Three Thousand Realms

The 'doctrine of tree thousand realms in a single moment of life' is the supreme philosophical achievement of the Great Teacher T'ien T'ai (538-97) of China. It is set forth in Great Concentration and Insight, a compilation of T'ien T'ai's lectures recorded by his disciple, Chang-an. Based on the Lotus Sutra, this doctrine clarifies the ultimate oneness of the absolute reality (Buddhahood) and the phenomenal world (environment).
A life-moment refers to the state of a person's life at any given moment and 3000 refers to the diversity of potential states it can assume at that moment. T'ien T'ai explained that a life-moment contains within it the Ten Worlds, their mutual possession, the ten factors and the three realms. In this way he demonstrated that all phenomena - body and mind, self and environment, sentient and insentient, cause and effect - are integrated in the life moment of the common mortal.
T'ien T'ai's doctrine is called the theoretical three thousand realms because it refers in theory to the life of Buddhahood latent in all people. In Nichiren Daishonin's Buddhism, three thousand realms refers to the method of attaining Buddhahood in this life, and is referred to as actual three thousand realms.