Falsifiability or refutability is the logical possibility that an assertion can be shown false by an observation or a physical experiment. That something is "falsifiable" does not mean it is false; rather, that if it is false, then this can be shown by observation or experiment. The term "testability" is related but more specific; it means that an assertion can be falsified through experimentation alone.
For example, "all men are mortal" is unfalsifiable, since no finite amount of observation could ever demonstrate its falsehood: that one or more men can live forever. "All men are immortal," by contrast, is falsifiable, by the presentation of just one dead man. Not all statements that are falsifiable in principle are falsifiable in practice. For example, "it will be raining here in one million years" is theoretically falsifiable, but not practically so.
Falsifiability is an important concept in science and the philosophy of science. The concept was made popular by Karl Popper, who, in his philosophical analysis of the scientific method, concluded that a hypothesis, proposition, or theory is "scientific" only if it is falsifiable. Popper however stressed that unfalsifiable statements are still important in science, and are often implied by falsifiable theories. For example, while "all men are mortal" is unfalsifiable, it is a logical consequence of the falsifiable theory that "every man dies before he reaches the age of 150 years". Similarly, the ancient metaphysical and unfalsifiable idea of the existence of atoms has led to corresponding falsifiable modern theories.ion leads to a false conclusion, then the assumption must also be false.