Background to Popper's ideas Popper's rejection of
Marxism during his teenage years left a profound mark on his thought. He had at one point joined a socialist association, and for a few months in 1919 considered himself a
communist. Although it is known that Popper worked as an office boy at the communist headquarters, whether or not he ever became a member of the Communist Party is unclear. During this time he became familiar with the Marxist view of economics,
class conflict, and history. Although he quickly became disillusioned with the views expounded by Marxists, his flirtation with the ideology led him to distance himself from those who believed that spilling blood for the sake of a revolution was necessary. He then took the view that when it came to sacrificing human lives, one was to think and act with extreme prudence. The failure of democratic parties to prevent fascism from taking over Austrian politics in the 1920s and 1930s traumatised Popper. He suffered from the direct consequences of this failure since events after the
Anschluss (the annexation of
Austria by the
German Reich in 1938) forced him into permanent exile. His most important works in the field of
social science—
The Poverty of Historicism (1944) and
The Open Society and Its Enemies (1945)—were inspired by his reflection on the events of his time and represented, in a sense, a reaction to the prevalent
totalitarian ideologies that then dominated Central European politics. His books defended democratic liberalism as a social and
political philosophy. They also represented extensive critiques of the philosophical presuppositions underpinning all forms of
totalitarianism. Popper believed that there was a contrast between the theories of
Sigmund Freud and
Alfred Adler, which he considered non-scientific, and
Albert Einstein's
theory of relativity which set off the revolution in
physics in the early 20th century. Popper thought that Einstein's theory, as a theory properly grounded in scientific thought and method, was highly "risky", in the sense that it was possible to deduce consequences from it which differed considerably from those of the then-dominant
Newtonian physics; one such prediction, that gravity could deflect light, was verified by
Eddington's experiments in 1919. In contrast he thought that nothing could, even in principle, falsify psychoanalytic theories. He thus came to the conclusion that they had more in common with primitive myths than with genuine science. This led Popper to conclude that what was regarded as the remarkable strengths of psychoanalytical theories were actually their weaknesses. Psychoanalytical theories were crafted in a way that made them able to refute any criticism and to give an explanation for every possible form of human behaviour. The nature of such theories made it impossible for any criticism or experiment—even in principle—to show them to be false. When Popper tackled the
problem of demarcation in the philosophy of science, this conclusion led him to posit that the strength of a scientific theory lies in its being susceptible to falsification. He considered that if a theory cannot, in principle, be falsified by criticism, it is not a scientific theory.
Philosophy of science Falsifiability and the problem of demarcation Popper coined the term "critical rationalism" to describe his philosophy. Popper rejected the empiricist view (following from Kant) that
basic statements are infallible; rather, according to Popper, they are descriptions in relation to a theoretical framework. Concerning the method of science, the term "critical rationalism" indicates his rejection of classical
empiricism, and the classical
observationalist-inductivist account of science that had grown out of it. Popper argued strongly against the latter, holding that
scientific theories are abstract in nature and can be tested only indirectly, by reference to their implications. He also held that scientific theory, and human knowledge generally, is irreducibly conjectural or hypothetical, and is generated by the creative imagination to solve problems that have arisen in specific historico-cultural settings. Logically, no number of positive outcomes at the level of experimental testing can confirm a scientific theory, but a single counterexample is logically decisive; it shows the theory, from which the implication is derived, to be false. Popper's account of the logical asymmetry between
verification and
falsifiability lies at the heart of his philosophy of science. It also inspired him to take falsifiability as his criterion of
demarcation between
metaphysics and science: a law should be considered scientific if, and only if, it makes predictions, irrespective of their validity or of our capacity to show them false. This led him to attack the claims of both
psychoanalysis and contemporary
Marxism to scientific status, on the basis that they do not make predictions, but instead are compatible with any possible observations. In
All Life is Problem Solving, Popper sought to explain the apparent progress of scientific knowledge—that is, how it is that our understanding of the universe seems to improve over time. This problem arises from his position that the truth content of our theories, even the best of them, cannot be verified by scientific testing, but can only be falsified. With only falsifications being possible logically, how can we explain the
growth of knowledge? In Popper's view, the advance of scientific knowledge is an
evolutionary process characterised by his formula: \mathrm{PS}_1 \rightarrow \mathrm{TT}_1 \rightarrow \mathrm{EE}_1 \rightarrow \mathrm{PS}_2. \, In response to a given problem situation (\mathrm{PS}_1), a number of competing conjectures, or tentative theories (\mathrm{TT}), are systematically subjected to the most rigorous attempts at falsification possible. This process, error elimination (\mathrm{EE}), performs a similar function for science that
natural selection performs for
biological evolution. Theories that better survive the process of refutation are not more true, but rather, more "fit"—in other words, more applicable to the problem situation at hand (\mathrm{PS}_1). Consequently, just as a species' biological fitness does not ensure continued survival, neither does rigorous testing protect a scientific theory from refutation in the future. Yet, as it appears that the engine of biological evolution has, over many generations, produced adaptive traits equipped to deal with more and more complex problems of survival, likewise, the evolution of theories through the scientific method may, in Popper's view, reflect a certain type of progress: toward more and more interesting problems (\mathrm{PS}_2). For Popper, it is in the interplay between the tentative theories (conjectures) and error elimination (refutation) that scientific knowledge advances toward greater and greater problems; in a process very much akin to the interplay between genetic variation and natural selection. Popper also wrote extensively against the famous
Copenhagen interpretation of
quantum mechanics. He strongly disagreed with
Niels Bohr's
instrumentalism and supported
Albert Einstein's
scientific realist approach to scientific theories about the universe. He found that Bohr's interpretation introduced subjectivity into physics, claiming later in his life that: This Popper's falsifiability resembles
Charles Peirce's nineteenth-century
fallibilism. In
Of Clocks and Clouds (1966), Popper remarked that he wished he had known of Peirce's work earlier.
Falsification and the problem of induction Among his contributions to philosophy is his claim to have solved the philosophical
problem of induction. He states that while there is no way to prove that the sun will rise, it is possible to formulate the theory that every day the sun will rise; if it does not rise on some particular day, the theory will be falsified and will have to be replaced by a different one. Until such a falsifying instance occurs, there is, for Popper, no need to reject the tentative acceptance of the theory, even though it is never justified in an inductive sense. Nor is it rational according to Popper to make instead the more complex assumption that the sun will rise until a given day, but will stop doing so the day after, or similar statements with additional conditions. Such a theory would be true with higher probability because it cannot be attacked so easily: • to falsify the first one, it is sufficient to find that the sun has stopped rising; • to falsify the second one, one additionally needs the assumption that the given day has not yet been reached. Popper held that it is the least likely, or most easily falsifiable, or simplest theory (attributes which he identified as all the same thing) that explains known facts that one should rationally prefer. His opposition to positivism, which held that it is the theory most likely to be true that one should prefer, here becomes very apparent. It is impossible, Popper argues, to ensure a theory to be true; it is more important that it is falsifiable. Popper agreed with
David Hume that there is often a psychological belief that the sun will rise tomorrow and that there is no logical justification for the supposition that it will, simply because it always has in the past. Popper writes,
Rationality Popper held that rationality is not restricted to the realm of empirical or scientific theories, but that it is merely a special case of the general method of criticism, the method of finding and eliminating contradictions in knowledge without ad-hoc measures. According to this view, rational discussion about metaphysical ideas, about moral values and even about purposes is possible. Popper's student
W.W. Bartley III tried to radicalise this idea and made the controversial claim that not only can criticism go beyond empirical knowledge but that everything can be rationally criticised. To Popper, who was an anti-
justificationist, traditional philosophy is misled by the false
principle of sufficient reason. He thinks that no assumption can ever be or needs ever to be justified, so a lack of justification is not a justification for doubt. Instead, theories should be tested and scrutinised. It is not the goal to bless theories with claims of certainty or justification, but to eliminate errors in them. He writes,
Philosophy of arithmetic Popper's principle of falsifiability runs into
prima facie difficulties when the epistemological status of mathematics is considered. It is difficult to conceive how simple statements of arithmetic, such as "2 + 2 = 4", could ever be shown to be false. If they are not open to falsification they can not be scientific. If they are not scientific, it needs to be explained how they can be informative about real world objects and events. Popper's solution was an original contribution in the
philosophy of mathematics. His idea was that a number statement such as "2 apples + 2 apples = 4 apples" can be taken in two senses. In its
pure mathematics sense, "2 + 2 = 4" is
logically true and cannot be refuted. Contrastingly, in its
applied mathematics sense of it describing the physical behaviour of apples, it can be falsified. This can be done by placing two apples in a container, then proceeding to place another two apples in the same container. If there are five, three, or a number of apples that is not four in said container, the theory that "2 apples + 2 apples = 4 apples" is shown to be false. On the contrary, if there are four apples in the container, the theory of numbers is shown to be applicable to reality.
Political philosophy In
The Open Society and Its Enemies and
The Poverty of Historicism, Popper developed a critique of
historicism and a defence of the "Open Society". Popper considered historicism to be the theory that history develops inexorably and necessarily according to knowable general laws towards a determinate end. He argued that this view is the principal theoretical presupposition underpinning most forms of
authoritarianism and
totalitarianism. He argued that historicism is founded upon mistaken assumptions regarding the nature of scientific law and prediction. Since the growth of human knowledge is a causal factor in the evolution of human history, and since "no society can predict, scientifically, its own future states of knowledge", it follows, he argued, that there can be no predictive science of human history. For Popper, metaphysical and historical indeterminism go hand in hand. In his early years Popper was impressed by Marxism, whether of Communists or socialists. An event that happened in 1919 had a profound effect on him: During a riot, caused by the Communists, the police shot several unarmed people, including some of Popper's friends, when they tried to free party comrades from prison. The riot had, in fact, been part of a plan by which leaders of the Communist party with connections to
Béla Kun tried to take power by a coup; Popper did not know about this at that time. However, he knew that the riot instigators were swayed by the Marxist doctrine that class struggle would produce vastly more dead men than the inevitable revolution brought about as quickly as possible, and so had no scruples to put the life of the rioters at risk to achieve their selfish goal of becoming the future leaders of the working class. This was the start of his later criticism of historicism. Popper began to reject Marxist historicism, which he associated with questionable means, and later
socialism, which he associated with placing equality before freedom (to the possible disadvantage of equality). Popper said that he was a socialist for "several years", and maintained an interest in egalitarianism, although as early as 1945 in
The Open Society he was describing Communist parties as giving a weak opposition to fascism due to shared historicism with fascism. Over time, primarily in defence of liberal democracy, Popper began to see
Soviet-type communism as a form of totalitarianism,
The paradox of tolerance Although Popper was an advocate of toleration, he also warned against unlimited tolerance. In
The Open Society and Its Enemies, he argued:
The "conspiracy theory of society" Popper criticized what he termed the "conspiracy theory of society", the view that powerful people or groups, godlike in their efficacy, are responsible for purposely bringing about all the ills of society. This view cannot be right, Popper argued, because "nothing ever comes off exactly as intended." According to philosopher David Coady, "Popper has often been cited by critics of conspiracy theories, and his views on the topic continue to constitute an orthodoxy in some circles." However, philosopher Charles Pigden has pointed out that Popper's argument only applies to a very extreme kind of conspiracy theory, not to conspiracy theories generally.
Metaphysics Truth As early as 1934, Popper wrote of the search for truth as "one of the strongest motives for scientific discovery." Still, he describes in
Objective Knowledge (1972) early concerns about the much-criticised notion of
truth as correspondence. Then came the
semantic theory of truth formulated by the logician
Alfred Tarski and published in 1933. Popper wrote of learning in 1935 of the consequences of Tarski's theory, to his intense joy. The theory met critical objections to
truth as correspondence and thereby rehabilitated it. The theory also seemed, in Popper's eyes, to support
metaphysical realism and the regulative idea of a search for truth. According to this theory, the conditions for the truth of a sentence as well as the sentences themselves are part of a
metalanguage. So, for example, the sentence "Snow is white" is true if and only if snow is white. Although many philosophers have interpreted, and continue to interpret, Tarski's theory as a
deflationary theory, Popper refers to it as a theory in which "is true" is replaced with "
corresponds to the facts". He bases this interpretation on the fact that examples such as the one described above refer to two things: assertions and the facts to which they refer. He identifies Tarski's formulation of the truth conditions of sentences as the introduction of a "metalinguistic predicate" and distinguishes the following cases: • "John called" is true. • "It is true that John called." The first case belongs to the metalanguage whereas the second is more likely to belong to the object language. Hence, "it is true that" possesses the logical status of a redundancy. "Is true", on the other hand, is a predicate necessary for making general observations such as "John was telling the truth about Phillip." Upon this basis, along with that of the logical content of assertions (where logical content is inversely proportional to probability), Popper went on to develop his important notion of
verisimilitude or "truthlikeness". The intuitive idea behind verisimilitude is that the assertions or hypotheses of scientific theories can be objectively measured with respect to the amount of truth and falsity that they imply. And, in this way, one theory can be evaluated as more or less true than another on a quantitative basis which, Popper emphasises forcefully, has nothing to do with "subjective probabilities" or other merely "epistemic" considerations. The simplest mathematical formulation that Popper gives of this concept can be found in the tenth chapter of
Conjectures and Refutations. Here he defines it as: : \mathit{Vs}(a)=\mathit{CT}_v(a)-\mathit{CT}_f(a) \, where \mathit{Vs}(a) is the verisimilitude of
a, \mathit{CT}_v(a) is a measure of the content of the truth of
a, and \mathit{CT}_f(a) is a measure of the content of the falsity of
a. Popper's original attempt to define not just verisimilitude, but an actual measure of it, turned out to be inadequate. However, it inspired a wealth of new attempts.
Popper's three worlds Knowledge, for Popper, was objective, both in the sense that it is objectively true (or truthlike), and also in the sense that knowledge has an ontological status (i.e., knowledge as object) independent of the knowing subject (
Objective Knowledge: An Evolutionary Approach, 1972). He proposed
three worlds:
World One, being the physical world, or physical states;
World Two, being the world of mind, or individuals' private mental states, ideas and perceptions; and
World Three, being the
public body of human knowledge expressed in its manifold forms (e.g., "scientific theories, ethical principles, characters in novels, philosophy, art, poetry, in short our entire cultural heritage"), or the products of World Two made manifest in the materials of World One (e.g., books, papers, paintings, symphonies, cathedrals,
particle accelerators). World Three, Popper argued, was the product of individual human beings in exactly the same sense that an animal path in the jungle is the creation of many individual animals, but not planned or intended by any of them. World Three thus has an existence and an evolution independent of any individually known subjects. The influence of World Three on the individual human mind (World Two) is in Popper's view at least as strong as the influence of World One. In other words, the knowledge held by a given individual mind owes at least as much to the total, accumulated wealth of human knowledge made manifest as to the world of direct experience. As such, the growth of human knowledge could be said to be a function of the independent evolution of World Three. Many contemporary philosophers, such as
Daniel Dennett, have not embraced Popper's Three World conjecture, mainly owing to what they see as its resemblance to
mind–body dualism. Popper himself, however, saw it rather as a form of pluralistic realism, with greater explanatory power than either physicalism or dualism.
Origin and evolution of life The
creation–evolution controversy raised the issue of whether creationistic ideas may be legitimately called science. In the debate, both sides and even courts in their decisions have invoked Popper's criterion of falsifiability (see
Daubert standard). In this context, passages written by Popper are frequently quoted in which he speaks about such issues himself. For example, he famously stated "
Darwinism is not a testable scientific theory, but a
metaphysical research program—a possible framework for testable scientific theories." He continued: He noted that
theism, presented as explaining adaptation, "was worse than an open admission of failure, for it created the impression that an ultimate explanation had been reached". Popper later said: He explained that the difficulty of testing had led some people to describe natural selection as a
tautology, and that he too had in the past described the theory as "almost tautological", and had tried to explain how the theory could be untestable (as is a tautology) and yet of great scientific interest: Popper summarised his new view as follows: These frequently quoted passages are only a small part of what Popper wrote on evolution, however, and may give the wrong impression that he mainly discussed questions of its falsifiability. Popper never invented this criterion to give justifiable use of words like science. In fact, Popper stressed that "the last thing I wish to do, however, is to advocate another dogma" and that "what is to be called a 'science' and who is to be called a 'scientist' must always remain a matter of convention or decision." He quotes Menger's dictum that "Definitions are dogmas; only the conclusions drawn from them can afford us any new insight" and notes that different definitions of science can be rationally debated and compared: Popper had his own sophisticated views on evolution that go much beyond what the frequently-quoted passages say. In effect, Popper agreed with some points of both creationists and naturalists, but disagreed with both on crucial aspects. Popper understood the universe as a creative entity that invents new things, including life, but without the necessity of something like a god, especially not one who is pulling strings from behind the curtain. He said that evolution of the genotype must, as the creationists say, work in a goal-directed way but disagreed with their view that it must necessarily be the hand of god that imposes these goals onto the stage of life. Instead, he formulated the spearhead model of evolution, a version of genetic pluralism. According to this, living organisms have goals, and act according to these goals, each guided by a central control. In its most sophisticated form, this is the brain of humans, but controls also exist in much less sophisticated ways for species of lower complexity, such as the
amoeba. This control organ plays a special role in evolution—it is the "spearhead of evolution". The goals bring the purpose into the world. Mutations in the genes that determine the structure of the control may then cause drastic changes in behaviour, preferences and goals, without having an impact on the organism's
phenotype. Popper postulates that such purely behavioural changes are less likely to be lethal for the organism compared to drastic changes of the phenotype. Popper contrasts his views with the notion of the "hopeful monster" that has large phenotype mutations and calls it the "hopeful behavioural monster". After behaviour has changed radically, small but quick changes of the phenotype follow to make the organism fitter to its changed goals. This way it looks as if the phenotype were changing guided by some invisible hand, while it is merely natural selection working in combination with the new behaviour. For example, according to this hypothesis, the eating habits of the giraffe must have changed before its elongated neck evolved. Popper contrasted this view as "evolution from within" or "active Darwinism" (the organism actively trying to discover new ways of life and being on a quest for conquering new ecological niches), with the naturalistic "evolution from without" (which has the picture of a hostile environment only trying to kill the mostly passive organism, or perhaps segregate some of its groups). Popper was a key figure encouraging patent lawyer
Günter Wächtershäuser to publish his
iron–sulfur world hypothesis on
abiogenesis and his criticism of
"soup" theory. On the creation-evolution controversy, Popper initially wrote that he considered it with a footnote to the effect that he In his later work, however, when he had developed his own "spearhead model" and "active Darwinism" theories, Popper revised this view and found some validity in the controversy:
Free will Popper and
John Eccles speculated on the problem of
free will for many years, generally agreeing on an
interactionist dualist theory of mind. However, although Popper was a body-mind dualist, he did not think that the mind is
a substance separate from the body: he thought that mental or psychological properties or aspects of people
are distinct from physical ones. When he gave the second
Arthur Holly Compton Memorial Lecture in 1965, Popper revisited the idea of
quantum indeterminacy as a source of human freedom. Eccles had suggested that "critically poised neurons" might be influenced by the mind to assist in a decision. Popper criticised Compton's idea of amplified quantum events affecting the decision. He wrote: Popper called not for something between chance and necessity but for a combination of randomness and control to explain freedom, though not yet explicitly in two stages with random chance before the controlled decision, saying, "freedom is not just chance but, rather, the result of a subtle interplay between something almost random or haphazard, and something like a restrictive or selective control." Then in his 1977 book with John Eccles,
The Self and its Brain, Popper finally formulates the two-stage model in a temporal sequence. And he compares free will to Darwinian evolution and natural selection:
Religion and God Popper was not a religious man in the formal sense of the word. He neither maintained any link with his Jewish ancestry nor was he an observant Lutheran. However, he did consider that every person including himself, was religious in the sense of believing in something more important and beyond us through which we can transcend ourselves. Popper called this something a
Third World. In an interview that Popper gave in 1969 with the condition that it should be kept secret until after his death, he summarised his position on God as follows: "I don't know whether God exists or not (...) Some forms of atheism are arrogant and ignorant and should be rejected, but
agnosticism—to admit that we don't know and to search—is all right. (...) When I look at what I call the gift of life, I feel a gratitude which is in tune with some religious ideas of God. However, the moment I even speak of it, I am embarrassed that I may do something wrong to God in talking about God." Aged fifteen, after reading
Spinoza (at the suggestion of his father), Popper recounts that "it gave me a lifetime's dislike of theorizing about God". In 1936, applying to the
Academic Assistance Council to leave Austria, he described himself as "Protestant, namely evangelical but of Jewish origin." Responding to the question of whether he wanted religious communities approached on his behalf, opposite the Jewish Orthodox section he wrote "NO", underlining it twice. Popper objected to organised religion, saying "it tends to use the name of God in vain", noting the danger of fanaticism because of religious conflicts: "The whole thing goes back to myths which, though they may have a kernel of truth, are untrue. Why then should the Jewish myth be true and the Indian and Egyptian myths not be true?" Ethical issues always constituted an important part of the background to Popper's philosophy. He was a Patron and Honorary Advisor of the Humanist Society of New Zealand and affiliated with the
British Humanist Association in the UK (later known as Humanists UK) as a member of the organisation Advisory Council; his concept of the open society provided the "ideological backbone" of humanist campaigning across the 1960s and 1970s. In this capacity, he also contributed an essay to the 1968 collection
The Humanist Outlook. In later life he discussed ethics rarely, and religious questions hardly at all. While he maintained his affiliation with the British Humanist Association, he sympathized with the religious views of others where they were honestly held, and commented that he could not endorse "various humanist and secular offensives". In a letter unrelated to the interview, he stressed his tolerant attitude: "Although I am not for religion, I do think that we should show respect for anybody who believes honestly." While he did not believe that religious convictions could be rationally justified, he recognised that religions were a source of comfort for their adherents: "because something isn't science, however, does not mean it is meaningless".
Criticism Most criticisms of Popper's philosophy are
of the falsification, or error elimination, element in his account of problem solving. Popper presents falsifiability as both an ideal and as an important principle in a practical method of effective human problem solving; as such, the current conclusions of science are stronger than pseudo-sciences or
non-sciences, insofar as they have survived this particularly vigorous selection method. He does not argue that any such conclusions are therefore true, or that this describes the actual methods of any particular scientist. Rather, it is recommended as an essential principle of methodology that, if enacted by a system or community, will lead to slow but steady progress of a sort (relative to how well the system or community enacts the method). It has been suggested that Popper's ideas are often mistaken for a hard logical account of truth because of the historical co-incidence of their appearing at the same time as
logical positivism, the followers of which mistook his aims for their own. The
Quine–Duhem thesis argues that it is impossible to test a single hypothesis on its own, since each one comes as part of an environment of theories. Thus we can only say that the whole package of relevant theories has been collectively
falsified, but cannot conclusively say which element of the package must be replaced. An example of this is given by the discovery of the planet
Neptune: when the motion of
Uranus was found not to match the predictions of
Newton's laws, the theory "There are seven planets in the solar system" was rejected, and not Newton's laws themselves. Popper discussed this critique of
naive falsificationism in Chapters 3 and 4 of
The Logic of Scientific Discovery. The philosopher
Thomas Kuhn writes in
The Structure of Scientific Revolutions (1962) that he places an emphasis on anomalous experiences similar to that which Popper places on falsification. However, he adds that anomalous experiences cannot be identified with falsification, and questions whether theories could be falsified in the manner suggested by Popper. Kuhn argues in
The Essential Tension (1977) that while Popper was correct that
psychoanalysis cannot be considered a science, there are better reasons for drawing that conclusion than those Popper provided. Popper's student
Imre Lakatos attempted to reconcile Kuhn's work with
falsificationism by arguing that science progresses by the falsification of
research programs rather than the more specific
universal statements of naive falsificationism. Popper claimed to have recognised already in the 1934 version of his
Logic of Discovery a fact later stressed by Kuhn, "that scientists necessarily develop their ideas within a definite theoretical framework", and to that extent to have anticipated Kuhn's central point about "normal science". However, Popper criticised what he saw as Kuhn's relativism, this criticism being at the heart of the
Kuhn-Popper debate. Also, in his collection
Conjectures and Refutations: The Growth of Scientific Knowledge (Harper & Row, 1963), Popper writes, Another objection is that it is not always possible to demonstrate falsehood definitively, especially if one is using
statistical criteria to evaluate a
null hypothesis. More generally it is not always clear, if evidence contradicts a hypothesis, that this is a sign of flaws in the hypothesis rather than of flaws in the evidence. However, this is a misunderstanding of what Popper's philosophy of science sets out to do. Rather than offering a set of instructions that merely need to be followed diligently to achieve science, Popper makes it clear in
The Logic of Scientific Discovery that his belief is that the resolution of conflicts between hypotheses and observations can only be a matter of the collective judgment of scientists, in each individual case. In
Science Versus Crime, Houck writes that Popper's falsificationism can be questioned logically: it is not clear how Popper would deal with a statement like "for every metal, there is a temperature at which it will melt". The hypothesis cannot be falsified by any possible observation, for there will always be a higher temperature than tested at which the metal may in fact melt, yet it seems to be a valid scientific hypothesis. These examples were pointed out by
Carl Gustav Hempel. Hempel came to acknowledge that logical positivism's verificationism was untenable, but argued that falsificationism was equally untenable on logical grounds alone. The simplest response to this is that, because Popper describes how theories attain, maintain and lose scientific status, individual consequences of currently accepted scientific theories are scientific in the sense of being part of tentative scientific knowledge, and both of Hempel's examples fall under this category. For instance,
atomic theory implies that all metals melt at some temperature. An early adversary of Popper's critical rationalism,
Karl-Otto Apel attempted a comprehensive refutation of Popper's philosophy. In
Transformation der Philosophie (1973), Apel charged Popper with being guilty of, amongst other things, a pragmatic contradiction. The philosopher
Adolf Grünbaum argues in
The Foundations of Psychoanalysis (1984) that Popper's view that psychoanalytic theories, even in principle, cannot be falsified is incorrect. The philosopher
Roger Scruton argues in
Sexual Desire (1986) that Popper was mistaken to claim that Freudian theory implies no testable observation and therefore does not have genuine predictive power. Scruton maintains that Freudian theory has both "theoretical terms" and "empirical content". He points to the example of Freud's theory of
repression, which in his view has "strong empirical content" and implies testable consequences. Nevertheless, Scruton also concluded that Freudian theory is not genuinely scientific. The philosopher
Charles Taylor accuses Popper of exploiting his worldwide fame as an
epistemologist to diminish the importance of philosophers of the 20th-century
continental tradition. According to Taylor, Popper's criticisms are completely baseless, but they are received with an attention and respect that Popper's "intrinsic worth hardly merits". The philosopher
John Gray argues that Popper's account of scientific method would have prevented the theories of
Charles Darwin and
Albert Einstein from being accepted. However, Gray's criticism with regards to Einstein is at odds with the fact that Popper frequently used Einstein's theory of general relativity as a case study of how the principle of falsifiability works in practice. Popper, the historian of ideas and his scholarship, is criticised in some academic quarters for his treatment of Plato and Hegel. The philosopher and psychologist Michel ter Hark wrote in
Popper, Otto Selz and the Rise of Evolutionary Epistemology (2004) that Popper took some of his ideas from his tutor, the German psychologist
Otto Selz. Selz never published his ideas, partly because of the rise of
Nazism, which forced him to quit his work in 1933 and prohibited any reference to his ideas. == Influence ==