The focus of this dissertation is Myoan Eisai, considered by scholarship as the founder of the Rinzai Zen lineage in Japan. This work aims to answer two interrelated questions: what is Eisai's Zen? and how does Eisai's Zen relate to other schools of Buddhism? Through an analysis of Eisai's texts composed following his return from his second trip to China in 1187, I illustrate the link between Eisai's understanding of Zen and the practice of morality in Buddhism; moreover this dissertation shows clearly that, for Eisai, Zen is compatible with both Tendai and the study of the precepts. This work analyzes the Eisai's use of doctrinal debates found in Chinese sources to argue for the introduction of Zen to Japan. Through this analysis, we see how Eisai views Zen, based on his experience in Chinese monasteries, not as a distinct group of practitioners rebelling against traditional forms of practice, but rather as a return to fundamental Buddhist positions concerning the importance of morality and its relationship to meditative practices.

The conclusions reached in this dissertation have doctrinal, historical and philosophical implications. Doctrinally and historically, this work brings into focus the origins of Zen in Japan and its relationship with other schools; we see how a comprehensive textual analysis of Eisai's Chinese sources highlights the key relationship between Zen and morality, and how this relationship relates to Buddhist practice. Philosophically, this work provides an example of a new methodological approach to comparative philosophical problems in Buddhism, and reveals potential areas of interest for a revival in comparative studies of Buddhist ethics.