空海と四国遍路の隠された目的Kukai's Secret Purpose For The Shikoku Pilgrimage

The approximately 1,400-kilometer pilgrimage route known as the “Shikoku 88-temple pilgrimage” (Shikoku Pilgrimage, 四国八十八ヶ所), commonly referred to as the Henro (遍路), has long been traversed by countless Buddhist devotees. However, it is said that Kukai (空海, 774–835), the founder of the Shingon school of Buddhism, traditionally credited with establishing this pilgrimage route, had a hidden purpose. Rather than solely opening the pilgrimage for religious devotion, Kukai was also believed to have been tasked by imperial decree with conducting surveys and traversing the entirety of Shikoku. Consequently, there remain alternative theories about attributing the pilgrimage’s origins solely to Kukai. Personally, I also feel that the pilgrimage was less a matter of faith and more likely traces of a survey mission that later became beautified from a religious perspective by subsequent generations.

Of course, this is also related to the fact that Kukai himself was originally from Shikoku. Born in the coastal region of Sanuki Province (present-day Kagawa Prefecture) in 774, Kukai went on to accomplish numerous extraordinary feats, eventually retreating to Mount Koya in Wakayama, where he founded Shingon Buddhism (真言密教). A key question arises: Why was Mount Koya chosen as the sacred site for Shingon Buddhism? A sage as intelligent as Kukai surely had substantial reasons for selecting this location. With numerous legends surrounding him, the true nature of Kukai remains elusive. Therefore, this article will examine him from a different angle—not merely from the perspective of esoteric Buddhist faith, but also considering the deep political intentions of the emperor and the imperial court at the time.

In a previous article, I wrote about how Kukai, initially just an unknown monk, was suddenly selected to participate in the state-sponsored diplomatic mission to Tang China (遣唐使), significantly shortening his required 20-year study period as an overseas monk and returning after only two years. Moreover, within that brief period, Kukai received full transmission from Huiguo (恵果阿闍梨), the most distinguished Chinese monk of his era. It is said that Huiguo personally chose Kukai as his successor, urging him to “return to your homeland immediately and establish Esoteric Buddhism there.” This event may be difficult to fully appreciate today, but it was unprecedented compared to Saicho (最澄), another prominent Japanese monk aboard the same diplomatic voyage. (Saicho later became the founder of the Tendai school, establishing Enryaku-ji Temple on Mount Hiei.)

Notably, Kukai eagerly collected and purchased numerous Buddhist texts unknown in Japan while in Tang China, bringing them back home and presenting them to the imperial court. According to some accounts, the vast number of scriptures Kukai introduced did not overlap at all with existing Japanese texts, suggesting he may have read every Buddhist work available in Japan before his journey. Such a feat would be impossible for an obscure monk without privileged access to state-held documents.

Additionally, given Kukai’s lack of personal funds, the fact he was able to purchase numerous scriptures and Buddhist statues suggests he received substantial financial backing, hinting at his involvement in some undisclosed, state-sponsored project. However, the full extent remains unknown, leaving room for many intriguing hypotheses. Thus, Kukai, a mysterious superhuman figure, continues to fascinate many Japanese people today.

Considering the possibility that the pilgrimage route was established in 815 after Kukai’s return from Tang China, it becomes clear that during his time abroad, he likely acquired diverse knowledge—not only of Esoteric Buddhism but also of Confucianism, Taoism, and other disciplines, integrating them comprehensively in pursuit of something specific. That something was mercury. In fact, Kukai is associated with numerous legends involving cinnabar (朱, shu; chemically mercuric sulfide, 辰砂, shinsha), and historical records often suggest a connection between his activities and mercury. Setting aside the legendary notion of “Sokushin Jobutsu” (即身成仏), some records state Kukai secluded himself at Mount Koya in 831 due to illness, which has led some scholars to speculate mercury poisoning as a possible cause.

However, readers may find it challenging to understand the immense value of mercury at the time or why it was so crucial. Therefore, let us closely examine the relationship between Kukai and mercury-producing regions.

Mercury (朱, shu or cinnabar) has been prized as a mysterious metal worldwide since ancient times. Particularly during the Tang dynasty, when Kukai studied in China, Taoist-inspired immortalist ideas flourished, and it is recorded that emperors seeking eternal life frequently consumed mercury. Especially in China, within the secret arts of creating “elixirs of immortality” known as alchemical techniques (錬丹術), mercury was regarded as an even more valuable and prestigious material than gold or silver, prioritized in a hierarchy not as “gold, silver, copper,” but rather as “cinnabar (朱), gold, silver.” Consequently, for people at that time, entering mountainous areas in search of cinnabar as a key ingredient for elixirs would have felt entirely natural.

Putting aside the spiritual ideals of immortality, mercury was also an extremely important material within Buddhism itself. This is because cinnabar pigment (水銀丹) was used extensively not only in the coloration of Buddhist paintings and temples but was also essential for the gilding (鍍金) of Buddhist statues, which made them glow with a golden brilliance. A prime example of this is the Great Buddha (盧舎那仏) at Todaiji Temple in Nara. When first completed, an immense amount of mercury was used in the gold plating covering this giant statue. Historical records indicate that approximately 437 kilograms of gold required about 2,459 kilograms of mercury—a remarkable ratio of roughly “1 part gold to 5.6 parts mercury.” Thus, mercury was an indispensable resource both religiously and practically, from the Taoist elixirs of immortality to the majestic construction of Buddhist statues and temples. This fact provides a key to unraveling the mysteries surrounding Kukai. In other words, while mercury had long been valued in Japan as a mystical substance and elixir, its demand increased dramatically alongside the flourishing of Buddhism.

Since the construction of the Great Buddha as a national project during the Nara period, this trend continued into the Heian period, where temples and large Buddhist statues continued to be built throughout the nation as part of state-sponsored Buddhist policies. Particularly during the reign of Emperor Kanmu (桓武天皇, r. 781–806), the relocation of the capital from Nagaoka-kyo to Heian-kyo necessitated constructing large temples to demonstrate national prestige and pray for national protection. However, the imperial court at that time suffered severe financial difficulties, compelling them to seek financial aid from powerful patrons. By the era of Emperor Saga (嵯峨天皇, r. 809–823), Buddhist projects flourished even further alongside the rise of newer forms of Buddhism, such as Shingon and Tendai esoteric teachings. Riding this new wave was none other than the previously unknown monk, Kukai.

During this historically significant period of Buddhist prosperity in Japan, the demand for mercury skyrocketed due to extensive gilding projects for Buddhist statues and ceremonial implements. Thus, the imperial court faced the necessity of expanding its sphere of control and venturing throughout the country, even resorting to conflict to secure essential materials and resources, notably gold and mercury. In a way, the securing and procurement of mercury became a chronic strategic issue integral to the nation’s continued existence.

Examining this matter from a linguistic and cultural perspective in Japan, the term “Niu” (丹生) becomes highly significant. Mercury (cinnabar, 辰砂, shinsha) mining sites have been scattered throughout Japan, and many of these locations still carry names including the characters “Niu” (丹生). Archaeological evidence has demonstrated a strong connection between ancient place names or shrines bearing “Niu” and cinnabar deposits. Personally, I have visited several shrines and sacred sites related to “Niu,” observing that in these areas, cinnabar mining occurred alongside the development of unique mercury-related religious practices.

Interestingly, looking broadly at major cinnabar-producing regions in Japan reveals why Kukai established the pilgrimage route in Shikoku and based his headquarters at Mount Koya. Here, the significant geographical feature known as Japan’s Median Tectonic Line (中央構造線) becomes crucial. This is Japan’s largest fault zone, stretching roughly 1,000 kilometers east-west across the middle of the Japanese archipelago, from Kyushu to the Kanto region. Notably, major cinnabar mining sites situated along this Median Tectonic Line, following their ancient names, include:

・Ise Province Niu: Niu Mines in present-day Mie Prefecture

・Yoshino and Kumano areas in Kii Province: around Mount Koya in present-day Wakayama Prefecture

・Central Shikoku’s Awa Province, Niudani: around Anan City in present-day Tokushima Prefecture

・Bungo Province, central Kyushu: present-day Usuki City, Oita Prefecture

Considering the historical fact that the Japanese imperial court established its foundations in ancient Yamato (southern Nara Prefecture) and later moved its capital to Heian-kyo (present-day Kyoto) in 794, the closest and most sustainable cinnabar sources for the court would have been Wakayama and Tokushima. Wakayama had active land routes, while Tokushima facilitated maritime trade with the imperial court. From this perspective, by setting aside religious considerations and focusing solely on practical resource demands, one can begin to uncover Kukai’s hidden intentions in establishing the Shikoku pilgrimage and founding the headquarters at Mount Koya.

In the next article, I will further delve into the unique religious practices that revered “Niu” as a deity, aiming to shed even more light on the figure of Kukai.