Traditional foundationalism, initially put forward by Aristotle, also received attention in the Middle Ages. Thomas Aquinas sustained the theory with a little bit adaptation. But, becuase of his unique conception of the mechanism of knowledge acquisition, William of Ockham suggested a theory of foundationalism based on direct realism. In order to avoid the criticisms levelled at Aristotle's foundationalism, he also rejected the theory of form and put emphasis on the certainty of emperience. In this article, first we will review the foundationalism of Aristotle and Aquinas very brietly and then we will examine that of William of Ockham in detail. Finally, we will show that Ockham's theory is more compatible with the theory of acquaintance than that of Aristotle and Aquinas. Nowadays, many leading proponents of traditional foundationalism believe that in order to maintain this theory they should take hold of the theory of acauaintance.