In a letter to a follower Nichiren wrote:

“There are two methods of contemplating the three thousand realms in one thought-moment. One is that of principle (ri), and the other that of actuality (ji). In the time of T’ien-t’ai [Chih-i] and Dengyo [Saichō], that of principle was appropriate. Now is the time for that of actuality. Because this [form of] contemplation is superior, the great ordeals [attending its propagation] are also more severe. One [i.e., that of principle] is ichinen sanzen of the trace teaching (shakumon), while the other is ichinen sanzen of the origin teaching (honmon). They differ, differ vastly, like heaven and earth.”

“Now” is of course the Final Dharma age: Nichiren’s “three thousand realms in one thought-moment” represents teaching and practice expressly for the time of Mappō. But what exactly is the “vast difference” he claims between his teaching of ichinen sanzen and that of the earlier T’ient’ai/Tendai masters?

First of all, the passage just cited alludes to a difference in textual locus. … Chih-i derived the concept of the three thousand realms in one thought-moment from the trace teaching or first fourteen chapters of the Lotus Sūtra; it represents his attempt to conceptualize the “true aspect of the dharmas” (shohō jissō) referred to in the “Skillful Means” chapter. For Nichiren, however, this doctrine “is found only in the origin teaching of the Lotus Sūtra, hidden in the depths of the text of the ‘Fathoming the Lifespan’ chapter.” This shift of textual grounding for the ichinen sanzen doctrine is related to the larger context of medieval Tendai thought, which … saw the latter part of the sūtra as superior to the former in its revelation of nonlinear, original enlightenment. (Page 264)