne out, the theory wassaved from falsification by the addition ofad hoc hypotheseswhich made it compatible with the facts. By this means, Popperasserted, a theory which was initially genuinely scientificdegenerated into pseudo-scientific dogma.

These factors combined to make Popper takefalsifiability ashis criterion for demarcating science from non-science: if a theory isincompatible 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 solelyto accommodate such observations, or because, as in the case ofpsychoanalytic theories, it is consistent with all possibleobservations, is unscientific. For Popper, however, to assert that atheory is unscientific, is not necessarily to hold that it isunenlightening, still less that it is meaningless, for it sometimeshappens that a theory which is unscientific (because it isunfalsifiable) at a given time may become falsifiable, and thusscientific, with the development of technology, or with the furtherarticulation and refinement of the theory. Further, even purelymythogenic explanations have performed a valuable function in the pastin expediting our understanding of the nature of reality.

3. The Problem of Demarcation

As Popper represents it, the central problem in the philosophy ofscience is that of demarcation, i.e., of distinguishing betweenscience and what he terms ‘non-science’, under whichheading he ranks, amongst others, logic, metaphysics, psychoanalysis,and Adler’s individual psychology. Popper is unusual amongstcontemporary philosophers in that heaccepts the validity ofthe Humean critique of induction, and indeed, goes beyond it inarguing that induction is never actually used in science. However, hedoes not concede that this entails the scepticism which is associatedwith Hume, and argues that the Baconian/Newtonian insistence on theprimacy of ‘pure’ observation, as the initial step in theformation of theories, is completely misguided: all observation isselective and theory-laden—there are no pure or theory-freeobservations. In this way he destabilises the traditional view thatscience can be distinguished from non-science on the basis of itsinductive methodology; in contradistinction to this, Popper holds thatthere is no unique methodology specific to science. Science, likevirtually every other human, and indeed organic, activity, Popperbelieves, consists largely of problem-solving.

Popper accordingly repudiates induction and rejects the view that itis the characteristic method of scientific investigation andinference, substitutingfalsifiability in its place. It iseasy, he argues, to obtain evidence in favour of virtually any theory,and he consequently holds that such ‘corroboration’, as heterms it, should count scientifically only if it is the positiveresult of a genuinely ‘risky’ prediction, which mightconceivably have been false. For Popper, a theory is scientific onlyif it is refutable by a conceivable event. Every genuine test of ascientific theory, then, is logically an attempt to refute or tofalsify it, and one genuine counter-instance falsifies the wholetheory. In a critical sense, Popper’s theory of demarcation is basedupon his perception of the logical asymmetry which holds betweenverification and falsification: it is logically impossible toconclusively verify a universal proposition by reference to experience(as Hume saw clearly), but a single counter-instance conclusivelyfalsifies 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, isprohibitive, in the sense that it forbids, by implication,particular events or occurrences. As such it can be tested andfalsified, but never logically verified. Thus Popper stresses that itshould not be inferred from the fact that a theory has withstood themost rigorous testing, for however long a period of time, that it hasbeen verified; rather we should recognise that such a theory hasreceived a high measure of corroboration. and may be provisionallyretained as the best available theory until it is finally falsified(if indeed it is ever falsified), and/or is superseded by a bettertheory.

Popper has always drawn a clear distinction between thelogic of falsifiability and itsapplied methodology. Thelogic of his theory is utterly simple: if a single ferrous metal isunaffected by a magnetic field it cannot be the case that all ferrousmetals are affected by magnetic fields. Logically speaking, ascientific law is conclusively falsifiable although it is notconclusively verifiable. Methodologically, however, the situation ismuch more complex: no observation is free from the possibility oferror—consequently we may question whether our experimentalresult was what it appeared to be.

Thus, while advocating falsifiability as the criterion of demarcationfor science, Popper explicitly allows for the fact that in practice asingle conflicting or counter-instance is never sufficientmethodologically to falsify a theory, and that scientific theories areoften retained even though much of the available evidence conflictswith them, or is anomalous with respect to them. Scientific theoriesmay, and do, arise genetically in many different ways, and the mannerin which a particular scientist comes to formulate a particular theorymay be of biographical interest, but it is of no consequence as far asthe philosophy of science is concerned. Popper stresses in particularthat there is no unique way, no single method such as induction, whichfunctions as the route to scientific theory, a view which Einsteinpersonally endorsed with his affirmation that ‘There is nological path leading to [the highly universal laws of science]. Theycan only be reached by intuition, based upon something like anintellectual love of the objects of experience’. Science, inPopper’s view, starts with problems rather than withobservations—it is, indeed, precisely in the context ofgrappling with a problem that the scientist makes observations in thefirst instance: his observations are selectively designed to test theextent to which a given theory functions as a satisfactory solution toa 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 usefuland informative truths, but until such time as psychoanalyticaltheories can be formulated in such a manner as to be falsifiable, theywill not attain the status of scientific theories), and astrology andphrenology are pseudo-sciences. Formally, then, Popper’s theory ofdemarcation may be articulated as follows: where a ‘basicstatement’ is to be understood as a particularobservation-report, then we may say that a theory is scientific if andonly if it divides the class of basic statements into the followingtwo non-empty sub-classes: (a) the class of all those basic statementswith which it is inconsistent, or which it prohibits—this is theclass of itspotential falsifiers (i.e., those statementswhich, if true, falsify the whole theory), and (b) the class of thosebasic statements with which it is consistent, or which it permits(i.e., those statements which, if true, corroborate it, or bear itout).

4. The Growth of Human Knowledge

For Popper accordingly, the growth of human knowledge proceeds fromour problems and from our attempts to solve them. These attemptsinvolve the formulation of theories which, if they are to explainanomalies which exist with respect to earlier theories, must go beyondexisting knowledge and therefore require a leap of the imagination.For this reason, Popper places special emphasis on the role played bythe independent creative imagination in the formulation of theory. Thecentrality and priority ofproblems in Popper’s account ofscience is paramount, and it is this which leads him to characterisescientists as ‘problem-solvers’. Further, since thescientist begins with problems rather than with observations or‘bare facts’, Popper argues that the only logicaltechnique which is an integral part of scientific method is that ofthe deductive testing of theories which are not themselves the productof any logical operation. In this deductive procedure conclusions areinferred from a tentative hypothesis. These conclusions are thencompared with one another and with other relevant statements todetermine whether they falsify or corroborate the hypothesis. Suchconclusions are not directly compared with the facts, Popper stresses,simply because there are no ‘pure’ facts available; allobservation-statements are theory-laden, and are as much a function ofpurely subjective factors (interests, expectations, wishes, etc.) asthey are a function of what is objectively real.

How then does the deductive procedure work? Popper specifies foursteps (Logic of Scientific Discovery, 1.3, 9):

(a) The first isformal, a testing of the internalconsistency of the theoretical system to see if it involves anycontradictions.

(b) The second step issemi-formal, the axiomatising of thetheory to distinguish between its empirical and its logical elements.In performing this step the scientist makes the logical form of thetheory explicit. Failure to do this can lead tocategory-mistakes—the scientist ends up asking the wrongquestions, 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 inorder to distinguish the two clearly.

(c) The third step is the comparing of the new theory with existingones to determine whether it constitutes an advance upon them. If itdoes not constitute such an advance, it will not be adopted. If, onthe other hand, its explanatory success matches that of the existingtheories, and additionally, it explains some hitherto anomalousphenomenon, or solves some hitherto unsolvable problems, it will bedeemed to constitute an advance upon the existing theories, and willbe adopted. Thus science involves theoretical progress. However,Popper stresses that we ascertain whether one theory is better thananother by deductively testing both theories, rather than byinduction. For this reason, he argues that a theory is deemed to bebetter than another if (while unfalsified) it has greater empiricalcontent, and therefore greater predictive power than its rival. Theclassic illustration of this in physics was the replacement ofNewton’s theory of universal gravitation by Einstein’s theory ofrelativity. This elucidates the nature of science as Popper sees it:at any given time there will be a number of conflicting theories orconjectures, some of which will explain more than others. The latterwill consequently be provisionally adopted. In short, for Popper anytheory \(X\) is better than a ‘rival’ theory\(Y\) if \(X\) hasgreater empirical content, andhence greater predictive power, than \(Y\).

(d) The fourth and final step is the testing of a theory by theempirical application of the conclusions derived from it. If suchconclusions are shown to be true, the theory is corroborated (butnever verified). If the conclusion is shown to be false, then this istaken as a signal that the theory cannot be completely correct(logically the theory is falsified), and the scientist begins hisquest for a better theory. He does not, however,abandon thepresent theory until such time as he has a better one to substitutefor it. More precisely, the method of theory-testing is as follows:certain singular propositions are deduced from the newtheory—these are predictions, and of special interest are thosepredictions which are ‘risky’ (in the sense of beingintuitively implausible or of being startlingly novel) andexperimentally testable. From amongst the latter the scientist nextselects those which are not derivable from the current or existingtheory—of particular importance are those which contradict thecurrent or existing theory. He then seeks a decision as regards theseand other derived statements by comparing them with the results ofpractical applications and experimentation. If the new predictions areborne out, then the new theory iscorroborated (and the oldone falsified), and is adopted as a working hypothesis. If thepredictions are not borne out, then they falsify the theory from whichthey are derived. Thus Popper retains an element of empiricism: forhim scientific method does involve making an appeal to experience. Butunlike traditional empiricists, Popper holds that experience cannotdetermine theory (i.e., we do not argue or infer fromobservation to theory), it ratherdelimits it: it shows whichtheories are false, not which theories are true. Moreover, Popper alsorejects the empiricist doctrine that empirical observations are, orcan be, infallible, in view of the fact that they are themselvestheory-laden.

The general picture of Popper’s philosophy of science, then is this:Hume’s philosophy demonstrates that there is a contradiction implicitin traditional empiricism, which holds both that all knowledge isderived from experienceand that universal propositions(including scientific laws) are verifiable by reference to experience.The contradiction, which Hume himself saw clearly, derives from theattempt to show that, notwithstanding the open-ended nature ofexperience, scientific laws may be construed as empiricalgeneralisations which are in some way finally confirmable by a‘positive’ experience. Popper eliminates the contradictionby rejecting the first of these principles and removing the demand forempirical verification in favour of empirical falsification in thesecond. Scientific theories, for him, are not inductively inferredfrom experience, nor is scientific experimentation carried out with aview to verifying or finally establishing the truth of theories;rather,all knowledge is provisional, conjectural,hypothetical—we can never finally prove our scientifictheories, we can merely (provisionally) confirm or (conclusively)refute them; hence at any given time we have to choose between thepotentially infinite number of theories which will explain the set ofphenomena under investigation. Faced with this choice, we can onlyeliminate those theories which are demonstrably false, and rationallychoose between the remaining, unfalsified theories. Hence Popper’semphasis on the importance of the critical spirit to science—forhim critical thinking is the very essence of rationality. For it isonly by critical thought that we can eliminate false theories, anddetermine which of the remaining theories is the best available one,in the sense of possessing the highest level of explanatory force andpredictive power. It is precisely this kind of critical thinking whichis conspicuous by its absence in contemporary Marxism and inpsychoanalysis.

5. Probability, Knowledge and Verisimilitude

In the view of many social scientists, the more probable a theory is,thebetter it is, and if we have to choose between twotheories which are equally 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 tobe precise, the working scientist, is interested, in Popper’s view, intheories with a high informative content, because such theoriespossess a high predictive power and are consequently highly testable.But if this is true, Popper argues, then, paradoxical as it may sound,the moreimprobable a theory is the better it isscientifically, because the probability and informative content of atheory vary inversely—the higher the informative content of atheory the lower will be its probability, for the more information astatement contains, the greater will be the number of ways in which itmay turn out to be false. Thus the statements which are of specialinterest to the scientist are those with a high informative contentand (consequentially) a low probability, which nevertheless come closeto the truth. Informative content, which is in inverse proportion toprobability, is in direct proportion to testability. Consequently theseverity of the test to which a theory can be subjected, and by meansof which it is falsified or corroborated, is all-important.

For Popper, all scientific criticism must be piecemeal, i.e., he holdsthat it is not possible to question every aspect of a theory at once.More precisely, while attempting to resolve a particular problem ascientist of necessity accepts all kinds of things as unproblematic.These things constitute what Popper terms the ‘backgroundknowledge’. However, he stresses that the background knowledgeisnot knowledge in the sense of being conclusivelyestablished; it may be challenged at any time, especially if it issuspected that its uncritical acceptance may be responsible fordifficulties which are subsequently encountered. Nevertheless, it isclearly not possible to question both the theory and the backgroundknowledge at the same time (e.g., in conducting an experiment thescientist of necessity assumes that the apparatus used is in workingorder).

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 repeated tests usually show where the trouble lies. Evenobservation statements, Popper maintains, are fallible, and science inhis view is not a quest for certain knowledge, but an evolutionaryprocess in which hypotheses or conjectures are imaginatively proposedand tested in order to explain facts or to solve problems. Popperemphasises both the importance of questioning the background knowledgewhen the need arises, and the significance of the fact thatobservation-statements are theory-laden, and hence fallible. For whilefalsifiability is simple as a logical principle, in practice it isexceedingly complicated—no single observation can ever be takento falsify a theory, for there is always the possibility (a) that theobservation itself is mistaken, or (b) that the assumed backgroundknowledge is faulty or defective.

Popper was initially uneasy with the concept of truth, and in hisearliest writings he avoided asserting that a theory which iscorroborated is true—for clearly if every theory is anopen-ended hypothesis, as he maintains, thenipso facto ithas to be at least potentially false. For this reason Popperrestricted himself to the contention that a theory which is falsifiedis false and is known to be such, and that a theory which replaces afalsified theory (because it has a higher empirical content than thelatter, and explains what has falsified it) is a ‘bettertheory’ than its predecessor. However, he came to acceptTarski’s reformulation of the correspondence theory of truth, and inConjectures and Refutations (1963) he integrated the conceptsof truth and content to frame the metalogical concept of‘truthlikeness’ or ‘verisimilitude’.A ‘good’ scientific theory, Popper thus argued, has ahigher level of verisimilitude than its rivals, and he explicated thisconcept by reference to the logical consequences of theories. Atheory’s content is the totality of its logical consequences, whichcan be divided into two classes: there is the‘truth-content’ of a theory, which is the classof true propositions which may be derived from it, on the one hand,and the ‘falsity-content’ of a theory, on theother 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 theorywhich is true is necessarily empty).

Popper offered two methods of comparing theories in terms ofverisimilitude, the qualitative and quantitative definitions. On thequalitative account, Popper asserted:

Assuming that the truth-content and the falsity-content of twotheories \(t_1\) and \(t_2\) arecomparable, we can say that \(t_2\) is more closelysimilar to the truth, or corresponds better to the facts, than\(t_1\), if and only if either:

(a) the truth-content but not the falsity-content of\(t_2\) exceeds that of \(t_1\), or

(b) the falsity-content of \(t_1\), but not itstruth-content, exceeds that of \(t_2\).(Conjectures and Refutations, 233).

Here, verisimilitude is defined in terms of subclass relationships:\(t_2\) has a higher level of verisimilitude than\(t_1\) if and only if their truth- andfalsity-contents are comparable through subclass relationships, andeither (a) \(t_2\)’s truth-content includes\(t_1\)’s and \(t_2\)’s falsity-content,if it exists, is included in, or is the same as,\(t_1\)’s, \(or\) (b) \(t_2\)’struth-content includes or is the same as \(t_1\)’s and\(t_2\)’s falsity-content, if it exists, is included in\(t_1\)’s.

On the quantitative account, verisimilitude is defined by assigningquantities to contents, where the index of the content of a giventheory is its logical improbability (given again that content andprobability vary inversely). Formally, then, Popper defines thequantitative verisimilitude which a statement ‘a’possesses by means of a formula:

where \(Vs(a)\) represents the verisimilitude of \(a\), \(Ct_T (a)\)is a measure of the truth-content of \(a\), and \(Ct_F (a)\) is ameasure of its falsity-content.

The utilisation of either method of computing verisimilitude shows,Popper held, that even if a theory \(t_2\) with ahigher content than a rival theory \(t_1\) issubsequently falsified, it can still legitimately be regarded as abetter theory than \(t_1\), and ‘better’ ishere now understood to mean \(t_2\) iscloser tothe truth than \(t_1\). Thus scientific progressinvolves, on this view, the abandonment of partially true, butfalsified, theories, for theories with a higher level ofverisimilitude, i.e., which approach more closely to the truth. Inthis way, verisimilitude allowed Popper to mitigate what many saw asthe pessimism of an anti-inductivist philosophy of science which heldthat most, if not all scientific theories are false, and that a truetheory, even if discovered, could not beknown to be such.With the introduction of the new concept, Popper was able to representthis as an essentially optimistic position in terms of which we canlegitimately be said to have reason to believe that science makesprogress towards the truth through the falsification and corroborationof theories. Scientific progress, in other words, could now berepresented as progresstowards the truth, and experimentalcorroboration could be seen anindicator ofverisimilitude.

However, in the 1970s a series of papers published by researcherssuch as Miller, Tichý, and Grünbaum in particular revealedfundamental defects in Popper’s formal definitions of verisimilitude.The significance of this work was that verisimilitude is largelyimportant in Popper’s system because of its application to theorieswhich are known to befalse. In this connection, Popper hadwritten:

Ultimately, the idea of verisimilitude is most important in caseswhere we know that we have to work with theories which areatbest approximations—that is to say, theories of which weknow that they cannot be true. (This is often the case in the socialsciences). In these cases we can still speak of better or worseapproximations to the truth (and we therefore do not need to interpretthese cases in an instrumentalist sense). (Conjectures andRefutations, 235).

For these reasons, the deficiencies discovered by the critics inPopper’s formal definitions were seen by many as critical, preciselybecause the most significant of these related to the levels ofverisimilitude offalse theories. In 1974, Miller andTichý, working independently of each other, demonstrated thatthe conditions specified by Popper in his accounts of both qualitativeand quantitative verisimilitude for comparing the truth- andfalsity-contents of theories can be satisfied only when the theoriesaretrue. In the crucially important case of false theories,however, Popper’s definitions are formally defective. For while Popperhad believed that verisimilitude intersected positively with hisaccount of corroboration, in the sense that he viewed an improbabletheory which had withstood critical testing as one the truth-contentof which is great relative to rival theories, while itsfalsity-content (if it exists) would be relatively low, Miller andTichý proved, on the contrary, that in the case of a falsetheory \(t_2\) which has excess content over a rivaltheory false \(t_1\) both the truth-contentand the falsity-content of \(t_2\) will exceedthat of \(t_1\). With respect to theories which arefalse, therefore, Popper’s conditions for comparing levels ofverisimilitude, whether in quantitative and qualitative terms, cannever be met.

Commentators on Popper, with few exceptions, had initially attachedlittle importance to his theory of verisimilitude. However, after thefailure of Popper’s definitions in 1974, some critics came to see itas central to his philosophy of science, and consequentially held thatthe whole edifice of the latter had been subverted. For his part,Popper’s response was two-fold. In the first place, whileacknowledging the deficiencies in his own formal account (“mymain mistake was my failure to see at once that … if thecontent of a false statement \( a\) exceeds that of a statement\(b\), then the truth-content of \(a\) exceeds thetruth-content of \(b\), and the same holds of theirfalsity-contents”,Objective Knowledge, 371), Popperargued that “I do think that we should not conclude from thefailure of my attempts to solve the problem [of definingverisimilitude] that the problem cannot be solved”(Objective Knowledge, 372), a point of view which was toprecipitate more than two decades of important technical research inthis field. At another, more fundamental level, he moved the task offormally defining the concept from centre-stage in his philosophy ofscience, by protesting that he had never intended to imply “thatdegrees 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 isheuristic and intuitive, in which the absence of an adequate formaldefinition is not an insuperable impediment to its utilisation in theactual appraisal of theories relativised to problems in which we havean interest. The thrust of the latter strategy seems to many togenuinely reflect the significance of the concept of verisimilitude inPopper’s system, but it has not satisfied all of his critics.

6. Social and Political Thought—The Critique of Historicism and Holism

Given Popper’s personal history and background, it is hardlysurprising that he developed a deep and abiding interest in social andpolitical philosophy. However, it is worth emphasising that his angleof approach to these fields is through a consideration of the natureof the social sciences which seek to describe and explicate themsystematically, particularly history. It is in this context that heoffers an account of the nature of scientific prediction, which inturn allows him a point of departure for his attack upontotalitarianism and all its intellectual supports, especially holismand historicism. In this context holism is to be understood as theview that human social groupings are greater than the sum of theirmembers, that such groupings are ‘organic’ entities intheir own right, that they act on their human members and shape theirdestinies, and that they are subject to their own independent laws ofdevelopment. Historicism, which is closely associated with holism, isthe belief that history develops inexorably and necessarily accordingto certain principles or rules towards a determinate end (as forexample in the dialectic of Hegel, which was adopted and implementedby Marx). The link between holism and historicism is that the holistbelieves that individuals are essentially formed by the socialgroupings to which they belong, while the historicist—who isusually also a holist—holds that we can understand such a socialgrouping only in terms of the internal principles which determine itsdevelopment.

These beliefs lead to what Popper calls ‘The HistoricistDoctrine of the Social Sciences’, the views (a) that theprincipal task of the social sciences is to make predictions about thesocial and political development of man, and (b) that the task ofpolitics, once the key predictions have been made, is, in Marx’swords, to lessen the ‘birth pangs’ of future social andpolitical developments. Popper thinks that this view of the socialsciences is both theoretically misconceived (in the sense of beingbased upon a view of natural science and its methodology which istotally wrong), and socially dangerous, as it leads inevitably tototalitarianism and authoritarianism—to centralised governmentalcontrol of the individual and the attempted imposition of large-scalesocial planning. Against this Popper strongly advances the view thatany human social grouping is no more (or less) than the sum of itsindividual members, that what happens in history is the (largelyunplanned and unforeseeable) result of the actions of suchindividuals, and that large scale social planning to an antecedentlyconceived blueprint is inherently misconceived—and inevitablydisastrous—precisely because human actions have consequenceswhich cannot be foreseen. Popper, then, is an historicalindeterminist, insofar as he holds that history does not evolvein accordance with intrinsic laws or principles, that in the absenceof such laws and principles unconditional prediction in the socialsciences is an impossibility, and that there is no such thing ashistorical necessity.

The link between Popper’s theory of knowledge and his socialphilosophy is his fallibilism—just as we make theoreticalprogress in science by deliberately subjecting our theories tocritical scrutiny, and abandoning those which have been falsified, sotoo, Popper holds, the critical spirit can and should be sustained atthe social level. More specifically, the open society can be broughtabout only if it is possible for the individual citizen to evaluatecritically the consequences of the implementation of governmentpolicies, which can then be abandoned or modified in the light of suchcritical scrutiny—in such a society, the rights of theindividual to criticise administrative policies will be formallysafeguarded and upheld, undesirable policies will be eliminated in amanner analogous to the elimination of falsified scientific theories,and differences between people on social policy will be resolved bycritical discussion and argument rather than by force. The opensociety as thus conceived of by Popper may be defined as ‘anassociation of free individuals respecting each other’s rights withinthe framework of mutual protection supplied by the state, andachieving, through the making of responsible, rational decisions, agrowing measure of humane and enlightened life’ (Levinson, R.B.In Defense of Plato, 17). As such, Popper holds, it is not autopian ideal, but an empirically realised form of social organisationwhich, he argues, is in every respect superior to its (real orpotential) totalitarian rivals. But he does not engage in a moraldefence of the ideology of liberalism; rather his strategy is the muchdeeper one of showing that totalitarianism is typically based uponhistoricist and holist presuppositions, and of demonstrating thatthese presuppositions are fundamentally incoherent.

7. Scientific Knowledge, History, and Prediction

At a very general level, Popper argues that historicism and holismhave their origins in what he terms ‘one of the oldest dreams ofmankind—the dream of prophecy, the idea that we can know whatthe future has in store for us, and that we can profit from suchknowledge by adjusting our policy to it.’ (Conjectures andRefutations, 338). This dream was given further impetus, hespeculates, by the emergence of a genuine predictive capabilityregarding such events as solar and lunar eclipses at an early stage inhuman civilisation, which has of course become increasingly refinedwith the development of the natural sciences and their concomitanttechnologies. The kind of reasoning which has made, and continues tomake, historicism plausible may, on this account, be reconstructed asfollows: if the application of the laws of the natural sciences canlead to the successful prediction of such future events as eclipses,then surely it is reasonable to infer that knowledge of the laws ofhistory as yielded by a social science or sciences (assuming that suchlaws exist) would lead to the successful prediction of such futuresocial phenomena as revolutions? Why should it be possible to predictan eclipse, but not a revolution? Why can we not conceive of a socialscience which could and would function as the theoretical naturalsciences function, and yield precise unconditional predictions in theappropriate sphere of application? These are amongst the questionswhich Popper seeks to answer, and in doing so, to show that they arebased upon a series of misconceptions about the nature of science, andabout the relationship between scientific laws and scientificprediction.

His first argument may be summarised as follows: in relation to thecritically important concept of prediction, Popper makes a distinctionbetween what he terms ‘conditional scientificpredictions’, which have the form ‘If \(X\) takesplace, 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 thenatural sciences, which means that typically prediction in naturalscience is conditional and limited in scope—it takes the form ofhypothetical assertions stating that certain specified changes willcome about if particular specified events antecedently take place.This is not to deny that ‘unconditional scientificprophecies’, such as the prediction of eclipses, for example, dotake place in science, and that the theoretical natural sciences makethem possible. However, Popper argues that (a) these unconditionalprophecies are notcharacteristic of the natural sciences,and (b) that the mechanism whereby they occur, in the very limited wayin which they do, is not understood by the historicist.

What is the mechanism which makes unconditional scientific propheciespossible? The answer is that such prophecies can sometimes be derivedfrom a combination of conditional predictions (themselves derived fromscientific laws)and existential statements specifying thatthe conditions in relation to the system being investigated arefulfilled. Schematically, this can be represented as follows:

where \(\mathrm{C.P.} =\) Conditional Prediction; \(\mathrm{E.S.} =\)Existential Statement; \(\mathrm{U.P.} = \) UnconditionalProphecy. The most common examples of unconditional scientificprophecies in science relate to the prediction of such phenomena aslunar and solar eclipses and comets.

Given, then, that this is the mechanism which generates unconditionalscientific prophecies, Popper makes two related claims abouthistoricism: (a) That the historicist does not in fact derive hisunconditional scientific prophecies in this manner from conditionalpredictions, and (b) the historicistcannot do so becauselong-term unconditional scientific prophecies can be derived fromconditional predictions only if they apply to systems which arewell-isolated, stationary, and recurrent (like our solar system). Suchsystems are quite rare in nature, and human society is mostemphatically not one of them.

This, then, Popper argues, is the reason why it is a fundamentalmistake for the historicist to take the unconditional scientificprophecies of eclipses as being typical and characteristic of thepredictions of natural science—in fact such predictions arepossible only because our solar system is a stationary and repetitivesystem which is isolated from other such systems by immense expansesof empty space. The solar system aside, there are very few suchsystems around for scientific investigation—most of the othersare confined to the field of biology, where unconditional propheciesabout the life-cycles of organisms are made possible by the existenceof precisely the same factors. Thus one of the fallacies committed bythe historicist is to take the (relatively rare) instances ofunconditional prophecies in the natural science as constituting theessence of what scientific prediction is, to fail to see that suchprophecies apply only to systems which are isolated, stationary, andrepetitive, and to seek to apply the method of scientific prophecy tohuman society and human history. The latter, of course, isnot an isolated system (in fact it’s not a system at all), itis 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, andontologically distinct from every other historical event. For thisreason, it is impossible in principle that unconditional scientificprophecies could be made in relation to human history—the ideathat the successful unconditional prediction of eclipses provides uswith reasonable grounds for the hope of successful unconditionalprediction regarding the evolution of human history turns out to bebased upon a gross misconception, and is quite false. As Popperhimself concludes, “The fact that we predict eclipses does not,therefore, provide a valid reason for expecting that we can predictrevolutions.” (Conjectures and Refutations, 340).

8. Immutable Laws and Contingent Trends

This argument is one of the strongest that has ever been broughtagainst historicism, cutting, as it does, right to the heart of one ofits main theoretical presuppositions. However, it is not Popper’s onlyargument against it. An additional mistake which he detects inhistoricism is the failure of the historicist to distinguish betweenscientificlaws andtrends, which is also frequentlyaccompanied by a simple logical fallacy. The fallacy is that ofinferring from the fact that our understanding of any (past)historical event—such as, for example, the FrenchRevolution—is in direct proportion to our knowledge of theantecedent conditions which led to that event, that knowledge of allthe antecedent conditions of some future event is possible, and thatsuch knowledge would make that future event precisely predictable. Forthe truth is that the number of factors which predate and lead to theoccurrence of any event, past, present, or future, is indefinitelylarge, and therefore knowledge of all of these factors is impossible,even in principle. What gives rise to the fallacy is the manner inwhich the historian (necessarily) selectively isolates a finite numberof the antecedent conditions of some past event as being of particularimportance, which are then somewhat misleadingly termed ‘thecauses’ of that event, when in fact what this means is that theyare the specific conditions which a particular historian or group ofhistorians take to be more relevant than any other of theindefinitely large number of such conditions (for this reason, mosthistorical debates range over the question as to whether theconditions thus specified are the right ones). While thiskind of selectivity may be justifiable in relation to the treatment ofany past event, it has no basis whatsoever in relation to thefuture—if we now select, as Marx did, the ‘relevant’antecedent conditions for some future event, the likelihood is that wewill select wrongly.

The historicist’s failure to distinguish between scientific laws andtrends is equally destructive of his cause. This failure makes himthink it possible to explain change by discovering trends runningthrough past history, and to anticipate and predict future occurrenceson the basis of such observations. Here Popper points out that thereis a critical difference between a trend and a scientific law, thefailure to observe which is fatal. For a scientific law is universalin form, while a trend can be expressed only as a singular existentialstatement. This logical difference is crucial because unconditionalpredictions, as we have already seen, can be based only uponconditional ones, which themselves must be derived from scientificlaws. Neither conditional nor unconditional predictions can be basedupon trends, because these may change or be reversed with a change inthe conditions which gave rise to them in the first instance. AsPopper puts it, there can be no doubt that “the habit ofconfusing trends with laws, together with the intuitive observation oftrends 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, nordoes he deny that the observation of trends can be of practicalutility value—but the essential point is that a trend issomething whichitself ultimately stands in need ofscientific explanation, and it cannot therefore function as the frameof reference in terms of which anything else can be scientificallyexplained or predicted.

A point which connects with this has to do with the role which theevolution of human knowledge has played in the historical developmentof human society. It is incontestable that, as Marx himself observed,there has been a causal link between the two, in the sense thatadvances in scientific and technological knowledge have given rise towidespread global changes in patterns of human social organisation andsocial interaction, which in turn have led to social structures (e.g.educational systems) which further growth in human knowledge. Inshort, the evolution of human history has been strongly influenced bythe growth of human knowledge, and it is extremely likelythat this will continue to be the case—all the empiricalevidence suggests that the link between the two is progressivelyconsolidating. However, this gives rise to further problems for thehistoricist. In the first place, the statement that ‘if there issuch a thing as growing human knowledge, then we cannot anticipatetoday what we shall know only tomorrow’ is, Popper holds,intuitively highly plausible. Moreover, he argues, it is logicallydemonstrable by a consideration of the implications of the fact thatno scientific predictor, human or otherwise, can possibly predict, byscientific methods, its own future results. From this it follows, heholds, that ‘no society can predict, scientifically, its ownfuture states of knowledge’. (The Poverty ofHistoricism, vii). Thus, while the future evolution of humanhistory is extremely likely to be influenced by new developments inhuman knowledge, as it always has in the past, we cannot nowscientifically determine what such knowledge will be. From this itfollows that if the future holds any new discoveries or any newdevelopments in the growth of our knowledge (and given the falliblenature of the latter, it is inconceivable that it does not), then itis impossible for us to predict them now, and it is thereforeimpossible for us to predict the future development of human historynow, given that the latter will, at least in part, be determined bythe future growth of our knowledge. Thus once again historicismcollapses—the dream of a theoretical, predictive science ofhistory is unrealisable, because it is an impossible dream.

Popper’s arguments against holism, and in particular his argumentsagainst the propriety of large-scale planning of social structures,are interconnected with his demonstration of the logical shortcomingsof the presuppositions of historicism. Popper points out that suchplanning (which actually took place, of course, in the USSR, in China,and in Cambodia, for example, under totalitarian regimes whichaccepted forms of historicism and holism) is necessarily structured inthe light of the predictions which have been made about future historyon the basis of the so-called ‘laws’ which historicistssuch as Marx and Mao claimed to have discovered in relation to humanhistory. Accordingly, recognition that there are no such laws, andthat unconditional predictions about future history are based, atbest, upon nothing more substantial than the observation of contingenttrends, shows that, from a purely theoretical as well as a practicalpoint of view, large-scale social planning is indeed a recipe fordisaster. In summary, unconditional large-scale planning for thefuture is theoretically as well as practically misguided, because,again, part of what we are planning for is our future knowledge, andour future knowledge is not something which we can in principle nowpossess—we cannot adequately plan for unexpected advances in ourfuture knowledge, or for the effects which such advances will haveupon society as a whole. The acceptance of historical indeterminism,then, as the only philosophy of history which is commensurate with aproper understanding of the nature of scientific knowledge, fatallyundermines both historicism and holism.

Popper’s critique of both historicism and holism is balanced, on thepositive side, by his affirmation of the ideals of individualism andmarket economics and his strong defence of the open society—theview, again, that a society is equivalent to the sum of its members,that the actions of the members of society serve to fashion and toshape it, and that the social consequences of intentional actions arevery often, and very largely, unintentional. This part of his socialphilosophy was influenced by the economist Friedrich Hayek, who workedwith him at the London School of Economics and who was a life-longfriend. Popper advocated what he (rather unfortunately) terms‘piecemeal social engineering’ as the central mechanismfor social planning—for in utilising this mechanism intentionalactions are directed to the achievement of one specific goal at atime, which makes it possible to monitor the situation to determinewhether adverse unintended effects of intentional actions occur, inorder to correct and readjust when this proves necessary. This, ofcourse, parallels precisely the critical testing of theories inscientific investigation. This approach to social planning (which isexplicitly based upon the premise that we do not, because we cannot,know what the future will be like) encourages attempts to put rightwhat is problematic in society—generally-acknowledged socialills—rather than attempts to impose some preconceived idea ofthe ‘good’ upon society as a whole. For this reason, in agenuinely open society piecemeal social engineering goes hand-in-handfor Popper withnegative utilitarianism (the attempt tominimise the amount of misery, rather than, as with positiveutilitarianism, the attempt to maximise the amount of happiness). Thestate, he holds, should concern itself with the task of progressivelyformulating and implementing policies designed to deal with the socialproblems which actually confront it, with the goal of eliminatinghuman misery and suffering to the highest possible degree. Thepositive task of increasing social and personal happiness, bycontrast, can and should be left to individual citizens (whomay, of course, act collectively to this end), who, unlike the state,have at least a chance of achieving this goal, but who in a freesociety are rarely in a position to systematically subvert the rightsof others in the pursuit of idealised objectives. Thus in the finalanalysis for Popper the activity of problem-solving is as definitiveof our humanity at the level of social and political organisation asit is at the level of science, and it is this key insight whichunifies and integrates the broad spectrum of his thought.

9. Critical Evaluation

While it cannot be said that Popper was a modest man, he tookcriticism of his theories very seriously, and spent much of his timein his later years trying to show that such criticisms were eitherbased upon misunderstandings, or that his theories could, without lossof integrity, be made compatible with new and important insights. Thefollowing is a summary of some of the main criticisms which he has hadto address. (For Popper’s responses to critical commentary, see his‘Replies to My Critics’, in P.A. Schilpp (ed.),ThePhilosophy of Karl Popper, Volume 2, and hisRealism and theAim of Science, edited by W.W. Bartley III.)

1. Popper professes to be anti-conventionalist, and his commitment tothe correspondence theory of truth places him firmly within therealist’s camp. Yet, following Kant, he strongly repudiates thepositivist/empiricist view that basic statements (i.e., present-tenseobservation statements about sense-data) are infallible, and arguesconvincingly that such basic statements are not mere‘reports’ of passively registered sensations. Rather theyare descriptions of what is observed as interpreted by the observerwith reference to a determinate theoretical framework. This is whyPopper 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 activeprocess, in which the mind assimilates data by reference to an assumedtheoretical backdrop. He accordingly asserts that basic statementsthemselves are open-ended hypotheses: they have a certain causalrelationship with experience, but they are not determined byexperience, and they cannot be verified or confirmed by experience.However, this poses a difficulty regarding the consistency of Popper’stheory: if a theory \(X\) is to be genuinely testable (and soscientific) it must be possible to determine whether or not the basicpropositions which would, if true, falsify it, areactuallytrue or false (i.e., whether its potential falsifiers are actualfalsifiers). But how can this be known, if such basic statementscannot be verified by experience? Popper’s answer is that ‘basicstatements are not justifiable by our immediate experiences, but are… accepted by an act, a free decision’. (Logic ofScientific Discovery, 109). However, and notwithstanding Popper’sclaims to the contrary, this itself seems to be a refined form ofconventionalism—it implies that it is almost entirely anarbitrary matter whether it is accepted that a potential falsifier isan actual one, and consequently that the falsification of a theory isitself the function of a ‘free’ and arbitrary act. It alsoseems very difficult to reconcile this with Popper’s view that scienceprogressively moves closer to the truth, conceived of in terms of thecorrespondence theory, for this kind of conventionalism is inimical tothis (classical) conception of truth.

2. As Lakatos has pointed out, Popper’s theory of demarcation hingesquite fundamentally on the assumption that there are such things ascritical tests, which either falsify a theory, or give it a strongmeasure of corroboration. Popper himself is fond of citing, as anexample of such a critical test, the resolution, by Adams andLeverrier, of the problem which the anomalous orbit of Uranus posedfor nineteenth century astronomers. Both men independently came to theconclusion that, assuming Newtonian mechanics to be precisely correct,the observed divergence in the elliptical orbit of Uranus could beexplained if the existence of a seventh, as yet unobserved outerplanet was posited. Further, they were able, again within theframework of Newtonian mechanics, to calculate the precise position ofthe ‘new’ planet. Thus when subsequent research by Galleat the Berlin observatory revealed that such a planet (Neptune) did infact exist, and was situated precisely where Adams and Leverrier hadcalculated, this was hailed as by all and sundry as a magnificenttriumph for Newtonian physics: in Popperian terms, Newton’s theory hadbeen subjected to a critical test, and had passed with flying colours.Popper himself refers to this strong corroboration of Newtonianphysics as ‘the most startling and convincing success of anyhuman intellectual achievement’. Yet Lakatos flatly denies thatthere are critical tests, in the Popperian sense, in science, andargues the point convincingly by turning the above example of analleged critical test on its head. What, he asks, would have happenedif Galle hadnot found the planet Neptune? Would Newtonianphysics have been abandoned, or would Newton’s theory have beenfalsified? The answer is clearly not, for Galle’s failure could havebeen attributed to any number of causes other than the falsity ofNewtonian physics (e.g., the interference of the earth’s atmospherewith the telescope, the existence of an asteroid belt which hides thenew planet from the earth, etc). The point here is that the‘falsification/corroboration’ disjunction offered byPopper is far too logically neat: non-corroboration isnotnecessarily falsification, and falsification of a high-levelscientific theory is never brought about by an isolated observation orset 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 theelaborate context of the research programmes associated with themgradually grinding to a halt, with the result that an ever-wideninggap opens up between the facts to be explained, and the researchprogrammes themselves (Lakatos 1978, passim). Popper’s distinctionbetween the logic of falsifiability and its applied methodology doesnot in the end do full justice to the fact that all high-leveltheories grow and live despite the existence of anomalies (i.e.,events/phenomena which are incompatible with the theories). Theexistence of such anomalies is not usually taken by the workingscientist as an indication that the theory in question is false; onthe contrary, he will usually, and necessarily, assume that theauxiliary hypotheses which are associated with the theory can bemodified to incorporate, and explain, existing anomalies.

3. Scientific laws are expressed by universal statements (i.e., theytake the logical form ‘All \(A\)s are \(X\)’, orsome equivalent) which are therefore concealed conditionals—theyhave to be understood as hypothetical statements asserting what wouldbe the case under certain ideal conditions. In themselves they are notexistential in nature. Thus ‘All \(A\)s are\(X\)’ means ‘If anything is an \(A\), then itis \(X\)’. Since scientific laws are non-existential innature, they logically cannot imply any basic statements, since thelatter are explicitly existential. The question arises, then, as tohow any basic statement can falsify a scientific law, given that basicstatements are not deducible from scientific laws in themselves?Popper answers that scientific laws are always taken inconjunction with statements outlining the ‘initialconditions’ of the system under investigation; these latter,which are singular existential statements, do, when combined with thescientific law, yield hard and fast implications. Thus, the law‘All \(A\)s are \(X\)’, together with theinitial condition statement ‘There is an \(A\) at\(Y\)’, yields the implication ‘The \(A\) at\(Y\) is \(X\)’, which, if false, falsifies theoriginal law.

This reply is adequate only if it is true, as Popper assumes, thatsingular existential statements will always do the work ofbridging the gap between a universal theory and a prediction. HilaryPutnam in particular has argued that this assumption is false, in thatin some cases at least the statements required to bridge this gap(which he calls ‘auxiliary hypotheses’) are general ratherthan particular, and consequently that when the prediction turns outto be false we have no way of knowing whether this is due to thefalsity of the scientific lawor the falsity of the auxiliaryhypotheses. The working scientist, Putnam argues, always initiallyassumes that it is the latter, which shows not only that scientificlaws 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 impossibleto discriminate science from non-science on the basis of thefalsifiability of the scientific statementsalone; herecognizes that scientific theories are predictive, and consequentlyprohibitive,only when taken in conjunction with auxiliaryhypotheses, and he also recognizes that readjustment or modificationof the latter is an integral part of scientific practice. Hence hisfinal concern is to outline conditions which indicate when suchmodification is genuinely scientific, and when it is merelyadhoc. This is itself clearly a major alteration in his position,and arguably represents a substantial retraction on his part: Marxismcan no longer be dismissed as ‘unscientific’ simplybecause its advocates preserved the theory from falsification bymodifying it (for in general terms, such a procedure, it nowtranspires, is perfectly respectable scientific practice). It is nowcondemned as unscientific by Popper because the onlyrationale for the modifications which were made to theoriginal theory was to ensure that it evaded falsification, and sosuch modifications weread hoc, rather than scientific. Thiscontention—though not at all implausible—has, to hostileeyes, a somewhat contrived air about it, and is unlikely to worry theconvinced Marxist. On the other hand, the shift in Popper’s own basicposition is taken by some critics as an indicator thatfalsificationism, for all its apparent merits, fares no better in thefinal analysis than verificationism.

Bibliography

Primary Literature: Works By Popper

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