Life
Born in Vienna (then Austria-Hungary) in 1902 to middle-class parents of Jewish origins, Karl Popper was educated at the University of Vienna. He took a Ph.D. in philosophy in 1928, and taught in secondary school from 1930 to 1936. In 1934 he published his first book, Logik der Forschung (The Logic of Scientific Discovery), in which he criticized psychologism, naturalism, inductionism, and logical positivism, and put forth his theory of potential falsifiability being the criterion for what should be considered science. In 1937, the rise of Nazism and the threat of the Anschluss led Popper to emigrate to New Zealand, where he became lecturer in philosophy at Canterbury University College New Zealand (at Christchurch).

In 1946, he moved to England to become reader in logic and scientific method at the London School of Economics, where he was appointed professor in 1949. He was knighted by Queen Elizabeth II in 1965, and was elected a Fellow of the Royal Society in 1976. He retired from academic life in 1969, though he remained intellectually active until his death in 1994. He was invested with the Insignia of a Companion of Honour in 1982.

Popper won many awards and honors in his field, including the Lippincott Award of the American Political Science Association, the Sonning Prize, and fellowships in the Royal Society, British Academy, London School of Economics, King's College London, and Darwin College Cambridge. Austria awarded him the Grand Decoration of Honour in Gold.

Popper's philosophy

Philosophy of Science
Popper coined the term critical rationalism to describe his philosophy. This designation is significant, and indicates his rejection of classical empiricism, and of the observationalist-inductivist account of science that had grown out of it. Popper argued strongly against the latter, holding that scientific theories are universal in nature, and can be tested only indirectly, by reference to their implications.

He also held that scientific theory, and human knowledge generally, is irreducibly conjectural or hypothetical, and is generated by the creative imagination in order to solve problems that have arisen in specific historico-cultural settings. Logically, no number of positive outcomes at the level of experimental testing can confirm a scientific theory, but a single genuine counter-instance is logically decisive: it shows the theory, from which the implication is derived, to be false.

Popper's account of the logical asymmetry between verification and falsification lies at the heart of his philosophy of science. It also inspired him to take falsifiability as his criterion of demarcation between what is and is not genuinely scientific: a theory should be considered scientific if and only if it is falsifiable. This led him to attack the claims of both psychoanalysis and contemporary Marxism to scientific status, on the basis that the theories enshrined by them are not falsifiable. His scientific work was influenced by his study of quantum mechanics and by Albert Einstein's approach to scientific theories. Popper's falsificationism resembles Charles Peirce's fallibilism. In Of Clocks and Clouds (1966), Popper said he wished he had known of Peirce's work earlier.

In All Life is Problem Solving (1999), Popper sought to explain the apparent progress of scientific knowledge—how it is that our understanding of the universe seems to improve over time. This problem arises from his position that the truth content of our theories, even the best of them, cannot be verified by scientific testing, but can only be falsified. If so, then how is it that the growth of science appears to result in a growth in knowledge? In Popper's view, the advance of scientific knowledge is an evolutionary process characterized by his formula:

In response to a given problem situation (PS1), a number of competing conjectures, or tentative theories (TT), are systematically subjected to the most rigorous attempts at falsification possible. This process, error elimination (EE), performs a similar function for science that natural selection performs for biological evolution. Theories that better survive the process of refutation are not more true, but rather, more "fit"—in other words, more applicable to the problem situation at hand (PS1). Consequently, just as a species' "biological fit" does not predict continued survival, neither does rigorous testing protect a scientific theory from refutation in the future.

Yet, as it appears that the engine of biological evolution has produced, over time, adaptive traits equipped to deal with more and more complex problems of survival, likewise, the evolution of theories through the scientific method may, in Popper's view, reflect a certain type of progress: toward more and more interesting problems (PS2). For Popper, it is in the interplay between the tentative theories (conjectures) and error elimination (refutation) that scientific knowledge advances toward greater and greater problems; in a process very much akin to the interplay between genetic variation and natural selection.

In his earlier work Conjectures and Refutations (1963), Popper abandoned his previous rejection of the idea that truth is not a particularly relevant goal for science to pursue. He adopted the semantic theory of truth formulated by the logician Alfred Tarski in 1948. According to this theory, the conditions for the truth of a sentence as well as the sentences themselves are part of a metalanguage. So, for example, the sentence "Snow is white" is true if and only if snow is white. Although many philosophers have interpreted, and continue to interpret, Tarski's theory as a deflationary theory, Popper explicitly refers to it as a theory in which "truth" must be replaced with "corresponds to the facts."

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:

1) "John called" is true.
2) "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. In words, the intuitive idea behind verisimilitude is that the assertions or hypotheses of scientific theories can be objectively measured with respect to the amount of truth and falsity that they imply. And, in this way, one theory can be evaluated as more or less true than another on a quantitative basis which, Popper emphasizes forcefully, has nothing to do with "subjective probabilities" or other merely "epistemic" considerations.

The simplest mathemical formulation that Popper gives of this concept can be found in the tenth chapter of Conjectures and Refutations.. Here he defines it as:

where Vs(a) is the verisimilitude (or truthlikeness) of a, Ctv(a) is a measure of the content of truth of a, and CTf(a) is a measure of the content of the falsity of a.

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:

1. World One , being the phenomenal world, or the world of direct experience;
2. World Two , being the world of mind, or mental states, ideas, and perceptions; and
3. World Three, being the body of human knowledge expressed in its manifold forms, or the products of the second world made manifest in the materials of the first world (i.e.–books, papers, paintings, symphonies, and all the products of the human mind).

World Three, he argued, was the product of individual human beings in exactly the same sense that an animal path is the product of individual animals, and that, as such, has an existence and evolution independent of any individual knowing subjects. The influence of World Three, in his view, on the individual human mind (World Two) is at least as strong as the influence of World One.

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, than to the world of direct experience. As such, the growth of human knowledge could be said to be a function of the independent evolution of World Three. Compare with Memetics. Contemporary philosophers have not embraced Popper's Three World conjecture, due mostly, it seems, to its resemblance to Cartesian dualism.

Politics
In The Open Society and Its Enemies and The Poverty of Historicism, Popper developed a critique of historicism and a defence of the 'Open Society' and liberal democracy. Historicism is the theory that history develops inexorably and necessarily according to knowable general laws towards a determinate end. Popper argued that this view is the principal theoretical presupposition underpinning most forms of authoritarianism and totalitarianism.

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.

Problem of Induction
Among his contributions to philosophy is his answer to David Hume's Problem of Induction. Hume stated that just because the sun has risen every day for as long as anyone can remember, doesn't mean that there is any rational reason to believe it will come up tomorrow. There is no rational way to prove that a pattern will continue on just because it has before.

Popper's reply is characteristic, and ties in with his criterion of falsifiability. He states that while there is no way to prove that the sun will come up, we can theorize that it will. If it does not come up, then it will be disproven, but since right now it seems to be consistent with our theory, the theory is not disproven. Thus, Popper's demarcation between science and non-science serves as an answer to an old logical problem as well. This approach was criticised by Peter Singer for masking the role induction plays in empirical discovery.

Influence
By all accounts, Popper has played a vital role in establishing the philosophy of science as a vigorous, autonomous discipline within analytic philosophy, through his own prolific and influential works, and also through his influence on his own contemporaries and students -- chief among them, Imre Lakatos and Paul Feyerabend, two of the foremost philosophers of science in the next generation of analytic philosophy. (Lakatos's work drastically modifies Popper's position, and Feyerabend's repudiates it entirely, but the work of both is deeply influenced by Popper and engaged with many of the problems that Popper set.)

Popper is also widely credited as one of the leading voices in the revolt against the logical positivism of the Vienna Circle in the middle of the 20th century; his revival of Humean skepticism toward induction and his introduction of the principle of falsifiability as a non-inductive solution to the puzzle of scientific knowledge, played a major role in the criticism of the view of science that was central to the positivist programme. Popper entitled a chapter of his memoirs "Who Killed Logical Positivism?" — a question which he answered by saying "I fear that I must admit responsibility". Popper may have overstated his own influence somewhat vis-a-vis fellow critics of positivism, such as W.V.O. Quine and Popper's philosophical nemesis Ludwig Wittgenstein.

While there is some dispute as to the matter of influence, Popper had a long standing and close friendship with economist Friedrich Hayek, also from Vienna. Each found support and similarities in each other's work, citing each other often, though not without qualification. In a letter to Hayek in 1944, Popper stated, "I think I have learnt more from you than from any other living thinker, except perhaps Alfred Tarski." (See Hacohen, 2000). Popper dedicated his Conjectures and Refutations to Hayek. For his part, Hayek dedicated a collection of papers, Studies in Philosophy, Politics, and Economics, to Popper, and in 1982 said, "...ever since his Logik der Forschung first came out in 1934, I have been a complete adherent to his general theory of methodology." (See Weimer and Palermo, 1982).

Popper's influence, both through his work in philosophy of science and through his political philosophy, has also extended beyond the academy. Among Popper's students and advocates is the multibillionaire investor George Soros, who says his investment strategies are modelled on Popper's understanding of the advancement of knowledge through falsification. Among Soros's philanthropic foundations is the Open Society Institute, a think-tank named in honor of Popper's The Open Society and Its Enemies, which Soros founded to advance the Popperian defense of the open society against authoritarianism and totalitarianism.

Critics
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 naïve falsificationism in Chapters 3 & 4 of The Logic of Scientific Discovery. For Popper, theories are accepted or rejected via a sort of 'natural selection'. Theories that say more about the way things appear are to be preferred over those that do not; the more generally applicable a theory is, the greater its value. Thus Newton’s laws, with their wide general application, are to be preferred over the much more specific “the solar system has seven planets”.

Thomas Kuhn’s influential book The Structure of Scientific Revolutions argued that scientists work in a series of paradigms, and found little evidence of scientists actually following a falsificationist methodology. Popper's student Imre Lakatos attempted to reconcile Kuhn’s work with falsificationism by arguing that science progresses by the falsification of research programs rather than the more specific universal statements of naïve falsificationism. Another of Popper’s students Paul Feyerabend ultimately rejected any prescriptive methodology, and argued that the only universal method characterizing scientific progress was anything goes.

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 that evidence contradicting a hypothesis is a sign of flaws in the hypothesis rather than of flaws in the procedures used to test it.

Other critics seek to vindicate the claims of historicism or holism to intellectual respectability, or psychoanalysis or Marxism to scientific status. It has been argued that Popper's student Imre Lakatos transformed Popper's philosophy using historicist and updated Hegelian historiographic ideas.

Charles Taylor accuses Popper of exploiting his fame as an epistemologist to diminish the importance of philosophers of the continental tradition. According to Taylor, Popper's criticims are completely baseless, but they are received with an attention and respect that Popper's "intrinsic worth hardly merits".

In 2004 philosopher and psychologist Michel ter Hark (Groningen, The Netherlands) published a book, called Popper, Otto Selz and the rise of evolutionary epistemology, in which he claims that Popper got a part of his ideas from his tutor, the German-Jewish psychologist Otto Selz. Selz himself never published his ideas, partly because of the rise of Nazism which ordered him to quit his work in 1933, and the prohibition of referencing to Selz' work.