Karl Popper on Falsifiability

It is often said that for a theory to be scientific it must be falsifiable.

This thinking comes from the work of philosopher Karl Popper.

Popper noticed that there were some theories that seemed to be able to explain everything within their scope. No matter what conceivable situation you imagined, the theory could explain it. True believers would consider this universal application as confirmation of the theory. Popper did not agree. He recognized that the theory might be true, but it can only be confirmed if it can pass a test where failure of the test would disprove the theory. This means a theory can only be confirmed if it can be falsified. If a theory can't be falsified, it might be true but it can't be confirmed.

Science involves testing hypotheses through empirical means, either through experiment or observation. For a theory to be scientific it must be confirmable by empirical means. According to Popper, for a theory to be confirmed it must pass a test in which failure would falsify the theory. Therefore, a scientific theory must be falsifiable, otherwise it cannot be confirmed by empirical means.

This view distinguishes theories that are simply consistent with the evidence from theories that are supported by evidence. It is about whether a theory can be confirmed and how to confirm it. It is not about whether a theory is true or false - an unfalsifiable theory might be true. Unfalsifiability simply means a theory is not supported by empirical evidence even if there is a vast amount of evidence consistent with the theory.

Popper described how he developed this philosophy in the article:

Science, Pseudo-Science, and Falsifiability by Karl Popper, 1962

In the article Popper, explained how vacuous theories are always confirmed:

I may illustrate this by two very different examples of human behaviour: that of a man who pushes a child into the water with the intention of drowning him; and that of a man who sacrifices his life in an attempt to save the child. Each of these two cases can be explained with equal ease in Freudian and Adlerian terms. According to Freud the first man suffered from repression (say, of some component of his Oedipus complex), while the second man had achieved sublimation. According to Adler the first man suffered from feelings of inferiority (producing perhaps the need to prove to himself that he dared to commit some crime), and so did the second man (whose need was to prove to himself that he dared to rescue the child). I could not think of any human behaviour which could not be interpreted in terms of either theory. It was precisely this fact -- that they always fitted, that they were always confirmed -- which in the eyes of their admirers constituted the strongest argument in favour of these theories. It began to dawn on me that this apparent strength was in fact their weakness.

He contrasted this with Einstein's theory of relativity which could be tested. If the theory didn't pass the test, it would be proven false.

With Einstein's theory the situation was strikingly different. Take one typical instance -- Einstein's prediction, just then confirmed by the findings of Eddington's expedition. Einstein's gravitational theory had led to the result that light must be attracted by heavy bodies (such as the sun), precisely as material bodies were attracted. As a consequence it could be calculated that light from a distant fixed star whose apparent position was close to the sun would reach the earth from such a direction that the star would seem to be slightly shifted away from the sun; or, in other words, that stars close to the sun would look as if they had moved a little away from the sun, and from one another. This is a thing which cannot normally be observed since such stars are rendered invisible in daytime by the sun's overwhelming brightness; but during an eclipse it is possible to take pictures of them. If the same constellation is photographed at night one can measure the distances on the two photographs, and check the predicted effect.

Popper explained that a theory is only confirmed by a passing test that if failed would falsify the theory.

Every genuine test of a theory is an attempt to falsify it, or to refute it. Testability is falsifiability; but there are degrees of testability; some theories are more testable, more exposed to refutation than others; they take, as it were, greater risks.

Confirming evidence should not count except when it is the result of a genuine test of the theory; and this means that it can be presented as a serious but unsuccessful attempt to falsify the theory.

He compared the psycho-analytic theories to relativity.

The two psycho-analytic theories were in a different class. They were simply non-testable, irrefutable. There was no conceivable human behaviour which could contradict them. This does not mean that Freud and Adler were not seeing certain things correctly; I personally do not doubt that much of what they say is of considerable importance, and may well play its part one day in a psychological science which is testable. But it does mean that those "clinical observations" which analysts naïvely believe confirm their theory cannot do this any more than the daily confirmations which astrologers find in their practice.

He concludes:

Thus the problem which I tried to solve by proposing the criterion of falsifiability was neither a problem of meaningfulness or significance, nor a problem of truth or acceptability. It was the problem of drawing a line (as well as this can be done) between the statements, or systems of statements, of the empirical sciences, and all other statements -- whether they are of a religious or of a metaphysical character, or simply pseudo-scientific. Years later -- it must have been in 1928 or 1929 -- I called this first problem of mine the "problem of demarcation." The criterion of falsifiability is a solution to this problem of demarcation, for it says that statements or systems of statements, in order to be ranked as scientific, must be capable of conflicting with possible, or conceivable, observations.

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