Although in casual speech it often

Although in casual speech it often matters very little whether a cause is termed " reason," " motive," " grounds," or " explanation," there are occasions when distinction among these meanings is essential; to understand as exactly as possible an author's usage of words for cause may be vital to the understanding of his whole approach to his subject. This is eminently true of Thucydides. When he uses prophasis and aitia in reference to the causes of the war, it is clearly necessary to know, if we can, whether he means his own explanation, or the reasons, motives, etc. of the participants. The question of the precise meaning of the words for cause in these places is not just a lexicographical minutia, but has a bearing on the question of Thucydides' historical method. The present paper is based on a study of the three words prophasis, aitia, and aitiai in Thucydides and in writers who may have influenced, or have been thought to have influenced, the usage of Thucydides. Its purpose is to establish the range of meaning that these words have in Thucydides, and to show that these patterns of meaning are in harmony with the general method of Thucydides' presentation.