In the field of epistemology, the problem of the criterion is an issue regarding the starting point of knowledge. This is a separate and more fundamental issue than the regress argument found in discussions on justification of knowledge.
American philosopher Roderick M. Chisholm in his Theory of Knowledge details the problem of the criterion with two sets of questions:
1. What do we know? or What is the extent of our knowledge?
2. How do we know? or What is the criterion of knowing?
An answer to either set of questions will allow us to devise a means of answering the other. Answering the former question set first is called particularism, whereas answering the latter set first is called methodism. A third solution, found untenable by many philosophers for its inadequacy to give explanation is skepticism. A skeptic will proclaim that since one cannot have an answer to the first set of questions without first answering the second set, and one cannot hope to answer the second set of questions without first knowing the answers to the first set, we are, therefore, unable to answer either. This has the result of our being unable to justify any of our beliefs.
Particularist theories organize things already known and attempt to use these particulars of knowledge to find a method of how we know, thus answering the second question set. Methodist theories propose an answer to question set two and proceed to use this to establish what we, in fact, know. Classical empiricism embraces the Methodist approach.