Falsifiability
Falsifiability is a standard of evaluation of scientific statements, including theories and hypotheses. A statement is falsifiable if it belongs to a language or logical structure capable of describing an empirical observation that contradicts it. In the case of a theory, it says that, given an initial condition, the theory must theoretically prohibit some observations, that is, it must make formal predictions. It was introduced by the philosopher of science Karl Popper in his book The Logic of Scientific Discovery. Popper emphasized that the contradiction is to be found in the logical structure alone, without having to worry about methodological considerations external to this structure. He proposed falsifiability as the cornerstone solution to both the problem of induction and the problem of demarcation.
Popper also emphasized the related asymmetry created by the relation of a universal law with basic observation statements and contrasted falsifiability with the intuitively similar concept of verifiability that was then current in the philosophical discipline of logical positivism. He argued that the only way to verify a claim such as "All swans are white" would be if one could empirically observe all swans, which is not possible. On the other hand, the observation of a single black swan is enough to refute this claim.
This asymmetry can only be seen rigorously when methodological falsification issues are put aside. Otherwise, a stated observation of one or even more black swans constitute at best a problematic refutation of the claim. Accordingly, to be rigorous, falsifiability is a logical criterion within an empirical language that is accepted by convention and allows these methodological considerations to be avoided. Only then the asymmetry and falsifiability are rigorous. Popper argued that it should not be conflated with falsificationism, which is a methodological approach where scientists actively try to find evidence to disprove theories. Falsifiability is distinct from [|Lakatos' falsificationism]. Its purpose is to make theory predictive, testable and useful in practice.
By contrast, the Duhem–Quine thesis says that definitive experimental falsifications are impossible and that no scientific hypothesis is by itself capable of making predictions, because an empirical test of the hypothesis requires background assumptions, which acceptations are methodological decisions in Lakatos' falsificationism.
Popper's response was that falsifiability is a logical criterion. Experimental research has the Duhem problem and other problems, such as the problem of induction, but, according to Popper, logical induction is a fallacy and statistical tests, which are possible only when a theory is falsifiable, are useful within a critical discussion.
Popper's distinction between logic and methodology has not allowed falsifiability to escape some criticisms aimed at methodology. For example, Popper's rejection of Marxism as unscientific because of its resistance to negative evidence is a methodological position, but the problems with this position are nevertheless presented as a limitation of falsifiability. Others, despite the unsuccessful proposals of Russell, the Vienna Circle, Lakatos, and others to establish a rigorous way of justifying scientific theories or research programs and thus demarcating them from non-science and pseudoscience, criticize falsifiability for not following a similar proposal and for supporting instead only a methodology based on critical discussion.
As a key notion in the separation of science from non-science and pseudoscience, falsifiability has featured prominently in many controversies and applications, used as legal precedent.
Induction and demarcation
One concern about the scientific method is how to move from observations to scientific laws. This is the problem of induction. Considering the hypothesis that all swans are white, given an observation of a white swan, there is no logical path from "here is a white swan" to "all swans are white"; doing so would involve a logical fallacy such as, for example, affirming the consequent.Popper's idea to solve this problem was that while it is impossible to verify that every swan is white, finding a single black swan shows that not every swan is white. Such falsification uses the valid inference modus tollens: if, from a law, can be logically deduced, but is observed, is false. Thus, given "all swans are white", "the specific swan here is white", but if what is observed is "the observed swan is not white", then "all swans are white" is false. More precisely, the deducible statement can be broken into an initial condition and a prediction as in in which "the thing here is a swan" and "the thing here is a white swan". If what is observed is C being true while P is false, the law is false by modus tollens.
Popper claimed that induction is not needed in science, that is, he rejected that we learn by the repetition of observations and considered that logical induction was a fallacy. Instead, laws are conjectured in a non-logical manner on the basis of expectations and predispositions and trials continue as long as there are problems. This led Popper's student and collaborator David Miller to write "the mission is to classify truths, not to certify them". In contrast, the logical empiricism movement, led by philosophers such as Moritz Schlick, Rudolf Carnap, Otto Neurath, and A. J. Ayer, wanted to formalize the idea that, for a law to be scientific, it must be possible to argue on the basis of observations in favor of its truth or falsity. No consensus emerged about how to achieve that, but the thought expressed by Mach's dictum that "where neither confirmation nor refutation is possible, science is not concerned" was accepted as a scientific precept.
Popper said that a demarcation criterion for the laws of science was possible, but that what matters is the logical possibility of falsification of these laws, which is falsifiability. He cited his encounter with psychoanalysis in the 1910s, especially with Alfred Adler. It did not matter what observation was presented, psychoanalysis could explain it. The reason it could explain everything is that it did not exclude anything. Popper claimed that this was a failure, because it meant that the criterion could not lead to a prediction. From a logical standpoint, observations that do not contradict a law does not mean that the law is true. A verification has no value in itself. But, if a hypothesis makes risky predictions and these are corroborated, Popper stated, that was a reason to prefer this hypothesis over others that makes less risky predictions or no predictions at all. In the [|definition of falsifiability], contradictions with observations are not used to support falsifications, but for logical "falsifications" that show that the law makes risky predictions.
Popper said that some philosophers of the Vienna Circle had conflated two problems, that of meaning and that of demarcation, and had proposed in verificationism a single solution to both: a statement that could not be verified was considered meaningless. Popper instead said that meaningful non-scientific theories exist, and that, accordingly, a criterion of meaningfulness does not coincide with a criterion of demarcation.
Hume's problem
The problem of induction is often called Hume's problem. David Hume studied how human beings obtain new knowledge that goes beyond known laws and observations, including how to discover new laws. He understood that deductive logic could not explain this learning process and argued in favour of a mental or psychological process of learning that would not require deductive logic. He argued that this learning process cannot be justified by any general rules, deductive or not. Popper accepted Hume's argument and therefore viewed progress in science as the result of quasi-induction, which is induction without inference rules and which he also called the "path of science".Philip N. Johnson-Laird agreed with Hume that no general method of justification for induction is possible but that induction does not require justification. Instead, these steps use patterns of induction, which are not expected to have a general justification: they may or may not be applicable depending on context.
Johnson-Laird's view was that "induction is just something that animals, including human beings, do to make life possible".
Popper accepted the possibility of a psychological explanation for the learning process, especially when psychology is seen as an extension of biology, but claimed that biological explanations were not within the scope of epistemology. In line with Johnson-Laird's view, Popper proposed an evolutionary mechanism to explain science's success, but he did not consider it part of his epistemology. He referred to this as psychologism. He wrote that his interest was mainly in the logic of science and that epistemology should be concerned with logical aspects only. Instead of asking why science succeeds, he considered induction pragmatically. He asked what methodology should be used to accept one among multiple hypotheses. He proposed that it be the one that was the most tested: "the one, which in the light of our critical discussion, appears to be the best so far". By his own account, because only a negative approach was supported by logic, Popper adopted a negative methodology to prevent the "policy of immunizing our theories against refutation". It also supported a "dogmatic attitude" in defending theories against criticism, because this would allow the process to be more complete. This view was much criticized.