Tendai Lotus School Teachings, p 71Q: Does the mind of one thought merely contain the threefold truth and the ten suchnesses?
A: It also contains the trichiliocosm of one hundred realms and a thousand suchnesses.
Q: What is the trichiliocosm of one hundred realms and a thousand suchnesses?
A: One Dharma realm contains Ten Suchlikes, so the ten Dharma realms contain one hundred suchlikes. Also, each Dharma realm contains the other nine Dharma realms, so there are one hundred Dharma realms and one thousand suchlikes. Also, one Dharma realm contains three kinds of worlds, so the one hundred Dharma realms contain three thousand worlds, a trichiliocosm.
Q: Why do you establish a trichiliocosm of one hundred realms and a thousand suchnesses?
A: If we do not organize phenomena in terms of the one hundred realms, it would not be complete, the thousand suchnesses could not be verbalized, and causality would be excluded. Without the trichiliocosm, the world and we who live in it would not be exhaustively [explained].