The idea is that since it is impossible to achieve a satisfactory justification for our claims to knowledge we should cease to look for one, and construct a scientific account -in purely 'natural' terms and without reference to justification - of how in fact we come to hold the beliefs we do.
This issue of whether to pursue justification or scientific explanation must be distinguished, it has been claimed, from the question of whether justification itself should be sought by appeal to natural facts (such as causal relations), as opposed to, for example, rational considerations that would entail the conclusion to be justified. (Also see: causal theory of knowledge, externalism, internalism.)
A comparison can also be made between naturalism in epistemology and in ethics.
Also see: psychologism
J Kim, 'What is "Naturalized Epistemology"?', Philosophical Perspectives (1988);
WVO Quine, Ontological Relativity and Other Essays (1969)