 Kūkai (774–835CE) is one of the intellectual giants of Japan, who ought not to be ignored in any account of the history of Japanese thought. Among the traditional Buddhist thinkers of Japan, and perhaps even of the whole of East Asia, he is one of the most systematic and philosophical. He is most famous for being the founder of Shingon esoteric Buddhism in Japan. But he is also remembered not only for his contributions as a teacher and scholar of religion, but for his accomplishments and innovations in social welfare, public education, lexicography, language, literature and poetry, literary theory, calligraphy, art, painting, wood-carving, sculpture, music, civil engineering, architecture, etc. during a period when Japan was undergoing rapid change. In this essay, we shall restrict our account to his philosophy.