Falsifiability

Karl Popper (1902–1994) made falsifiability the key to his philosophy of science. It became the most commonly invoked "criterion of demarcation" of science from nonscience.

According to the simple, hypothetico-deductive (H-D) model of scientific inquiry, a law claim, theory, or hypothesis H is falsifiable when a potentially checkable prediction O can be logically deduced from it, that is, when H → O. If O is observed to be true, then H passes this predictive test (although it may fail other tests). If O tests false, then H must also be false, since no true statement can logicaly imply a falsehood. For example, Isaac Newton's (1642–1727) theory of gravitation predicts a slow rotation of the orbit of the planet Mercury different from what astronomers observe. Thus Newton's theory is not only falsifiable (empirically vulnerable) but also actually falsified (shown to be false). Albert Einstein's (1879–1955) general theory of relativity is subject to the same test, so it, too, is falsifiable; but it passes the test. "All life in the universe employs the same genetic coding found on Earth" is falsifiable in principle but not in current practice, since to date (2004) we have identified no examples of extraterrestrial life. By contrast, "The universe is recreated at each instant by a divine being" yields no predictive tests at all, so it is not falsifiable even in principle.

Note that falsifiable does not mean "falsified" or "false" any more than breakable means "broken." On the simple model, even if, per impossibile, an empirical law could be known to be absolutely true in our universe, it would still be falsifiable in the sense that it would be empirically testable and would test false were the world relevantly different. A falsifiable claim rules out some potentially observable situations; and the more it excludes, the greater is its empirical content, that is, the more it claims about the structure of our universe.