The Hossō school represents an intriguing case, in that its doctrinal position offered a steadfast minority opposition, not only to the Tiantai/Tendai schema of the five periods, but also to the entire notion of buddhahood as a universal possibility. Hossō thought distinguishes two kinds of buddha nature: buddha nature as suchness or principle (J. ri busshō), which is universal, and active buddha nature (gyō busshō), which is not. Buddha nature as principle is quiescent and does not manifest itself in the phenomenal world; thus its universality does not mean that all beings can become buddhas. Achieving buddhahood depends on the presence of “untainted seeds” originally inherent in the ālaya or storehouse consciousness, the root consciousness underlying samsaric existence in which all deeds and impressions are stored as “seeds” or latent potentials, later fructifying in the form of experience. According to Hossō doctrine, individuals can be divided into “five natures” according to what sort of seeds they possess. Some have “active buddha nature,” that is, seeds that enable them to practice the bodhisattva way and become buddhas. Others have seeds that allow them to practice the path of the śrāvaka or the pratyekabuddha. These individuals can reach the nirvāṇa of the arhat, but they cannot become buddhas. Another group has a mixture of two or more of these three kinds of seeds: bodhisattva, śrāvaka, pratyekabuddha. Which kind of seed will develop is not predetermined; such persons are therefore said to be of “indeterminate nature.” Last, there are those who possess no untainted seeds and thus can never escape saṃsāra. They can, however, better their condition by accumulating merit through Buddhist practice.

Against the Lotus Sūtra’s claim that the three vehicles are the Buddha’s skillful means while the one vehicle is true, Hossō thinkers put forth this division of human capacity into five natures; they argued that the three vehicles of the śrāvaka, pratyekabuddha, and bodhisattva are true, while the one vehicle is a skillful method, designed by the Buddha to lead persons of the indeterminate group to follow the bodhisattva path and become buddhas, rather than taking the lesser path of the two vehicles. To support this argument, they invoked the Lotus Sūtra’s theme of “resuscitating” śrāvakas and restoring them to the bodhisattva path — as when the Buddha, in Chapter Three, reminds Śāriputra of his long-forgotten bodhisattva vow. Saichō, the Japanese Tendai founder, countered in part by drawing on Huayan (J. Kegon) thinkers to argue that suchness has not only a quiescent aspect as universal principle (fuhen shinnyo), but also a dynamic aspect that manifests itself as the concrete forms of the phenomenal world (zuien shinnyo). He also maintained that suchness has the nature of realizing and knowing. Thus, there was no need to postulate seeds in the store consciousness of only certain individuals as the cause of buddhahood. Saichō equated suchness in its dynamic aspect with active buddha nature, and because suchness is universal, everyone has the potential to realize buddhahood.