A proposition or theory is falsifiable if it is hypothetically possible to conduct an experiment that proves it false. A proposal or theory that can never be shown to be false with any hypothetical experiment is not falsifiable.
As an incredibly simple example, it is a generally accepted scientific principle that dropped objects fall downward. (Ignore various issues about imparting initial upward velocity, that is, throwing the object, or unusual frames of reference, like centrifuges.) But it is possible to visualize what it would look like if one dropped an object and it fell upward. Therefore, the theory that dropped objects fall downward is falsifiable.
To be considered scientific, a hypothesis must be "falsifiable", i.e., capable of being proven false. If no one, not even the supporters of the hypothesis, can think of a way the hypothesis might be proven false, then most scientists would agree that it is not part of science (see pseudoscience). However, the history of science is full of examples whereby supporters of various theories refused to consider the prospect that someone might prove them wrong.
For example, the suggestion that I would be very happy if I were ten years younger is not falsifiable, because it is impossible to test the hypothesis by making myself ten years younger and seeing how I feel.
Falsifiability is commonly used as a criterion for whether a particular proposition is "scientific." Philosopher Karl Popper popularized this criterion, arguing that science progresses through the process of "Observation, speculation, falsification;" That is, we observe the universe, we develop a speculative hypothesis about the universe, and we test our hypothesis. If our hypothesis is proven false, we discard it. If our hypothesis is not proven false, we can consider it provisionally true. According to Popper, there is no such thing as scientific certainty; on the contrary, all our beliefs are properly held provisionally, and always subject to the possibility that they will someday be disproven by experiment.
Popper, and those who agree with him, categorize many types of ideas as unfalsifiable, particularly in psychology and religion. For example, the proposition "Joe beats his children because his father beat him" is unfalsifiable, because we cannot perform a test in which Joe was not beaten by his father, and see if Joe still beats his children. Because the proposition is not testable, according to falsificationism, it is no better than any other hypothesis, and therefore not useful, and not scientific. However, the broader hypothesis "Parents who were beaten as children are more likely to beat their own children" can be falsified - e.g. through surveys - and if that broader hypothesis was supported, it would also lend weight to the position that it was true in Joe's particular case also. Thus, while propositions about specific historical persons or events are not directly falsifiable, they can be indirectly falsifiable insofar as they depend on broader claims that are falsifiable.
A good example of an unfalsifiable concept is the nature of matter inside black holes. By definition, the interior of a black hole cannot be seen and thus assertions about the interior are unfalsifiable.
In his book, Demon-Haunted World Carl Sagan, a noted astronomer, gives an example of a statement with this sort of premise saying: "There is a dragon in my garage." In order for one to prove or disprove the statement they are allowed to ask questions.
- Q. Can I see the dragon?
- A. No, the dragon is invisible.
- Q. Can I touch the dragon?
- A. No, my dragon is incorporeal.
- Q. Can I hear the dragon?
- A. No, my dragon is silent.
and so on...
In this way, every question can be answered and therefore the existence of the dragon is unfalsifiable. However, in the absence of physical or other evidence of its existence, whether the dragon actually exists or not is a matter of faith.
Science is weak about these prehistoric things in a way that has hardly been noticed. The science whose modern marvels we all admire succeeds by incessantly adding to its data. In all practical inventions, in most natural discoveries, it can always increase evidence by experiment. But it cannot experiment in making men; or even in watching to see what the first men make. An inventor can advance step by step in the construction of an aeroplane, even if he is only experimenting with sticks and scraps of metal in his own back-yard. But he cannot watch the Missing Link evolving in his own back-yard. If he has made a mistake in his calculations, the aeroplane will correct it by crashing to the ground. But if he has made a mistake about the arboreal habitat of his ancestor, he cannot see his arboreal ancestor falling off the tree. He cannot keep a cave-man like a cat in the back-yard and watch him to see whether he does really practice cannibalism or carry off his mate on the principles of marriage by capture. He cannot keep a tribe of primitive men like a pack of hounds and notice how far they are influenced by the herd instinct. If he sees a particular bird behave in a particular way, he can get other birds and see if they behave in that way; but if he finds a skull, or the scrap of a skull, in the hollow of a hill, he cannot multiply it into a vision of the valley of dry bones. In dealing with a past that has almost entirely perished, he can only go by evidence and not by experiment. And there is hardly enough evidence to be even evidential. Thus while most science moves in a sort of curve, being constantly corrected by new evidence, this science flies off into space in a straight line uncorrected by anything. But the habit of forming conclusions, as they can really be formed in more fruitful fields, is so fixed in the scientific mind that it cannot resist talking like this. It talks about the idea suggested by one scrap of bone as if it were something like the aeroplane which is constructed at last out of whole scrapheaps of scraps of metal. The trouble with the professor of the prehistoric is that he cannot scrap his scrap. The marvellous and triumphant aeroplane is made out of a hundred mistakes. The student of origins can only make one mistake and stick to it.
Falsifiability of religious beliefs
Many atheists claim that religious beliefs are inherently unfalsifiable. However, on the contrary, very many religious beliefs are capable of being falsified. Many religions predict a particular sort of afterlife; if, after death, one encountered a rather different sort of afterlife, that would falsify that religion. For example, Islam would claim that in the afterlife, one will be told that Muhammad was a true prophet and the Quran a true scripture. If, upon dying, one was not told these things - and, on the contrary, was told that Muhammad was a false prophet - that experience would falsify Islam. Thus, once we are dead, such claims will be easily falsifiable. However, the living cannot ask the dead because God prohibits the dead from coming back to testify.
Also, many religions make assertions about matters within our ability to observe. If those assertions are falsifiable through observation, then so are the religions making the assertions. Examples of falsifiable religious assertions are assertions that the world would end within the lifetime of a certain religious figure who is now dead and assertions about the characteristics of various animals.
Furthermore, if a religion makes mutually exclusive claims, falsifying that religion is a simple matter of applying a reductio ad absurdum.