The expression "three thousands" (三千) does not indicate a numerical or substantical immensity, but intends to show the inter-permeation of all dharmas and the ultimate unity of the whole universe.
As the basis of "three thousands" the school sets forth a world system of ten realms (十界). That is to say，the world of living beings is devided into ten realms, of which the higher four are saintly (四聖) and the lower six are ordinary (六凡).
These ten realms are mutually immanent and mutually inclusive (十界互具)，each one having in it the remaining nine realms. This immanence of each of the ten worlds in all of them accounts for 100 worlds (百界).
Further，each of these realms has ten features of Thusness (十如是). That is form，nature，substance， force，action，cause，circumstance，effect， remuneration， and the ultimate state. By discovering these ten features in the 100 worlds, this school arrives at the doctrine of 1000 realms (千如).
Moreover，each realm consists of three divisions (三世間)：the species of living beings (眾生世間), the species of space-region (國土世間)，and the spacies of five aggregates (五蘊世間). Thus there are three thousands worlds (三千世間)，constituting the whole of manifested reality. The universe of all beings are the whole world of dynamic becoming.
In the end，all the dharmas of the universe are immanent in one moment of thought. Namely，that the one moment of thought is itself 3000 worlds (一念三 千).
