The main thesis of this paper is that it is not possible to determine the nature of epistemic justification apart from scientific psychological investigation. I call this view “the strong thesis of methodological psychologism.” Two sub-theses provide the primary support for this claim. The first sub-thesis is that no account of epistemic justification is correct which requires for the possession of at least one justified belief a psychological capacity which humans do not have. That is, the correct account of epistemic justification must be psychologically realistic. The second sub-thesis is that it is not possible to determine whether an account of epistemic justification is psychologically realistic apart from scientific psychological investigation. After defending these subtheses, I point out some interesting consequences of the overall thesis which present a challenge to traditional epistemology.