Fundamentally, Ichinen Sanzen can be explained as follows: every thought (ichinen) encompasses three thousand (san-zen) worlds which make up the entirety of existence and the universe, or more simply, “the three thousand realms are contained in one mind.” The number 3,000 derives from multiplying the conceptual categories of “the mutual possession of the ten realms,” “the ten aspects of existence” and “the three categories of realms.” These basic concepts describe the states of existence. While “3,000” theoretically represents the existing worlds in the universe, the number is meant to describe vastness, universality, or infinity. Ichinen Sanzen of Ri refers to the theoretical principles found in the theoretical or “shakumon” section of the Lotus Sutra.

What is attaining Buddhahood? It is to awaken to Ichinen Sanzen and to achieve a great life transcending the limits of time and space. Therefore, Nichiren Shōnin said,

“The only way seemingly that leads to Buddhahood is the ‘3,000 in one thought’ doctrine.
(Kaimoku-Shō, WNS2, p. 108)

and also,

“Ichinen-Sanzen is like father and mother of a Buddha.”
(Shōmitsubō-gosho 3)

In other words, all Buddhas were born from Ichinen-Sanzen. He also said,

“This is not an immediate attainment of Buddhahood in this world (referring to women having to change before reaching Buddhahood), which can only be possible through the ‘3,000 in one thought’ doctrine.”
(Kaimoku-Shō, WNS2, p.90)