[T]he “three thousand realms” denotes “all dharmas” or “all phenomena.” In that sense, the number “three thousand” might be considered somewhat arbitrary. Nonetheless, it refers to a constant set of patterns that for Zhiyi constituted the “real aspect of the dharmas.” Because each one [of the ten dharma-realms, from hell to buddhahood,] contains all ten within itself, there are a hundred realms, each endowed with the ten suchnesses. The resulting thousand realms each entail another “three realms” or three aspects of living beings: (1) the “five aggregates (skandhas),” or momentary mental and physical constituents that unite temporarily to form living beings; (2) living beings considered as individuals belonging to one or another of the ten realms, such as hell dwellers, hungry ghosts, humans, and others; and (3) the insentient container worlds, or environments, that living beings inhabit.

In translating, we often say “three thousand realms in a single thought-moment” because that is natural English, but, strictly speaking, it is not correct. As Zhiyi goes on to explain: “Were the mind to give rise to all phenomena, that would be a vertical [relationship]. Were all phenomena to be simultaneously contained within the mind, that would be a horizontal [relationship]. Neither horizontal nor vertical will do. It is simply that the mind is all phenomena and all phenomena are the mind. [This relationship] is subtle and profound in the extreme; it can neither be grasped conceptually nor expressed in words. Therefore, it is called the realm of the inconceivable.”

In essence, the most minute phenomenon (a single thought-moment) and the entire cosmos (three thousand realms) are mutually encompassing: the one and the many, good and evil, delusion and awakening, subject and object, self and other, and all sentient beings from hell dwellers, hungry ghosts, and animals up through buddhas and bodhisattvas as well as their corresponding insentient environments — indeed, all things in the entire cosmos — are inseparable from the mind at each moment. However, only in the state of buddhahood is this fully realized. Zhanran comments, “You should know that person and land both encompass three thousand realms in one thought-moment. Thus, when we attain the way, in accordance with this principle, our body and mind at each instant pervade the dharma-realm.”