Descartes’s Meditations on First Philosophy was criticized as committing the fallacy of circular logic from the moment of its publication. Therefore, the problem of Descartes’s vicious circle needs to be discussed in order to understand Descartes’s epistemology. This paper begins with the examination of 3 interpretations proposed by several critiques and reveals the fact that all of 3 interpretations are obliged to recognize explicitly or implicitly the meditator of Meditation III having knowledge in a certain sense. To solve this problem, knowledge in Meditation III can be distinguished by complete knowledge and limited knowledge according to its nature. This paper aims to argue that each of this knowledge is still problematic. If all clear and distinct perceptions are considered as complete knowledge, the casuality principle is troubled. On this account, all clear and distinct perceptions employed in the demonstration of God’s existence cannot be knowledge of complete certainty. In the second place, if all clear and distinct perceptions are understood as limited knowledge, these perceptions are unavoidably regarded as knowledge justified only by externalism. However, Descartes himself nor theorists claiming this interpretation did not want such consequence. This paper concludes that the attempts to solve Descartes’s vicious circle examined in this paper are not successful for the reason that it depends on the unconvincing concept of knowledge.