This doctrine forms the essence of the Makashikan (Mo-ho-chih-kuan), a twenty-section work in which T’ien-t’ai Chih-i of China systematized various teachings included in the Lotus Sutra. This was his new interpretation of the Lotus Sutra. Nichiren regarded Chih-i’s doctrine of the Three Thousand Realms in One Mind as the essence of the Buddha’s teachings.

In the second section his work Kaimoku-shō (Essay on the Eye-opener), Nichiren extolled this doctrine in the following words: “Unless man attains buddhahood through the teaching of the Three Thousand Realms in One Mind, his attaining nirvana and becoming a buddha will be little more than a mere name.” In the first section of the same work, he also praised the doctrine as follows: “The very doctrine of the Three Thousand Realms in One Mind of the Tendai sect appears to be the way to lead man to buddhahood.” Nichiren specifically extolled this doctrine a total of eighteen times in his works.

What is the teaching of the Three Thousand Realms in One Mind? As has been mentioned, our minds revolve ceaselessly in the six worlds of hell (anger), hungry spirits (covetousness), animals (ignorance), demons (dispute), human beings (normality), and heaven (joy). Thus our sufferings continue endlessly.

However, even an ordinary person sometimes rises to the level of the four realms of the saints. He conceives the desire to study the right way to live (śrāvaka), realizing it intuitively from his experiences (pratyekabuddha), and wishing to live for the benefit of people and society (bodhisattva). But he will seldom if ever reach such a mental state of absolute compassion that he completely forgets himself. It would be a great thing if he could maintain such a mental state constantly. But soon his mind returns to that of an ordinary person without his having made any lasting improvement. …

Everyone’s mind possesses the ten realms, the six of the ordinary person and the four of the saint. These ten realms exist in the minds of heavenly beings, as well. The ten realms exist in the mind of each person in each of the ten realms. This is jikkai-gogu, or “the ten realms of being found in one another.”

The seed of the buddha-nature is also possessed by those who are in the worlds of hell and demons, although it is very undeveloped. The doctrine of the Three Thousand Realms in One Mind teaches that even those who are in such a state of mind have the possibility of attaining buddhahood and that the chance of salvation can be found anywhere. In other words, this doctrine teaches plainly that the Buddha’s compassion extends to all living beings. On the other hand, even when one thinks he has realized and become free of the bonds of illusion and suffering, by studying the doctrine he will become able to reflect on his remaining seeds of illusion and will come to wish to devote himself still more to his practice. …

The Ten Suchnesses: This doctrine consists often words prefaced by “such a” or “such an” : “such a form” (nyoze sō), “such a nature” (nyoze shō), “such an embodiment” (nyoze tai), “such a potency” (nyoze riki), “such a function” (nyoze sa), “such a primary cause” (nyoze in), “such a secondary cause” (nyoze en), “such an effect” (nyoze ka), “such a recompense” (nyoze hō), and “such a complete fundamental whole” (nyoze hommatsu kukyō-tō). This doctrine reveals the deepest reality of the existence of all things in the universe, which is called the principle of the Reality of All Existence (shohō jissō). Modern science has analyzed physical substances to the extent of subatomic particles. But the principle of the Reality of All Existence is much more profound than such an analysis, extending even to the mental world. The Chinese character read nyo means shinnyo, meaning “that which is constant and unchanging.” Nyoze means “such” or “thus,” and also “invariably,” “without fail,” or “without error.” …

Buddhism teaches that there are three ways of thinking about society. The first is go’on seken, or the world where an individual mind exerts an influence upon others, in other words, environment in the narrow sense. The second is shujō seken, or the world of which its living beings are regarded as a constituent. Generally speaking, this is a society or nation. The third is kokudo seken, or the world consisting of many societies and countries. This is commonly considered to be the whole world.

We all form part of these three kinds of worlds, which coexist, whether we like it or not. The one thousand functions of our minds are spread over these three kinds of worlds. One thousand multiplied by three is three thousand; thus, all relations in the three constituents of the world (sanzen) are included in a single momentary thought (ichinen) of a human being in his daily life. Hence the term “Three Thousand Realms in One Mind” (ichinen sanzen).