How do you know what you know? To answer this, you need a criterion—a method for distinguishing knowledge from mere belief. But how do you know your criterion is correct? You'd need to already know things to evaluate it. This ancient puzzle, called the "wheel" or "diallelus", reveals that all epistemology runs in a circle.
Click on each node to see how the circle works:
What do we know?
Which of our beliefs count as genuine knowledge?
What is the criterion?
How do we distinguish knowledge from mere opinion?
Each question presupposes an answer to the other. We're trapped in a circle.
Philosophers have proposed three fundamentally different ways to break the circle:
Sextus Empiricus, Pyrrhonian Skeptics (~200 CE)
The circle is inescapable. Since we cannot establish either knowledge or a criterion without the other, we should suspend judgment on all claims to knowledge. This leads to ataraxia—tranquility through abandoning the quest for certainty.
Objection: But doesn't the skeptic know that the circle is problematic? Doesn't suspending judgment require knowing something?
The Problem of the Criterion connects to a deeper puzzle. Any attempt at justification faces three dead ends:
The justification eventually loops back on itself. A justifies B, B justifies C, C justifies A.
Every justification requires another justification, forever. A justifies B, C justifies A, D justifies C...
The chain simply stops at some unjustified belief. But then that foundation is dogmatic.
The Problem of the Criterion is the circular horn of this trilemma: we justify our criterion by what we know, and what we know by our criterion.
Let's apply the problem to a specific claim:
The Pyrrhonist philosopher formulates the problem in Outlines of Pyrrhonism, calling it the "wheel" (diallelus). He argues it shows knowledge is impossible.
The Stoics claimed katalepsis—certain impressions that are self-certifying and need no external criterion. Skeptics attacked this as arbitrary.
Descartes tries to break the circle with the cogito: "I think, therefore I am" provides a foundation that is self-verifying. Critics argue this begs the question.
Reid develops "common sense" philosophy: we start with particular intuitions that are self-evident, then derive criteria from them. This is early particularism.
Chisholm calls it "one of the most important problems of philosophy" and distinguishes methodism from particularism as the two main non-skeptical responses.
The Problem of the Criterion reveals that epistemology cannot start from nothing.
Every attempt to establish what we know presupposes something already known.
This doesn't mean knowledge is impossible—it means that any theory of knowledge must make
assumptions. The question becomes: which assumptions are legitimate?
Methodists assume a criterion (like empiricism or rationalism) and use it to
determine what we know. Particularists assume we know certain things
(like that we have hands) and use those to discover the criterion.
Perhaps the deepest lesson is humility: all knowledge rests on a foundation we cannot
fully justify. The circle doesn't close, but we must start somewhere.