Falsifiability is an important concept in the philosophy of science. For an assertion to be falsifiable, in principle it must be possible to make an observation or do a physical experiment that would show the assertion to be false. For example, the assertion "All crows are black" could be falsified by observing one red crow. The school of thought that emphasises the importance of falsifiability as a philosophical principle is known as Falsificationism.

Table of contents
1 NaÔve Falsification
2 The Criterion of Demarcation
3 Criticism
4 Some Examples
5 See also

NaÔve Falsification

Falsification was first developed by Karl Popper in the 1930s. Popper noticed that two types of statements are of particular value to scientists. The first are statements of observations, such as 'this is a white swan'. Logicians call these statements singular existential statements, since they assert the existence of some particular thing. They can be parsed in the form: there is an x which is a swan and is white.

The second type of statement of interest to scientists categorizes all instances of something, for example 'all swans are white'. Logicians call these statements universal. They are usually parsed in the form for all x, if x is a swan then x is white.

Scientific laws are commonly supposed to be of this form. Perhaps the most difficult question in the methodology of science is: how does one move from observations to laws? How can one validly infer a universal statement from any number of existential statements?

Inductivist methodology supposed that one can somehow move from a series of singular existential statements to a universal statement. That is, that one can move from Ďthis is a white swan', ďthat is a white swanĒ, and so on, to a universal statement such as 'all swans are white'. This method is clearly logically invalid, since it is always possible that there may be a non-white swan that has somehow avoided observation. Yet some philosophers of science claim that science is based on such an inductive method.

Popper held that science could not be grounded on such an invalid inference. He proposed falsification as a solution to the problem of induction. Popper noticed that although a singular existential statement such as 'there is a white swan' cannot be used to affirm a universal statement, it can be used to show that one is false: the singular existential statement 'there is a black swan' serves to show that the universal statement 'all swans are white' is false, by modus tollens. 'There is a black swan' implies 'there is a non-white swan' which in turn implies 'there is something which is a swan and which is not white'.

Although the logic of naÔve falsification is valid, it is rather limited. Popper drew attention to these limitations in The Logic of Scientific Discovery, in response to anticipated criticism from Duhem and Carnap. W. V. Quine is also well-known for his observation in his influential essay, "Two Dogmas of Empiricism" (which is reprinted in From a Logical Point of View), that nearly any statement can be made to fit with the data, so long as one makes the requisite "compensatory adjustments." In order to falsify a universal, one must find a true falsifying singular statement. But Popper pointed out that it is always possible to change the universal statement or the existential statement so that falsification does not occur. On hearing that a black swan has been observed in Australia, one might introduce ad hoc hypothesis, 'all swans are white except those found in Australia'; or one might adopt a skeptical attitude towards the observer, 'Australian ornithologists are incompetent'. As Popper put it, a decision is required on the part of the scientist to accept or reject the statements that go to make up a theory or that might falsify it. At some point, the weight of the ad hoc hypotheses and disregarded falsifying observations will become so great that it becomes unreasonable to support the theory any longer, and a decision will be made to reject it.

In place of naÔve falsification, Popper envisioned science as evolving by the successive rejection of falsified theories, rather than falsified statements. Falsified theories are replaced by theories of greater explanatory power. Aristotelian mechanics explained observations of objects in everyday situations, but was falsified by Galileoís experiments, and replaced by Newtonian mechanics. Newtonian mechanics extended the reach of the theory to the movement of the planets and the mechanics of gases, but in its turn was falsified by the Michelson-Morley experiment and replaced by special relativity. At each stage, a new theory was accepted that had greater explanatory power, and as a result provided greater opportunity for its own falsification.

NaÔve falsificationism is an unsuccessful attempt to proscribe a rationally unavoidable method for science. Falsificationism proper on the other hand is a prescription of a way in which scientists ought to behave as a matter of choice. Both can be seen as attempts to show that science has a special status because of the method that it employs.

The Criterion of Demarcation

Popper proposed falsification as a way of determining if a theory is scientific or not. If a theory is falsifiable, then it is scientific. If it is not, then it is not science. Popper uses this criterion of demarcation to draw a sharp line between scientific and unscientific theories. Some have taken this principle to an extreme to cast doubt on the scientific validity of many disciplines (such as macroevolution and Cosmology).

In the philosophy of science, verificationism (also known as the verifiability theory of meaning) held that a statement must be in principle empirically verifiable in order to be both meaningful and scientific. This was an essential feature of the logical empiricism of the so-called Vienna Circle that featured such philosophers as Moritz Schlick, Rudolf Carnap, Otto Neutrath and Hans Reichenbach. After Popper, verifiability came to be replaced by falsifiability as the criterion of demarcation. In other words, in order to be scientific a statement had to be in principle falsifiable. Popper noticed that the philosophers of the Vienna Circle had mixed two different problems, and had accordingly given a single solution to both of them: verificationism. In opposition to this view, Popper emphasized that a theory might well be meaningful without being scientific, and that, accordingly, a criterion of meaningfulness may not necessarily coincide with a criterion of demarcation. His own falsificationism, thus, is not only an alternative to verificationism; it also an acknowledgment of the conceptual distinction that previous theories had ignored.

Falsifiability is a property of statements and theories, and is itself neutral. As a demarcation criterion, it seeks to take this property and make it a base for affirming the superiority of falsifiable theories over non-falsifiable ones, in effect setting up a political position that might be called falsificationism. However, much that would be considered meaningful and useful is not falsifiable. Certainly non-falsifiable statements have a role in scientific theories themselves. The Popperian criterion fails as a way to distinguish meaningful statements from meaningless ones; and it provides a definition of science that excludes much that is of value.

It is nevertheless very useful to know if a statement or theory is falsifiable, if for no other reason than it provides us with an understanding the ways in which one might asses the theory. One might at the least be saved from attempting to falsify a non-falsifiable theory, or come to see an unfalsifiable theory as unsupportable.


Thomas Kuhnís influential book The Structure of Scientific Revolutions argued that scientists work within a paradigm that determines the way in which they view the world. Scientists will go to great length to defend their paradigm against falsification, by the addition of ad hoc hypothesis. It is only through great pain and angst that science changes paradigms.

Some falsificationists saw Kuhnís work as a vindication, since it showed that science progressed by rejecting inadequate theories. More commonly, it has been seen as showing that sociological factors, rather than adherence to a strict, logically obligatory method, play the determining role in deciding which scientific theory is accepted. This was seen as a profound threat to those who seek to show that science has a special authority in virtue of the methods that it employs.

Imre Lakatos attempted to explain Kuhnís work in falsificationist terms by arguing that science progresses by the falsification of research programs rather than the more specific universal statements of naÔve falsification. In Lakatos' approach, a scientist works within a research program that corresponds roughly with Kuhn's 'paradigm'. Whereas Popper rejected the use of ad hoc hypothesis as unscientific, Lakatos accepted their place in the development of new theories.

Paul Feyerabend examined the history of science with perhaps a more critical eye, and ultimately rejected any prescriptive methodology at all. He goes beyond Lakatosí argument for ad hoc hypothesis, to say that science would not have progressed without making use of any and all available methods to support new theorys. He rejects any reliance on a scientific method, along with any special authority for science that might derive from such a method. He replaces it with the aphorism anything goes. For Feyerabend, any special status that science might have derives from the social and physical value of the results of science rather than its method.

Although falsifiability does provide a way to replace invalid inductive thinking with deductive, falsifiable reasoning, it appears that doing so is neither necessary for, nor conducive to, scientific progress.

Some Examples

Claims about verifiability and falsifiability have been used to criticize various controversial views. Examination of these examples shows the usefulness of falsifiability in showing us where to look when criticising a theory.

Non-falsifiable theories can usually be reduced to a simple uncircumscribed existential statement, such as there exists a green swan. It is entirely possible to verify that the theory is true, simply by producing the green swan. But since this statement does not specify when or where the green swan exists, it is simply not possible to show that the swan does not exist, and so it is impossible to falsify the statement. That such theories are unfalsifiable says nothing about either their validity or truth. But it does assist us in determining how such statements might be evaluated. If evidence cannot be presented to support a case, and yet the case cannot be shown to be indeed false, how much credence should it be given?


Mathematical and logical statements are typically regarded as unfalsifiable, since they are tautologies, not existential or universal statements. For example, "all bachelors are male" and "all green things are green" are necessarily true (or given) without any knowledge of the world.

Proving mathematical theorems involves reducing them to tautologies, which can be mechanically proven as true given the axioms of the system or reducing the negative to a contradiction. Theorems are unfalsifiable, since this process, coupled with the notion of consistency, eliminates the possibility of counterexamples - a process that the philosophy of mathematics studies in depth as a separate matter. How a mathematical formula might apply to the physical world, however, is a physical question, and thus testable, within certain limits.


Many philosophers have held that claims about morality (such as "murder is evil" and "John was wrong to steal that money") are not part of scientific inquiry; their function in language is not even to state facts, but simply to express certain moral sentiments. Hence they are not falsifiable.


On the view of some, theism is not falsifiable; since the existence of God is typically asserted without sufficient conditions to allow a falsifying observation. God being a transcendental being, beyond the realm of the observable, claims about its existence can neither be supported nor undermined by observation. It is quite consistent for a theist to agree that the existence of god is unfalsifiable, and that the proposition is not scientific, but still to claim that god exists. This is, of course, a matter of interest for anyone who places stock in natural theology--the argument from design and other a posteriori arguments for the existence of God. See non-cognitivism.

Conspiracy theories

There are other examples of theories, however, that are much less controversial as examples of unfalsifiable claims. Some so-called "conspiracy theories," at least as defended by some people, are essentially unfalsifiable because of their logical structure. Conspiracy theories usually take the form of uncircumscribed existential statements, alleging the existence of some action or object without specifying the place or time at which it can be observed. Failure to observe the phenomenon can then always be the result of looking in the wrong place or looking at the wrong time. Conspiracy theorists defend their position by claiming that lying and other forms of fabrication are, in fact, a common tool of governments and other powerful players.


Theories of history or politics which allegedly predict the future course of history have a logical form that renders them neither falsifiable nor verifiable. They claim that for every historically significant event, there exists an historical or economic law that determines the way in which events proceeded. Failure to identify the law does not mean that it does not exist, yet an event that satisfies the law does not prove the general case. Evaluation of such claims is at best difficult. On this basis, Popper himself argued that neither Marxism nor psychoanalysis were science, although both made such claims. Again, this does not mean, that any of these types of theories are necessarily invalid. Popper considered falsifiability a test of whether theories are scientific, not of whether theories are valid.


In philosophy, solipsism is often dismissed as unfalsifiable. Solipsism has it that the Universe exists entirely in one's own mind. This can straightforwardly be seen not to be falsifiable, because whatever evidence one might adduce that is contrary to solipsism can be, after all, dismissed as something that is "in one's mind." In other words, there is no evidence that one could possibly adduce that would be inconsistent with the proposition that everything that exists, exists in one's own mind. This view is somewhat similar to Cartesian skepticism, and indeed, Cartesian skepticism has been rejected as unfalsifiable as well by many philosophers.

Physical Laws

The laws of physics are an interesting case. Occasionally it is suggested that the most fundamental laws of physics, such as "force equals mass times acceleration" (F=ma), are not falsifiable because they are definitions of basic physical concepts (in the example, of "force"). More usually, they are treated as falsifiable laws, but it is a matter of considerable controversy in the philosophy of science what to regard as evidence for or against the most fundamental laws of physics. Isaac Newton's original laws of motion in their original form were falsified by experiments in the twentieth century, and replaced by more exact theories that hold under broader conditions (though Newton's theories are still close enough to be used practically without the modern updates). In the case of less fundamental laws, their falsifiability is much easier to understand. If, for example, a biologist hypothesizes that, as a matter of scientific law (though practising scientists will rarely actually state it as such), only one certain gland produces a certain hormone, and then someone discovers an individual that has the hormone but lacks the gland, the hypothesis is falsified.

The range of available testing apparatus is also sometimes an issue - when Galileo showed Catholic Church scholars the moons of Jupiter , there was only one telescope on hand, and telescopes were a new technology, so it was a genuine question whether the moons were real or an artifact of the telescope, or type of telescope. Diversity of apparatus is quite important to concepts of falsifiability, because presumably any observer with any apparatus should be able to prove a thesis false.

See also

Occam's razor, philosophy of science, philosophy of mathematics, cognitive bias, infrastructure bias