Since Decartes made public his proposition of "Cogito, ergo sum," numerous arguments and objections have been made with regard to its historical back-ground and its logical contradictions. Especially in the modern ages, various studies on the proposition have been promoted including bibliographic study by E. Gilson; logical researches by O. Amelin and J. Lapport; studies from the religious standpoint by J. Maritain; researches from the angle of the so-called existentialism such as by K. Jaspers and J. P. Satre; in addition to the materialistic studies which purport, as having been tried by H. Lefevre in his "History of Thoughts," to solve Decartes' contradictions. In this short story, we would like to infer the proposition's significance by solving the contradictions in keeping with Decartes' methods -- the order of quest -- and the order of description. In Chapter I, an answer is give to the question what cannot be meant by "cogito" in general sense. Through this negative quest shown above, we intend to make positive solution of the proposition in Chapter II, because the negative solution would make it easier to solve positively the problem. To show what we can learn through the negative and positive solution in the above two chapters, it would be the best to quote the terms of Decartes himself as follows: "....For no one before me, so far as I know, asserted that it (i. e. the rational soul) consists in cogitation alone, or in the faculty of cogitation, or the internal principle." Chapter III treats of the significance of the establishment of a soul to distinguish the true from the false as the power of quest which is solved in chapters I and II. By regarding the establishment of the soul as a desperate challenge to Montaigne's scepticism, as E. Gilson did, we would like to appreciate highly the modernity declared by the proposition, whatsoever might be the contradictions of the proposition or the contradictions which would be interpreted as the reflection of the social dilemma in Decartes' minds. In addition, we believe that it would be one of the objects of our future researches what significance would be further shown before us by the "establishment of the soul" as well as "the modernity."