QUESTION: What is the scriptural proof for the “3,000 existences contained in one thought” doctrine?

ANSWER: I should say, there are two kinds. First, it is preached in the second chapter, “Expedients”: “Reality of all phenomena consists of their appearances, natures, bodies. … The Buddhas, the World Honored Ones, wish to open the gate to Buddha wisdom.” These are scriptural proofs showing the existence of 3,000 dharma worlds even in the moment of thought of unenlightened beings suffering at the bottom of delusions and evil passions.

Secondly, “The Life Span of the Buddha” chapter preaches, “Nevertheless, it has been numerous and limitless aeons since I actually attained Buddhahood. …” It represents the “3,000 existences contained in one thought” doctrine attained by Śākyamuni Buddha in the eternal past. Now in the Latter Age of Degeneration I, Nichiren, exert myself to disseminate this “actual 3,000 in one thought” (ji no ichinen sanzen) doctrine shown in “The Life Span of the Buddha” chapter. I have kept secret in my heart this doctrine of the three great secret dharmas. However, if I do not leave it in writing for the future, I am sure, my disciples would be sorry for my lack of compassion after my death. It is useless to regret then, so I am writing this to be sent to you. After reading it once, please keep it confidential. Do not show it to others or talk about it indiscriminately.

Sandai Hiho Honjo-ji, The Transmission of the Three Great Secret Dharmas, Writings of Nichiren Shōnin, Doctrine 2, Page 291