Classification of Buddhist doctrines is an integral part and one of the main characteristics of Buddhist philosophy. As a rule, a main objective of this classification is to illustrate the superiority of certain Buddhist doctrines over others through detailed analysis and discussion. The various descriptions of these Buddhist doctrine’s in various sources gave modern researchers the opportunity to study Buddhist philosophy. However, in analyzing them, it is possible to see not only specific differences between mutual perceptions and criticisms, but it also reveals some doctrinal aspects not emphasized earlier. In this respect, the study of the classification of Buddhist schools in Japan in the 8th and 9th centuries is especially interesting.

Six Buddhist schools in Japan were organized at the end of the 7th century but prior to the 8th century. They are known as the “six schools of the Nara period” (Nanto Rokushu 南都六宗). They were: Sanron-shu (三論宗) which taught madhyamika; Hosso-shu (法相宗) which taught yogacara; Kegon-shu (華嚴宗) which specialized in avatamsaka; Kusya-shu (倶舎宗), the school of abhidharma; Jojitsu-shu (成実宗), which was affiliated with the Sanron school and was based on the traditions of satyasiddhi; and Ritsu-shu (律宗) which interpreted vinaya. Later, at the end of the 8th century and beginning of the 9th, two other Buddhist schools appeared in Japan: Tendai-shu (天台宗) and Shingon-shu (真言宗). The doctrines of Tendai-shu were based on verses from the Lotus Sutra (Saddharma-pundarika sutra, Hokke-kyo 法華経), and the Shingon-shu school taught the esoteric Buddhism of Vajrayana. The appearance of Vajrayana had a special meaning for Japanese Buddhism as new forms of Vedic and Hinduism taught in Tantric Buddhism merged with it.

Like Tendai-shu, Shingon proclaimed that everyone is capable of becoming a Buddha. But, for the first time in Japanese Buddhist history, the Shingon doctrine also taught that enlightenment can be attained in one’s current life by means of magic rituals (Sokushin Jobutsu 即身成仏).

It is natural that the study of all the various Buddhist doctrines present in Japan at that time was the educational basis in these schools, and an important part of this education was the classification of Buddhist doctrines established by the various schools.

For example, the Hosso-shu Buddhists classified Buddhist schools by dividing Sakyamuni Buddha’s sermons into three distinct periods. In the first period, Buddha taught the doctrine of Dharma and the theory of causality, which then became the dogma of Kusya-shu (abhidharma). In the second period, Buddha taught his disciples that “all Dharmas are empty.” Therefore, according to Hosso-shu classification, the doctrine of this period is that of the Sanron school (madhyamika). In the third period, Buddha taught about overcoming the extremes of the first two periods, which is the basis of Hosso doctrine (yogacara) (Doju 1899, 13).

In the Ritsu system, the doctrines of the Nara schools were classified according to so-called “regulating doctrines,” i.e., vows and precepts. The doctrines of Buddhist schools were classified according to their vows: 1) doctrines in which vows regulate Dharmas of feelings, i.e., Kusya-shu; 2) doctrines in which vows are based on changing Dharmas, or Jojitsu-shu; 3) the perfect doctrine which comprehends the essence of vows―Hosso-shu  (Shakuryu 1899, 21).

In Kegon-shu, the doctrines of Buddhist schools are classified according to the “four shapes of the Dharma world.” The “Dharma world” is the basic philosophical foundation of Kegon. In this world, phenomena (ji 事) and the absolute (ri 理) both appear simultaneously. This appearance is the testament to the presence of the primary nature of the Buddha in the Dharma world (Kavano 1899, 16).

The classification of the “Dharma world’s forms” was developed in Kegon doctrine to reflect the basic identities of ji and ri:

According to Kegon doctrine, the doctrines of the following schools correspond to the four principal views of the Dharma world: Ji-hokkai is described in the doctrines of Hinayana: Ri-hokkai—Sanron and Hosso, Ji-ri muge hokkai—Tendai-shu, Ji-ji muge hokkai—Kegon-shu (Ui 1941, 160‑61).

Kavano H. supposed that this classification system revealed a main aspect of Kegon doctrine: there are no oppositions between subjects and phenomena in a phenomenal reality. Phenomena are all identical and can interpenetrate each other. The relationship between the absolute and any phenomenon are characterized in the same way (Kavano 1899, 17).

Another researcher, J. Chang, believes that this classification of Buddhist schools reveals a basic difference between Kegon doctrine and the doctrines of Sanron and Hosso. He believes the system of the “four shapes of the Dharma world” was taught to overcome the “negative” dialectic of Sanron-shu and the idealism of Hosso-shu. In Kegon-shu, the definition of “absolute” or “emptiness” (shunya) was interpreted positively as “completeness and generality,” unlike in Sanron. As to the characteristics of phenomena in the Kegon doctrine, it was considered as the identity and distinction of all objects at the same time, while in Hosso doctrine, phenomenal life was reduced exclusively to the signs of seven vijnyana (Chang 1971, 183).

In Shingon, Buddhism was divided into esoteric (Mikkyo 密教) and exoteric (Kenkyo 顯教). An exoteric doctrine is similar to “conditional truth” in madhyamika: it helps in the initial stage of comprehension of the “hollowness” of the phenomenal world, but it doesn’t give full enlightenment. In esoteric Buddhism, the “secret doctrine” (Mikkyo 密教) helps one to understand their own Buddha nature.

A more detailed classification of Buddhist doctrines is presented in the compositions of Kukai (空海, Kobo-daishi 弘法大師, 774‑835)―the founder of Singon-shu in Japan. In his treatise The Secret Key to the Prajnya-paramita hridaya-sutra (Hannya shingyo hiken 般若心経祕鍵), Kukai described the doctrines of the six Nara schools as “Five Vehicles.” He merged the Risshu and Jojitsu schools into the “Vehicle of Two Ways,” referring to the Hinayanist maintenance of their doctrines. According to Kukai, each school represents the activity (or samadhi) of various Tantric Buddhas and deities.

In Kukai’s system, the doctrine of Kegon is the samadhi of the Creating Tathagata, or Bodhisattva Samantabhadra (Kukai 1999a, 91). In his opinion, the school’s doctrine of undivided phenomenon (ri) and the absolute (ji) completely corresponds to the nature of Samantabhadra.

Sanron-shu is an emanation of Bodhisattva Manjushri (Tathagata, free from all groundless reasoning). As Kukai shows, the assertions of madhyamika theorists that “all Dharmas are initially empty,” which is the basis of Sanron, illustrate well the moment when Manjushri’s sharp sword of wisdom cut through all errors and defilements (ibid., 91).

The yogacara school of Hosso is actually the samadhi of Bodhisattva Maitreya, and the law of cause and effect studied in Hosso is a display of his great compassion. Arguing on distinctions between essence and visibility, it denies the duality of the world of form, declaring that there is only consciousness (ibid.).

Kukai also says that Ritsu and Jojitsu are the samadhi of the Shravakas and Pratyeka Buddhas respectively. Considered a branch of Sanron, Jojitsu-shu’s philosophy of “real-life Dharmas” corresponds to the spirit of the Pratyeka Buddhas, and according to Kukai, “The Vehicle of Shravakas” represents the doctrines of Ritsu which systematizes regulations for Buddhist monks and nuns (ibid., 93).

Tendai-shu, designated by Kukai as the school of a “United Way,” represents the samadhi of Avalokiteshvara (Tathagata, which nature is pure). It teaches all sentient beings that the Three Vehicles merge into the One Vehicle, whose way is as pure as a lotus flower which grows in mud (ibid.).

In another Kukai composition, The Precious Key to a Secret Treasury (Hizouhōuron 祕藏寶論) the doctrines of the Buddhist schools are likened to the various medicines which the “King of Healing” (Buddha) applies to heal the afflictions of sentient beings. Kukai writes:

Accordingly, these “medicines” should lead to healing which is the consciousness of Mahavairochana, the expression of true consciousness in the doctrine of Shingon.

In other words, according to Kukai, the Nara schools can be viewed through the prism of Shingon doctrine as a Tantric essence of the Tathagata through the samadhi of various Bodhisattvas. This view is reflected in the mandalas of Shingon-shu, particularly in the “Garbhamandala” (Taizoukai-mandara 胎蔵界曼荼羅).

In the mandala’s center, the Buddha Mahavairochana is sitting on a red seven-petaled lotus, symbolizing the heart of the universe. Around him is a circle symbolizing “The Wheel of Law.” In the circle are eight Bodhisattvas, embodying various samadhi. For ease of explanation, let’s imagine these eight figures to be the eight points on a compass. Among these eight, note the following four Bodhisattvas: Samantabhadra, Maitreya, Manjushri and Avalokiteshvara. Samantabhadra (Hugen 普賢) and Maitreya (Miroku 彌勒) are at the NW and NE positions respectively. Manjushri (Monju 文殊) and Avalokiteshvara (Kannon 観音) are at the SW and SE positions.

Their four positions actually form a square with Buddha Tathagata Mahavairochana in the center. Thus, according to Kukai’s view, in the top part of the square are the Hosso and Kegon schools with the Tendai and Sanron schools at the bottom. Shingon, embodying the doctrine of Mahavairochana, occupies the center.

Such positioning can be interpreted two ways. First, the designation of each pair can symbolize various interpretations of the Dharma at these schools, and their arrangement in a circle can symbolize instructions on the Wheel of Dharma which unites all contradictions. In the second interpretation, the arrangement of each school at the four corners of a square probably connects them with the “Four Deva Kings,” the Guardians of the World (Shitenno 四天王). The Four Deva Kings are Vaishravana, Dhartarashastra, Virudhaka and Virupaksha and details about them can be found in three sutras: The Sutra of Gold Light (Survanabhasottasama sutra, Konkomyo-kyo 金光明経), The Benevolent King Sutra (Karunika-rajah sutra, Ninno-kyo 仁王経), and The Lotus Sutra (Saddharma-pundarika sutra, Hokke-kyo 法華経). During the 8th century, these three sutras were highly esteemed in Japan as sacred texts which promoted protection and prosperity of the state and were known as State protecting sutras (Gokoku kyoten 護国経典).

The principal subject of the first two sutras is the mission of the terrestrial king―chakravartin. As it considered in this sutras this king is patronized by Sakyamuni Buddha and the inhabitants of the Buddhist pantheon (the Four Deva Kings).

According to the sutras “Konkomyo-kyo” and “Ninno-kyo,” if the king worships the Buddhist sutras, constantly arranges for their public readings and honors the Buddhist clergy and laymen by proclaiming these sutras, he and his subjects will be protected from sudden attacks by external enemies, and all the deities of the Buddhist pantheon will rise to protect his country. In the sixth chapter of the Sutra of Gold Light, it says:

Kukai also highly regarded the public recitation of this sutra because of its sacred and magical value.

A petition sent by Kukai to emperor Ninmei (833‑50) in 834 requesting the inclusion of esoteric Buddhist rituals on the eve of the court’s New Year ceremony, to which the most prominent Nara priests were invited, is evidence of his high esteem. In the petition, Kukai said that even though the Sutra of Gold Light is known for its capability to spare the king and his subjects from misfortune, a simple reading of this sutra would be as inefficient as reading medical treatises to a patient. To release the powerful force hidden in this sutra, it is necessary to perform rituals of worship to the deities as described in the Sutra of Gold Light (Abe 1999, 58).

Because the Four Deva Kings occupied a special position among the other Buddhist deities, from Kukai’s point of view, their positions in the Garbhamandala represented the emanations of Hosso, Sanron, Kegon and Tendai, which made them guardians of Shingon doctrine, which is embodied in the centrally located Buddha Mahavairochana (or Tathagata Mahavairochana) (Dainichi Nyorai 大日如來).

It is necessary to note that the Buddha Mahavairochana, who is the central object of worship in Shingon-shu, has some specific features. According to Shingon doctrine, he is the source of the universe and all beings occupying it. In his body, the dualism of the phenomenal and absolute worlds is overcome. The whole world and all beings occupying it are not distinct from the essence of Dainichi. In the light of Dainichi, everything, even the most insignificant of events, has Buddha nature. It is necessary to note that Buddha Vairochana (Birushana 毘盧舍那) is also the central object of worship in Kegon-shu. According to Kegon-shu doctrine, Buddha Vairochana is the “true body” of the Dharma world, i.e., the basis of all reality (Huan 1973, 39).

At first glance, Birushana’s characteristic of having a body which overcomes the dualism of the phenomenal and the absolute is similar to Shingon-shu’s description of Dainichi-nyorai. However, in Kukai’s commentary on the Mahavairochana-sutra, he gives a more detailed description of Buddha Mahavairochana. According to Kukai, Buddha Mahavairochana possesses three characteristics: 1) the Universal light which expels darkness; 2) promoting the formation of all that is real; 3) the Unborn and Undestroyed Light.

These three characteristics symbolize Mahavairochana’s omnipresence, his compassion for all live beings and his eternal nature (Aśvaghoṣa 1967, 12‑13).

Kukai’s system of school classification might also correspond to the five descriptions of Buddha Mahavairochana’s enlightenment: unborn, inexpressible, undefiled, uncaused and emptiness. They, in turn, correspond to the five great elements (earth, water, fire, wind and sky). The first quality, unborn, is reflected in Kegon doctrine about the identity of the visible and invisible aspects of the Buddha’s nature. The quality of being “inexpressible,” represented by a “Tathagata, free from all groundless reasoning,” corresponds to the “eight negations” of Sanron doctrine. “Undefiled” corresponds to the classifications of “true” and “false” dharmas in Hosso doctrine. “Uncaused,” which recognizes the Five Skandha and denies “The Four Noble Truths,” is embodied in the “chariots” of Shravakas (Jojitsu-shu) and Pratyeka Buddhas (Ritsu).

“Emptiness,” which unites all contrasts and describes the pure self-nature of the Tathagata, corresponds to the Tendai doctrine about the uniform nature of the Buddha.

Correlating the various Buddhist doctrines to the five characteristics of Tathagatas illustrates the presence of Buddha nature in the consciousness of all living beings, irrespective of their failings. Moreover, we can see that Kukai’s intention was to show the intrinsic unity of all Buddhist doctrines and, therefore, the indivisibility of the Buddhist canon.

The integrity of Buddhist doctrine is also discussed in another of Kukai’s compositions, The Treatise About Two Doctrines, Obvious and Secret, and About The Distinctions Between Them (Ben kenmitsu nikyōron 辯顯密二教論):

Here are the five categories of people for whom these sermons are intended. Sutras are intended for Buddhist hermits who practice in solitude in the forested mountains. Vinaya sermons are conducted for those who study, support and protect the Dharma of Buddha (“the True Law”) to attain inviolable rest. If we consider that following the vinaya precepts was one of the principal duties of the Nara clergy, in this sense, they are the “Guardians of Dharma,” and their aspiration to attain inviolable rest equates to attaining peace and prosperity for the state. The abhidharma sermons are intended for those who preach the law of the Buddha while discerning and categorizing form and nature in order to study it in depth, all the while diligently investigating and questioning. These are the followers of Hosso. The doctrines of Sanron-shu, Kegon-shu and Tendai-shu come under the prajnya-paramita sermons because they are based on the desire to disassemble, analyze and discern creation in order to know the Law of Buddha. The dharani sermons apply to the Shingon school. They stand apart from the other sermons because they are for people incapable of comprehending sutras, abhidharma and prajnya-paramita, and for the inveterate villains who indulge in all manner of sin and slander the Law of the Buddha.

This characterization of Shingon contradicts another statement by Kukai in The Precious Key to a Secret Treasury about people with “sheep consciousness” being on the lowermost step of spiritual development:

However, a more detailed analysis reveals a deeper meaning. Shingon and all the other doctrines together are a part of the “Treasure of Law” that preaches to all living beings who aspire to salvation. However, Shingon is superior, just as the sacrificial cream is the best product of milk. The superiority of Mantrayana over other doctrines is due to the fact that it is intended to save even the most dull-witted and low class beings; those whose consciousness is defiled. The other schools refuse to give such beings salvation, and their sermons are for those who have already realized their own pure nature.

Having studied Mahayana sutras and the Vajrayana treatises, Kukai described in detail how to unleash the latent force of sacred texts by means of various verbal rituals. In doing so, he showed the Nara clergy a new and truly Buddhist philosophical method employing the indissoluble communication of Buddhist texts and rituals. Henceforth, according to esoteric Buddhism, the basic method for increasing the efficiency of Buddhist sutras and rituals should be based on sacred verbal utterances (Tantric mantras).

Thus, that fact, that Shingon-shu was recognized as a school of Nara Buddhism is explained by Kukai’s relationship to the Six Nara Schools. He showed that various Buddhist doctrines, being the manifestations of various qualities of the Buddha Tathagata, could be integrally intertwined into the Tantric world view, and that these manifestations correspond to the various traditions of Buddhist doctrine (sutras, vinaya, abhidharma, prajnya-paramita and dharani). At the same time, the qualities of a Tathagata are embodied in various Bodhisattvas, whose vows also correspond to the doctrines of the Mahayana schools: Kegon, Sanron, Hosso and Tendai. And according to Kukai, their possible correlation to the Four Deva Kings also implies they are the Guardians of Buddhist Law and the state (Chingo kokka 鎮護 国家). As these Bodhisattvas are an integral part of the world of Mahavayrochana Buddha as depicted in Shingon mandalas, we can see Kukai’s inclusion of “the Obvious Doctrine” in Tantric tradition occurs not only in written textual tradition, but also in the iconography which represents schematically the cosmological character of the Dharma. Thus, Kukai’s statement about the superiority of Vajrayana is based on the introduction of the Mahayana “obvious” doctrines into the system, the most important being the “secret doctrine” of Vajrayana.