Critical rationalism
Critical rationalism is Karl Popper's answer to what he considered the most important problems of epistemology and philosophy of science: the problems of the growth of knowledge, notably by induction, and the demarcation of science. He adopted a fallibilist approach to these problems, especially that of induction, without falling into skepticism. His approach was to put in perspective the distinctive role of deductive logic in the development of knowledge, especially in science, in the context of a less rigorous methodology based on critical thinking. The central technical concept in the application of critical rationalism to science is falsifiabiity. Popper first mentioned the term "critical rationalism" in The Open Society and Its Enemies, and also later in Conjectures and Refutations, Unended Quest, and The Myth of the Framework.
Fallibilism, not skepticism
Popper admitted that the truth of statements cannot be obtained using only logical definitions and deductions, as this leads to an infinite regress. For Popper, this does not prevent statements from being useful for solving problems, because they can be logically analyzed to draw logical consequences, possibly contradictions with observation statements linked to real tests. Popper wrote that the bulk of scientific activities use deductive logic to evaluate theories.Popper accepted Hume's argument and the consequences of Duhem's thesis and insisted that there is no logical method for accessing empirical truth, no inductive rule, not even to a small extent. However, he rejected skepticism, the idea that the search for truth is futile. He admitted that, although logic alone says nothing about empirical truth, statements can be related to reality through problem solving, scientific observations and experiments. Popper always insisted on this distinction between the logical aspect and methodological aspect of science. In Realism and the Aim of Science, Popper speaks of a "preferred" theory, not of a "true" or a "false" theory, when one theory is chosen over another given experimental results.
Tarski's semantic theory of truth
Popper was always aware that empirical truth eludes logic alone, and he was therefore reluctant to refer to the truth of scientific theories. This, he wrote, changed after reading Tarski's semantic theory of truth. He saw this theory as a way of talking about truth as a correspondence with facts. A key aspect of Tarski's theory, which Popper considered important, is the separation between the logical aspect of language as an object and its semantic interpretation. He saw this as a way of explaining the distinction between the logic of science and its methodology, or rather between logic and the metaphysical component to which methodology refers when it aims, for example, to test theories. The difference is that in Tarski's theory, "facts" are mathematical structures, not an external reality beyond the reach of logic and its language, and which we can only describe artificially in a meta-language as in the argument "Snow is white" is true because snow is white. This use of Tarski's theory is accepted by some and sharply criticized by others. Popper used it, for example, in Realism and the Aim of Science, to explain the difference between the metaphysical versions of the problem of induction and its logical versions. He wrote that the metaphysical versions of the problem refer to the "meta-theory of physics" and compared this to what Tarski calls the "semantics" in its theory of truth.The role of methodology
For Popper, the bulk of activities in science use deductive logic on statements, but this logical part of science must be integrated within an adequate methodology. The logical part is considered incapable of justifying empirical knowledge on its own. For example, Popper and the members of the Vienna Circle agreed that only statements can be used to justify statements, that is, the use of logic alone in science will not be linked to evidence. Logic uses accepted or provisionally accepted observation statements to determine whether a theory is logically refuted or not, but the "accepted" observation statement could be empirically false and that will not concern the logical part.Popper wrote that the bulk of scientific activity takes place in the logical part, using deductive logic to check the consistency of a theory, compare theories, check their empirical nature and, most importantly, test a theory, which is possible only when it is falsifiable. He emphasized that, even when theories are tested against observations, deductive logic is largely used. Despite this intensive use of logic, Popper accepted, as do most philosophers and scientists, that logic alone does not connect by itself with evidence. Popper explained this dilemma by stating the existence of a natural separation between the logical and the methodological parts of science.
Popper wrote that any criterion, including his famous falsifiability criterion, that applies solely on the logical structure could not alone define science. In The Logic of Scientific Discovery, he wrote "it is impossible to decide, by analysing its logical form , whether a system of statements is a conventional system of irrefutable implicit definitions, or whether it is a system which is empirical in my sense; that is, a refutable system." Popper insisted that falsifiability is a logical criterion, which must be understood in the context of a proper methodology. The methodology can hardly be made precise. It is a set of informal implicit conventions that guide all the decisions that surround the logical work, which experiments to conduct, which apparatus to build, which domain will be financially supported, etc., aspects that were raised by Lakatos in The Methodology of Scientific Research Programmes.
Popper's philosophy was criticized as if the logical part existed alone. For example, Putnam attributed to Popper "the fantasy of doing science using only deductive logic". Putnam further criticized Popper's description of the logical part of science by referring to methodological problems. For example, he wrote "I claim: in a great many important cases, scientific theories do not imply predictions at all." Because Popper does not believe in inductive logic, Wesley Salmon wrote that, for Popper, "there is no ampliative form of scientific argument, and consequently, science provides no information whatever about the future". Regarding the methodological part, Feyerabend wrote that there is no method in science. He considered and rejected methodological rules, but they were those of a naive falsificationist.
In contrast, Popper emphasized both parts of science and spoke of methodology as a means of correctly using falsifiability and the usual logical work in science to make it useful in a method of conjectures and refutations to be used in usual critical discussions. Falsififiability says hypotheses should be consistent and they should logically lead to predictions, which confrontation with observations should be considered in critical thinking.
Marxism and politic
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. Events after the Anschluss prompted him to refocus his writings on social and political philosophy. His most important works in the field of social science—The Poverty of Historicism and The Open Society and Its Enemies —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.Earlier in his life, the death of friends in a demonstration instigated by the communists when he was about seventeen strongly contributed to Popper's position regarding the search for contradictions or criticisms and the attitude of taking them into account. 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. He blamed Marxism, which thesis, Popper recalls, "is that although the revolution may claim some victims, capitalism is claiming more victims than the whole socialist revolution". He asked himself "whether such a calculation could ever be supported by 'science'." He then decided that criticism was important in science. This, Popper wrote, made him "a fallibilist", and impressed on him "the value of intellectual modesty". It made him "most conscious of the differences between dogmatic and critical thinking".
Psychoanalysis
Popper saw a contrast between the theories of Sigmund Freud and Alfred Adler, which he considered unscientific, and Albert Einstein's theory of relativity which sparked the revolution in physics in the early 20th century. Popper believed 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 that 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. When he tackled the problem of demarcation in the philosophy of science, he realized that "what made a theory, or a statement, scientific was its power to rule out, or exclude, the occurrence of some possible events—to prohibit, or forbid, the occurrence of these events." He thought that, in contrast, nothing could, even in principle, falsify psychoanalytic theories. This led him to posit that "only attempted refutations which did not succeed qua refutations should count as 'verifications'."A little later, Popper realized that theories can be "immunized" against falsification using auxiliary hypotheses. In Logik der Forschung, he introduced the notion of " content". He proposed that only modifications that increase the empirical content of a theory should be considered.
In a series of articles beginning in 1979, Adolf Grünbaum argued, with examples, that Freudian psychoanalytic theories are in fact falsifiable. He criticized Popper's analysis of Freud's psychoanalytic theories and, on this basis, questioned the applicability of the demarcation criterion in general.