Falsifiability

 ( Analytic Philosophy ) 

Falsifiability (or refutability) is a deductive standard of evaluation of scientific theories and hypotheses, introduced by the philosopher of science Karl Popper in his book The Logic of Scientific Discovery (1934). A theory or hypothesis is falsifiable if it can be logically contradicted by an empirical test.

Popper emphasized the asymmetry created by the relation of a universal law with basic observation statements and contrasted falsifiability to the intuitively similar concept of verifiability that was then current in logical positivism. He argued that the only way to verify a claim such as "All swans are white" would be if one could theoretically observe all swans, which is not possible. On the other hand, the falsifiability requirement for an anomalous instance, such as the observation of a single black swan, is theoretically reasonable and sufficient to logically falsify the claim.

Popper proposed falsifiability as the cornerstone solution to both the problem of induction and the problem of demarcation. He insisted that, as a logical criterion, his falsifiability is distinct from the related concept "capacity to be proven wrong" discussed in Lakatos's falsificationism. Even being a logical criterion, its purpose is to make the theory predictive power and Testability, and thus 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 method test of the hypothesis requires one or more background assumptions.

Popper's response is that falsifiability does not have the Duhem problem because it is a logical criterion. Experimental research has the Duhem problem and other problems, such as the problem of induction, but, according to Popper, statistical tests, which are only possible when a theory is falsifiable, can still be useful within a critical discussion.

As a key notion in the separation of science from non-science and pseudoscience, falsifiability has featured prominently in many scientific controversies and applications, even being used as legal precedent. However, falsifiability is not a sufficient condition for demarcating science as theories have to actually be tested in order to eliminate theories that are wrong. In scientific practice, this can cause theories to change from being falsified back to unfalsified, such as when the once-falsified Geocentric model world view was restored as a viable reference frame within special relativity. There is ambiguity surrounding the status of theories that cannot currently be tested.

Popper's idea to solve this problem is 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 $L$ we logically deduce $Q$, but what is observed is $\backslash neg\; Q$, we infer that the law $L$ is false. For example, given the statement $L\; =$ "all swans are white", we can deduce $Q\; =$ "the specific swan here is white", but if what is observed is $\backslash neg\; Q\; =$ "the specific swan here is not white" (say black), then "all swans are white" is false. More accurately, the statement $Q$ that can be deduced is broken into an initial condition and a prediction as in $C\; \backslash Rightarrow\; P$ in which $C\; =$ "the thing here is a swan" and $P\; =$ "the thing here is a white swan". If what is observed is C being true while P is false (formally, $C\; \backslash wedge\; \backslash neg\; P$), we can infer that the law is false.

For Popper, induction is actually never needed in science. Instead, in Popper's view, laws are conjectured in a non-logical manner on the basis of expectations and predispositions. This has led David Miller, a student and collaborator of Popper, to write "the mission is to classify truths, not to certify them". In contrast, the logical empiricism movement, which included such philosophers 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 either in favor of its truth or its falsity. There was no consensus among these philosophers 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 basic precept of critical reflection about science.

Popper said that a demarcation criterion was possible, but we have to use the logical possibility of falsifications, which is falsifiability. He cited his encounter with psychoanalysis in the 1910s. 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 also. For Popper, this was a failure, because it meant that it could not make any prediction. From a logical standpoint, if one finds an observation that does not contradict a law, it does not mean that the law is true. A verification has no value in itself. But, if the law makes risky predictions and these are corroborated, Popper says, there is a reason to prefer this law over another law that makes less risky predictions or no predictions at all. In the definition of falsifiability, contradictions with observations are not used to support eventual falsifications, but for logical "falsifications" that show that the law makes risky predictions, which is completely different.

On the basic philosophical side of this issue, Popper said that some philosophers of the Vienna Circle had mixed two different 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. In opposition to this view, Popper said that there are meaningful theories that are not scientific, and that, accordingly, a criterion of meaningfulness does not coincide with a criterion of demarcation.

When Johnson-Laird says that no justification is needed, he does not refer to a general inductive method of justification that, to avoid a circular reasoning, would not itself require any justification. On the contrary, in agreement with Hume, he means that there is no general method of justification for induction and that's ok, because the induction steps do 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 the background knowledge. Johnson-Laird wrote: "Philosophers have worried about which properties of objects warrant inductive inferences. The answer rests on knowledge: we don't infer that all the passengers on a plane are male because the first ten off the plane are men. We know that this observation doesn't rule out the possibility of a woman passenger." The reasoning pattern that was not applied here is enumerative induction.

Popper was interested in the overall learning process in science, to quasi-induction, which he also called the "path of science". However, Popper did not show much interest in these reasoning patterns, which he globally referred to as psychologism. He did not deny the possibility of some kind of psychological explanation for the learning process, especially when psychology is seen as an extension of biology, but he felt that these biological explanations were not within the scope of epistemology. Popper proposed an evolutionary mechanism to explain the success of science, which is much in line with Johnson-Laird's view that "induction is just something that animals, including human beings, do to make life possible", but Popper did not consider it a part of his epistemology. 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 the pragmatic problem of induction. This problem is not how to justify a theory or what is the global mechanism for the success of science but only what methodology do we use to pick one theory among theories that are already conjectured. His methodological answer to the latter question is that we pick the theory that is the most tested with the available technology: "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.: "The fundamental difference between my approach and the approach for which I long ago introduced the label 'inductivist' is that I lay stress on negative arguments, such as negative instances or counter-examples, refutations, and attempted refutations—in short, criticism". The purpose of his methodology is to prevent "the policy of immunizing our theories against refutation". It also supports some "dogmatic attitude" in defending theories against criticism, because this allows the process to be more complete. This negative view of science was much criticized and not only by Johnson-Laird.

In practice, some steps based on observations can be justified under assumptions, which can be very natural. For example, Bayesian inductive logic is justified by theorems that make explicit assumptions. These theorems are obtained with deductive logic, not inductive logic. They are sometimes presented as steps of induction, because they refer to laws of probability, even though they do not go beyond deductive logic. This is yet a third notion of induction, which overlaps with deductive logic in the following sense that it is supported by it. These deductive steps are not really inductive, but the overall process that includes the creation of assumptions is inductive in the usual sense. In a fallibilist perspective, a perspective that is widely accepted by philosophers, including Popper, every logical step of learning only creates an assumption or reinstates one that was doubted—that is all that science logically does.

discussed statements such as "All men are mortal." This is not falsifiable, because it does not matter how old a man is, maybe he will die next year. Maxwell said that this statement is nevertheless useful, because it is often corroborated. He coined the term "corroboration without demarcation". Popper's view is that it is indeed useful, because Popper considers that metaphysical statements can be useful, but also because it is indirectly corroborated by the corroboration of the falsifiable law "All men die before the age of 150." For Popper, if no such falsifiable law exists, then the metaphysical law is less useful, because it is not indirectly corroborated. This kind of non-falsifiable statements in science was noticed by Carnap as early as 1937.