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Falsification is a criterion which was explicitly stated by Sir Karl Popper as a way of distinguishing science from pseudoscience. Popper's argument for a convention as a criterion of demarcation largely stemmed from his rejection of inductivist thought and the positivist tradition therein. Positivist usage of "meaningful statements" or similar attempts to show that the problem of demarcation or induction were merely pseudo-problems (e.g., Wittgenstein) were largely refuted by Popper in his search for a new criterion of demarcation. He had also been made uncomfortable by Marxism and Freudianism and how their advocates always seemed to have an answer for everything, even clear falsifications; thus he decided that a proper scientific theory ought to be "falsifiable", meaning that it ought to be possible for some observation to be contrary to it. In Conjectures and Refutations, Popper lays it out the principle and its correlaries more formally as:
- (1) It is easy to obtain confirmations, or verifications, for nearly every theory-if we look for confirmations
- (2) Confirmations should count only if they are the result of risky predictions; that is to say, if, unenlightened by the theory in question, we should have expected the event which was incompatible with the theory-an event which would have refuted the theory
- (3) Every 'good' scientific theory is a prohibition: it forbids certain things to happen. The more a theory forbids, the better it is.
- (4) A theory which is not refutable by any conceivable event is nonscientific. Irrefutability is not a virtue of a theory (as people often think) but a vice.
- (5) 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.
- (6) 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. (I now speak in such cases of 'corroborating evidence'.)
- (7) Some genuinely testable theories, when found to be false, are still upheld by their admirers-for example by introducing ad hoc some auxiliary assumption, or by re-interpreting the theory ad hoc in such a way that it escapes refutation. Such a procedure is always possible, but it rescues the theory from refutation only at the price of destroying, or at least lowering, its scientific status.
If I say, "it is daytime", that is falsifiable, since it is falsified by the fact that I'm writing this in local nighttime.
If I say "it is nighttime", that is falsifiable, because though it is true, it would be falsified if I was writing this in local daytime.
If I say "it is nighttime and it is daytime", that is falsifiable, since it is always falsified.
If I say "it is nighttime or it is daytime", that is unfalsifiable, since I will write in either local daytime or local nighttime, leaving out hairsplitting about twilight.
A more classic example is what the Oracle of Delphi told King Croesus of Lydia (southwest Anatolia, ~550 BCE) when he wanted advice on attacking the Persians. The oracle told him that if he crossed the river Halys, a great empire would fall. Croesus led his army across, but the empire that fell was his. Advancing 700 years, Lucian of Samosata related how Roman Emperor Marcus Aurelius wanted some advice on fighting the Marcomanni and Quadi, some Germanic tribes then encamped on the opposite side of the Danube River from him. A certain Alexander of Abonutichus, a pagan Benny Hinn, advised him that if he threw two lions into the Danube, a great victory would result. Marcus Aurelius faithfully complied, but it was the Marcomanni and Quadi who had the great victory. When MA asked A of A about that defeat, A of A told him that he had only predicted a victory, and not who would have it.
Both the Oracle of Delphi and Alexander of Abonutichus could clearly claim vindication by making ambiguous prophecies, but such prophecies are short of falsifiability, and are therefore worthless.
An interesting application of falsifiability is in software (ClustalW, etc.) that aligns different gene and protein sequences. In many cases, for the alignment to work, it is necessary to insert gaps in the sequences. But enough gaps will enable any sequence to be "perfectly" aligned against any other sequence, a violation of falsifiability. So such software imposes penalties for creating and lengthening gaps; this introduces falsifiability by giving such abundant gaps a high penalty. This penalty is adjustable, because excessively-high penalties would exclude reasonable alignments, and because excessively-low penalties would make the introduced falsifiability too weak to be meaningful.
The concept of falsification is closely related to the usefulness of a theory. In order to be useful, a theory has to tell us something we do not already know and which has an impact on our lives. Those are exactly the conditions for falsifiability. And a theory which is not falsifiable has no applications.
Theory of Evolution
In a direct application of the theory of evolution it tells us what fossils we are most likely to find. But it has also big implications for medicine. How germs become immune can only be understood from the perspective of the theory of evolution. This application of the theory of evolution is so important that even creationists, at least those with some scientific background, acknowledge this.
The predictions based on creationism all turned out to be utterly wrong. Based on creationism the discovery of, now no longer, missing links could not have been expected. Creationists also did and some still do assert a young age of Earth. This is in stark contrast to geology which is successfully used for discovering oil sources.