The regress argument (also known as the diallelus (Latin < Greek di allelon "through or by means of one another")) is a problem in epistemology and, in general, a problem in any situation where a statement has to be justified.
According to this argument, any proposition requires a justification. However, any justification itself requires support, since nothing is true “just because”. This means that any proposition whatsoever can be endlessly (infinitely) questioned, like a child who asks "why?" over and over again.
The argument is usually attributed to Sextus Empiricus, and has been restated by Agrippa as part of what has become known as "Agrippa's Trilemma". The argument can be seen as a response to the suggestion in Plato's Theaetetus that knowledge is justified true belief.
Assuming that knowledge is justified true belief. Then:
The above presents us with three possible counter-arguments: some statements do not need justification; the chain of reasoning loops back on itself; or the sequence never finishes.
Perhaps the chain begins with a belief that is justified, but which is not justified by another belief. Such beliefs are called basic beliefs. In this solution, which is called foundationalism, all beliefs are justified by basic beliefs. Foundationalism seeks to escape the regress argument by claiming that there are some beliefs for which it is improper to ask for a justification. (See also a priori.)
Foundationalism is the belief that a chain of justification begins with a belief that is justified, but which is not justified by another belief. Thus, a belief is justified if and only if:
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