QUESTION: It is not entirely clear that the six realms of illusion exist in the realms of human beings, but I am beginning to think they seem to as I listen to you. Nevertheless, I cannot see the four realms of holy ones at all. What do you say about this?
ANWSER: You doubted the existence of six realms of illusion in the realm of men, but I tried hard to explain it until you said you seemed to understand it. The same might happen with four realms of holy ones. Therefore, I shall try to explain as much as possible what the sūtras state, supplemented with reason. We see the so-called principle of impermanence everywhere in front of our eyes. We humans understand this principle, through which two groups of Hinayāna sages (Two Vehicles) called śrāvaka and pratyekabuddha try to obtain enlightenment (arhatship). How can we say then that the realms of two Hinayāna sage groups are not included in the realm of men? A man, no matter how inconsiderate he may be, loves his wife and children. It shows that he is partly in the bodhisattva realm.
The only realm contained in the realm of men and yet hard to see is that of Buddhas. However, since we see nine other realms included in the realm of human beings, we can conjecture that the realm of Buddhas, too, is contained therein. You should firmly believe this and have no doubt about it. On the existence of the realm of Buddhas contained in the human realm, the Lotus Sūtra states in the second “Expedients” chapter that the purpose of Buddhas appearing in the world was “to cause all living beings to open the gate to the insight of the Buddha.” And in the Nirvana Sūtra, we come across a passage which states: “Though having only human eye, those who study Mahāyāna Buddhism are regarded the same as having Buddha-eye because they see the truth of Buddhism.” The reason why we, ordinary people, born in the Latter Age, can put faith in the Lotus Sūtra is that the realm of Buddhas is included in the realm of human beings.
Kanjin Honzon-shō, A Treatise Revealing the Spiritual Contemplation and the Most Verable One, Writings of Nichiren Shōnin, Doctrine 2, Page 135