the observation of these black swans contradicts the law "all swans are white", but even if there were no black swans, the law would still be falsifiable, because identifying a swan and observing the color black would remain possible.
likewise, the law "all ravens are black" is falsifiable, although it is still held to be true. observing e.g. a red raven could show it to be false. in contrast, the sentence "some time, a red raven will appear" is not falsifiable since it is too unspecific to admit a contradicting observation.
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. (march 2020)
informally, a statement is falsifiable if some observation might show it to be false. for example, "all swans are white" is falsifiable because "here is a black swan" shows it to be false. formally, it is the same, except that the observations used to prove falsifiability are only logical constructions distinct from those that are truly possible.[a][b]
falsifiability differs from verifiability, which was held as fundamental by many philosophers such as those of the vienna circle. in order to verify the claim "all swans are white" one would have to observe every swan, which is not possible, whereas the single observation "here is a black swan" is sufficient to falsify it.
it was introduced by the philosopher of science karl popper in his book the logic of scientific discovery (1934), as an answer to both the problem of induction and the demarcation problem. he saw falsifiability as the cornerstone of critical rationalism, his theory of science.
as a key notion in the separation of science from non-science, it has featured prominently in many scientific controversies and applications, even being used as legal precedent.