thumb|200px|1700-luvulla löydetyt [[mustajoutsenet kumosivat käsityksen siitä, että kaikki joutsenet olisivat valkoisia.]]
'''Fallibilismi''' on tieteenfilosofinen näkökulma, jonka mukaan mitään tieteellisyyden kriteerinä.

Ensimmäisenä fallibilismin idean esitti 1930-luvulla tieteellisen tiedon mittapuuksi. Hän tuli tulokseen, jonka mukaan minkään teorian totuudenmukaisuutta ei loppujen lopuksi voida täysin todistaa: teorioita voidaan ainoastaan kumota, ja niihin voidaan uskoa "parhaana tämänhetkisenä tietona". Täten tulisi luottaa ainoastaan teorioihin, jotka ovat ainakin periaatteessa kumottavissa. Popper viehtyi ajatukseen kuultuaan paikallisessa yliopistossa Albert Einsteinin luennon, jossa Einstein esitteli keksimäänsä suhteellisuusteoriaa ja kertoi olevansa valmis hylkäämään sen, jos joku löytäisi ristiriitaisia todisteita.

Teoksessaan ''Arvauksia ja kumoamisia &ndash; tieteellisen tiedon kasvu'' (1963) Popper esitti fallibilismin teemaksi sen, että tiede oppii virheistään ja että oppiessamme tiedoistamme korjatessamme niitä tietomme kasvaa, vaikka emme koskaan voikaan olla aivan varmoja siitä että olemme lopullisesti oikeassa. Tietomme "korroboroituvat" kestäessään tutkimukset, joissa riskinä on koko teorian kumoutuminen. Näin tieteestä tulee totuudenkaltaisempaa. Popperin mukaan ei siis ole mitään syytä ryhtyä relativisteiksi tai skeptikoiksi, vaikka emme voikaan olla aivan varmoja siitä että olemme lopullisesti oikeassa. Olemme kuitenkin aina vain lähempänä lopputulosta, joten pitkällä tähtäyksellä lopputulos on aina vain lähempänä totuutta.

Jotta teoriaa voitaisiin pitää totena, sen pitäisi falsifioitavuusvaateen mukaan periaatteessa olla todistettavissa tutkimuksella vääräksi. Esimerkiksi teorian, jonka mukaan "kaikki korpit ovat mustia", voisi falsifioida löytämällä punaisen korpin. Sellainen teoria, jota ei voida osoittaa vääräksi, ei siis täytä fallibilismin mukaan tieteen kriteereitä. Esimerkiksi psykoanalyysia on käytetty esimerkkinä teoriasta, jota on vaikea kumota, koska jos psykoterapia ei tehoa, voidaan aina väittää, että terapiaa ei ole vielä annettu tarpeeksi. (Psykoanalyysissa ei kuitenkaan käytetä tuollaista perustelua.)

Toinen yksinkertainen esimerkki teoriasta, jota ei voida kumota, on väittää että vihreitä joutsenia on olemassa. Väittämä ei kuitenkaan kerro missä ja koska vihreitä joutsenia esiintyy, joten sitä on mahdoton osoittaa vääräksi ja näin ollen sitä ei voida pitää tieteelliset kriteerit täyttävänä.

Fallibilismin mukaan hyvä tieteentekijä ei siis ole niinkään kiinnostunut siitä havaintoaineistosta, joka on teorian kanssa sopusoinnussa, vaan nimenomaan siitä, jonka kanssa teorialla on ongelmia. Popperin mukaan on helppo saada havainnot sopimaan yhteen teorian kanssa jo sosiaalipsykologisista syistä. Fallibilismi siis korostaa kriittisyyden merkitystä tieteen kehityksessä.

Falsifikaatiokriteeri ei voi käsitellä olemassaoloväitteitä sellaisenaan, koska emme voi millään tavoin todistaa, että jokin asia ei olisi. Siksi olemassaoloa ei oleteta ennen kuin olemassaolemattomuus falsifioidaan.

'''Falsifiability''' (or '''refutability''' or '''testability''') 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. Falsifiability is an important concept in science and the philosophy of science.

Some philosophers and scientists, most notably Karl Popper, have asserted that a hypothesis, proposition or theory is scientific only if it is falsifiable. 

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. On the other hand, a statement like "there exist parallel universes which cannot interact with our universe" is not falsifiable even in principle; there is no way to test whether such a universe does or does not exist.

== Naïve falsification ==<!-- see Karl Popper -->
===Two types of statements: observational and categorical===
In work beginning in the 1930s, Popper gave falsifiability a renewed emphasis as a criterion of empirical statements in science.

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 that is a swan, and x is white''. 

The second are statements that categorize all instances of something, such as "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 type. One 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 'this is a white swan', 'that is a white swan', and so on, to a universal statement such as 'all swans are white'. This method is clearly ''deductively'' invalid, since it is always possible that there may be a non-white swan that has eluded observation. 

=== Inductive categorical inference ===
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, but it can be used to show that one is false: the singular existential observation of a black swan serves to show that the universal statement 'all swans are white' is false -- in logic this is called modus tollens. 'There is a black swan' implies 'there is a non-white swan,' which, in turn, implies 'there is something that is a swan and that is not white', hence 'all swans are white' is false, because that is the same as 'there is ''nothing'' that is a swan and that is not white'.

One notices a white swan. From this one can conclude:

:At least one swan is white.

From this, one may wish to conjecture:

:All swans are white.

It is impractical to observe all the swans in the world to verify that they are all white.

Even so, the statement ''all swans are white'' is testable by being falsifiable.  For, if in testing many swans, the researcher finds a single black swan, then the statement ''all swans are white''  would be falsified by the counterexample of the single black swan.

==== Deductive falsification ====
The falsification of statements occurs through modus tollens, via some observation. Suppose some universal statement ''U'' forbids some observation ''O'':

:<math>U \rightarrow \neg O</math>

Observation O, however, is made:

:<math>\ \ O</math>

So by modus tollens,

:<math>\neg U</math>

Although the logic of naïve falsification is valid, it is rather limited. Nearly any statement can be made to fit the data, so long as one makes the requisite 'compensatory adjustments'. Popper drew attention to these limitations in ''W. V. Quine expounded this argument in detail, calling it 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 the bird watchers are incompetent'.

Thus, naïve falsification ought to, but does not, supply any way of handling competing hypotheses for many subject controversies (for instance objective criteria, to present a definitive falsification of universal statements.

== Falsificationism ==
Naïve falsificationism is an unsuccessful attempt to prescribe a rationally unavoidable method for science. Sophisticated methodological falsification, on the other hand, is a prescription of a way in which scientists ought to behave as a matter of choice. The object of this is to arrive at an evolutionary process whereby theories become ''less bad''.

Naïve falsification considers scientific statements individually. Scientific theories are formed from groups of these sorts of statements, and it is these groups that must be accepted or rejected by scientists. Scientific theories can always be defended by the addition of ad hoc hypotheses. 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 base theory any longer, and a decision will be made to reject it.

In place of naïve falsification, Popper envisioned science as evolving by the successive rejection of falsified theories, rather than falsified statements.  Falsified theories are to be replaced by theories that can account for the phenomena that falsified the prior theory, that is, with greater explanatory power. For example, Aristotelian mechanics explained observations of  everyday situations, but were falsified by Galileo’s experiments, and were themselves replaced by Newtonian mechanics, which accounted for the phenomena noted by Galileo (and others). precession of the orbit of Mercury was not predicted by Newtonian mechanics, but was by Einstein's Maxwell's electrodynamics and Einstein's electron orbits could exist without the particles radiating away their energy and spiraling towards the centre. Thus the new theory had to posit the existence of unintuitive concepts such as Energy levels, Quanta and Heisenberg's Uncertainty Principle.

At each stage, experimental observation made a theory untenable (i.e., falsified it) and a new theory was found that had greater ''explanatory power'' (i.e., could account for the previously unexplained phenomena), and as a result, ''provided greater opportunity for its own falsification''.

== The criterion of demarcation ==
Popper uses falsification as a criterion of demarcation to draw a sharp line between those theories that are scientific and those that are unscientific.  It is useful to know if a statement or theory is falsifiable, if for no other reason than that it provides us with an understanding of the ways in which one might assess 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.

Popper claimed that, if a theory is falsifiable, then it is scientific; if it is not falsifiable, then it is not open to falsification. 

The Popperian criterion excludes from the domain of science not unfalsifiable ''statements'' but only ''whole theories'' that ''contain no'' falsifiable statements; thus it leaves us with the Duhemian problem of what constitutes a 'whole theory' as well as the problem of what makes a statement 'meaningful'.  Popper's own falsificationism, thus, is not only an alternative to verificationism, it is also an acknowledgement of the conceptual distinction that previous theories had ignored.

===Verificationism===

In the philosophy of science, verificationism (also known as the verifiability theory of meaning) holds 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 positivism of the so-called Vienna Circle that included such philosophers as Moritz Schlick, Rudolf Carnap, Otto Neurath, the Berlin philosopher Hans Reichenbach, and the logical empiricism of A.J. Ayer.

Popper noticed that the philosophers of the Vienna Circle had mixed two different problems, that of meaning and that of demarcation, and had proposed in verificationism a single solution to both. In opposition to this view, Popper emphasized that there are meaningful theories that are not scientific, and that, accordingly, a criterion of meaningfulness does not coincide with a criterion of demarcation. Verifiability came to be replaced by falsifiability as the criterion of demarcation.

Falsificationism strictly opposes the view that non-falsifiable statements are meaningless or otherwise inherently bad.<ref>''Logic of Scientific Discovery'', section 6, footnote *3</ref>

===Use in courts of law===
Falsifiability was one of the criteria used by Judge William Overton in the McLean v. Arkansas ruling to determine that 'creation science' was not scientific and should not be taught in Arkansas public schools. In his conclusion related to this criterion he stated that "While anybody is free to approach a scientific inquiry in any fashion they choose, they cannot properly describe the methodology as scientific, if they start with the conclusion and refuse to change it regardless of the evidence developed during the course of the investigation."<ref name=mclean>[http://www.talkorigins.org/faqs/mclean-v-arkansas.html McLean v. Arkansas Board of Education], Decision January 5, 1982.</ref>

It was also enshrined in United States law as part of the Daubert Standard set by the Supreme Court for whether scientific evidence is admissible in a jury trial.

== Criticisms ==
===Contemporary philosophers===
Many contemporary philosophers of science and analytic philosophers, notably W.H. Newton-Smith, John Worrall, Herbert Feigl, L. Jonathan Cohen, Wesley C. Salmon, Ilkka Niiniluoto, Raimo Tuomela, Colin Howson, Hilary Putnam, David Stove and Richard C. Jeffrey, are strongly critical of Popper's philosophy of science. Popper's mistrust of inductive reasoning has led to claims that he misrepresents scientific practice. Among the professional philosophers of science, the Popperian view has never been seriously preferred to probabilistic induction, which is the mainstream account of scientific reasoning.<ref>[http://www.lse.ac.uk/collections/CPNSS/events/Conferences/ConfirmationInductionandScience/Confirmation2007.htm Confirmation, Induction and Science<!-- Bot generated title -->]</ref> Adherents of Popper speak with disrespect of "professional philosophy", for example W. W. Bartley:
:''Sir Karl Popper is not really a participant in the contemporary professional philosophical dialogue; quite the contrary, he has ruined that dialogue. If he is on the right track, then the majority of professional philosophers the world over have wasted or are wasting their intellectual careers. The gulf between Popper's way of doing philosophy and that of the bulk of contemporary professional philosophers is as great as that between astronomy and astrology.''<ref>W. W. Bartley, III: Biology &amp; evolutionary epistemology. ''Philosophia'' '''6''':3&ndash;4 (September&ndash;December 1976), pp. 463&ndash;494</ref>
Rafe Champion:
:''Popper's ideas have failed to convince the majority of professional philosophers because his theory of conjectural knowledge does not even pretend to provide positively justified foundations of belief. Nobody else does better, but they keep trying, like chemists still in search of the Philosopher's Stone or physicists trying to build perpetual motion machines.<ref>Rafe Champion: [http://www.the-rathouse.com/bartagree.html Agreeing to Disagree: Bartley's Critique of Reason]. ''Melbourne Age Monthly Review'' (October 1985)</ref>
and David Miller:
:''‘What distinguishes science from all other human endeavours is that the accounts of the world that our best, mature sciences deliver are strongly supported by evidence and this evidence gives us the strongest reason to believe them.’  That anyway is what is said at the beginning of the advertisement for a recent conference on induction at a celebrated seat of learning in the UK.  It shows how much critical rationalists still have to do to make known the message of ''Logik der Forschung'' concerning what empirical evidence is able to do and what it does.''<ref>David Miller: Some hard questions for critical rationalism</ref>

=== Kuhn and Lakatos ===
Whereas Popper was concerned in the main with the ''logic'' of science, Thomas Kuhn’s influential book ''The Structure of Scientific Revolutions'' examined in detail the history of science. Kuhn argued that scientists work within a conceptual paradigm that strongly influences the way in which they see data. Scientists will go to great length to defend their paradigm against falsification, by the addition of ''ad hoc'' hypotheses to existing theories. Changing a 'paradigm' is difficult, as it requires an individual scientist to break with his or her peers and defend a heterodox theory. 

Some falsificationists saw Kuhn’s work as a vindication, since it provided historical evidence that science progressed by rejecting inadequate theories, and that it is the ''decision'', on the part of the scientist, to accept or reject a theory that is the crucial element of falsificationism. Foremost amongst these was Imre Lakatos.

Lakatos attempted to explain Kuhn’s work by arguing that science progresses by the falsification of ''research programs'' rather than the more specific universal statements of naïve 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'' hypotheses as unscientific, Lakatos accepted their place in the development of new theories.

Some philosophers of science, such as Paul Feyerabend, take Kuhn's work as showing that social factors, rather than adherence to a purely rational method, decide which scientific theories gain general acceptance.  Many other philosophers of science dispute such a view, such as Alan Sokal and Kuhn himself.