Fallibilism is the philosophical doctrine that absolute certainty about knowledge is impossible, or at least that all claims to knowledge could, in principle, be mistaken. Unlike Scepticism (the doctrine that true knowledge is by definition uncertain), Fallibilism does not imply the need to abandon our knowledge, in that it holds that we need not have logically conclusive justifications for what we know. Rather, it is an admission that, because empirical knowledge can always be revised by further observation, then any of the things we take as knowledge might possibly turn out to be false.

Some fallibilists make an exception for things that are axiomatically true (such as mathematical and logical knowledge); others remain fallibilists about even these, on the basis that, even if these axiomatic systems are in a sense infallible, we as humans are still capable of error when working with these systems. In addition, the incompleteness theorems of Kurt Gödel (1906 - 1978) purport to show that it is actually impossible to find a complete and consistent set of axioms for all of mathematics anyway, and that even mathematics has paradoxes, like Russell's Barber Paradox.

Fallibilism was arguably already present in the views of early Greek philosophers like Socrates and Plato, but as a formal doctrine it is most strongly associated with the late 19th Century philosopher Charles Sanders Peirce, who used it in his attack on Foundationalism. Other prominent proponents of Fallibilism include W. V. O. Quine and Karl Popper (1902 - 1994). It was also influential in the development of the Pragmatism of C. S. Peirce, William James and John Dewey.

In response to this regress problem, various alternative schools of thought have arisen:

• Foundationalism claims that some basic beliefs that support other beliefs are foundational and do not themselves require justification by other beliefs.
• Infinitism typically take the infinite series to be merely potential, and an individual need only have the ability to bring forth the relevant reasons when the need arises, so that an infinite regress can then be considered to be a valid justification.
• Coherentism holds that an individual belief is justified circularly by the way it fits together (coheres) with the rest of the belief system of which it is a part, so that the regress does not proceed according to a pattern of linear justification.
• Foundherentism is another position which is meant to be a unification of Foundationalism and Coherentism.

• Epistemological Fallibilism is the doctrine, as described above, that absolute certainty about knowledge is impossible.
• Moral Fallibilism is a related doctrine in Ethics which holds that objectively true moral standards do exist, but that they cannot be reliably or conclusively determined by humans. It presents a third possible stance between the two extremes of Moral Objectivism (the view that there are objective moral values which are independent of our perception of them or our stance towards them) and Moral Subjectivism (the view that ethical sentences reduce to factual statements about the attitudes and/or conventions of individual people).