mutual possession of the Ten Worlds ［十界互具］ ( jikkai-gogu): A principle formulated by T’ien-t’ai (538–597) on the basis of the Lotus Sutra stating that each of the Ten Worlds possesses the potential for all ten within itself. One of the component principles of T’ien-t’ai’s doctrine of three thousand realms in a single moment of life. “Mutual possession” means that life is not fixed in one or another of the Ten Worlds, but can manifest any of the ten, from hell to the state of Buddhahood, at any given moment. While one of the ten is manifest, the other nine remain latent, in the state of non-substantiality. The important point of this principle is that all beings in any of the nine worlds possess the Buddha nature. This means that every person has the potential to manifest Buddhahood, while a Buddha also possesses the nine worlds and in this sense is not separate or different from ordinary people.
From another viewpoint, the mutual possession of the Ten Worlds can be seen as indicating “the world of Buddhahood inherent in the nine worlds,” or “inclusion of Buddhahood in the nine worlds,” and “the nine worlds inherent in Buddhahood,” or “inclusion of the nine worlds in Buddhahood.” In his treatise The Object of Devotion for Observing the Mind, Nichiren (1222–1282) writes: “The ‘Expedient Means’ chapter in volume one of the Lotus Sutra states, ‘The Buddhas wish to open the door of Buddha wisdom to all living beings.’ This refers to the world of Buddhahood inherent in the nine worlds. The ‘Life Span’ chapter states: ‘Thus, since I attained Buddhahood, an extremely long period of time has passed. My life span is an immeasurable number of asamkhya kalpas, and during that time I have constantly abided here without entering extinction. Good men, originally I practiced the bodhisattva way, and the life span that I acquired then has yet to come to an end but will last twice the number of years that have already passed.’ Here the sutra refers to the nine worlds inherent in Buddhahood” (356–57).