Regress argument
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. This means that any proposition whatsoever can be endlessly (infinitely) questioned.
Origin
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.
Structure
Assuming that knowledge is justified true belief, then:
Suppose that P is some piece of knowledge. Then P is a justified true belief.
The only thing that can justify P is another statement – let's call it P1; so P1 justifies P.
But if P1 is to be a satisfactory justification for P, then we must know that P1.