The Kephalaia gnōstika rhas been very influential in the history of Orthodox monasticism, not least because it is the earliest detailed articulation of the practise of using the monologic prayer 'Lord Jesus' to achieve interior stillness and so be able to contemplate God without distraction. It is however a work of technical density and grammatical subtlety, and details of the precise theology of prayer which it contains have never been grasped with precision. What is clear is that it utilises, repeatedly and apparently with specific intent, terminology of the ascetical traditions associated with Evagrios Monachos on the one hand, and pseudo-Makarios on the other. These two traditions are usually characterised as that of the 'intellect' and that of the 'heart' respectively. It has always been assumed that in adopting both sorts of terminology Diadochos was attempting to synthesise the best of each without any heretical associations. Modern scholarship has been interested to determine the nature and extentof Diadochos's dependence on each tradition. This dissertation argues that before this question can be resolved the precise theological content of the Kephalaia gnōstika r must be clarified by undertaking a systematic terminological analysis. In addition, only a terminological analysis can indicate what Diadochos's synthesis of the two traditions really entails, and what the relationship is between his synthesis and his understanding of the prayer 'Lord Jesus' . Accordingly, the dissertation begins with a terminological analysis, and proceeds to an overall theological analysis, supported by an English translation of the Kephalaia gnōstika r which translates all the technical vocabulary consistently. The discussion then returns to the question of Diadochos's use of his sources. It is argued that Diadochos's dependence on Evagrios is superficial, and that his use of Messalian terms has been employed in order to wrest them from their heterodox theological associations. Concluding observations are made to the effect that the Diadochan synthesis is both philosophically respectable and of continuing importance for the understanding and practice of Christian prayer.