In this essay I shall argue for two theses: first, Peirce was a justificationist in epistemology, but, contrary to the classical or rationalist model of justification, he denied that individual consciousness is the source of justification or certainty. In fact, he inaugurated the semeiotic model of justification or certainty. Secondly, Peirce was a naturalist in epistemology. For him, man's thinking and inquiring, asking and answering, asserting and denying, doubting, believing, expecting and explaining are actions and reactions which can be observed, described and understood like actions and reactions of other higher animals. He was neither an empiricist nor a rationalist in the classical sense.