 Of Kūkai’s so-called theoretical works, the philosophically most significant are Benkenmitsu nikkyōron, Himitsu mandara jūjūshinron, Hizō hōyaku, Sokushinjōbutsugi, Shōjijissōgi, and Unjigi. The former three provide comparative analyses exploring what distinguishes Shingon esoteric Buddhism from other forms of religious thought and practice, including the exoteric forms of Buddhism. The latter three are expositions of the Shingon worldview and its concepts, and are grouped together to comprise the so-called Sanbu-sho (“Three Writings”). Benkenmitsu nikkyōron (2 vols.) written in 814/815 sets forth the differences between esoteric and exoteric Buddhism to emphasize the superiority of the former. Starting from this foundation (i.e. of the distinction and superiority of the esoteric standpoint), Kūkai composed the Sanbu-sho, in rapid succession from around 821 to 824. These “Three Writings” provide expositions of what commentators have called the “metaphysics” of Shingon, dealing in detail with the three so-called mysterious aspects of the cosmic truth (Dharma): its “body, speech, and mind.” Sokushinjōbutsugi explicates the concept of sokushinjōbutsu or enlightenment in one’s immediate embodied existence. Shōjijissōgi and Unjigi are unique works of a “metaphysical linguistics.” They examine the interrelationship (which is ultimately non-dualistic) between the phenomenal world, its sounds (or “voices”) as “signs” (or “letters”), and reality itself, the “meaning,” they convey. In doing so, those two works explicate the Shingon concept of hosshin seppō, the Buddha’s preaching of the Dharma via cosmic phenomena. In fact all three of these “Three Writings” deal with the embodied realization of the Dharma. But they do this from the different angles of the microcosm of the human body and the macrocosm of the cosmic body. Embodiment then plays an important role in Kūkai’s Buddhism. As we shall see below, the embodiment of the Dharma as such also involves the incorporation of the mantra and the mandala along with other esoteric features. The work that represents the culmination of Kūkai’s thought, completed at the age of 57 in 830, is his Jūjūshinron (10 vols.) along with its abridged version, Hizō hōyaku (3 vols.). These two works position Shingon esotericism as the culmination of all Buddhist and religious standpoints. They make the grand claim that Shingon provides the most comprehensive view to truth, the Dharma. All other religious views on the other hand are mere approximations of that truth in various degrees and from various perspectives. Each religious standpoint then corresponds to a specific level of development of the human mind, which in ten states leads up to the enlightened mind. The final culminating stage of enlightenment then is what corresponds to Shingon doctrine as the most comprehensive, and hence “most true.” Nevertheless, the pre-Shingon and exoteric teachings each have their rightful place within this scheme so that they are not utterly discounted. In addition to the above-mentioned six major works, there is also Kūkai’s first work – written prior to his trip to China –, the Sangōshīki. This was his first look at Buddhism in comparison with Taoism and Confucianism. In the following however, I shall focus on Kūkai’s thought in the six major works. These are his mature works, and also universally recognized as his most important theoretical works.