This paper explores the problems of human knowledge stemming from the medieval era to the present. In it I will grapple with the elusive question of the justification of knowledge that goes all the way back to Aristotle in the 4th century BC and beyond. Aristotle is a good subject to start with since in a real sense he was a medieval philosopher insofar as he was “reincarnated” into a Latin-speaking Europe that previously knew very little of his ideas until his writings were reintro- duced there. “For Aristotle…epistemology is based on the study of particular phenomena and rises to the knowledge of essences.” He believed that we can know things, and I agree. In this paper I argue that we can know things and know that we know them. Further, epistemology as a branch of philosophy is valid only when it includes the existential domain, i.e. data that is received through the senses. Perception and experience must not be proscribed, for they are God's means by which we know.