Excerpt from The Flower of Chinese Buddhism
[From the book The Flower of Chinese Buddhism, by Daisaku Ikeda]
Earlier Buddhist thought had described the Ten Worlds, or ten realms into which beings may be reborn depending upon the karma they have accumulated in their past existences. These realms range from the lowest states of being, such as hell or the realm of hungry spirits, to the highest states, those of bodhisattvas and Buddhas. Earlier Buddhism had seen these states as mutually exclusive--that is, the individual could occupy only one state in a lifetime and moved from one state to another after the conclusion of that lifetime. In Zhiyi's system of thought, the Ten Worlds are multiplied by various factors that condition them to produce a total of three thousand possible worlds, that is, three thousand realms according to which life may manifest itself. Zhiyi then goes on to state that all of these three thousand possible worlds are present within each instant or "moment of life" of the individual.
Within a single lifetime, the individual is capable of moving back and forth any number of times from one realm to another. Thus, one may move upward through religious practice and striving until reaching the ultimate goal, the state of Buddhahood, without going through a lengthy series of rebirths. Or, conversely, the individual may, because of evil deeds or neglect of spiritual concerns, move downward in the scale toward the lower realms of existence.
This concept explains why it is possible to attain Buddhahood in this present lifetime without having to go through countless existences of arduous spiritual striving, as had been asserted in earlier Buddhism. At the same time, however, it also implies that enlightenment, once attained, is not a permanent condition but must constantly be supported and actively sustained to avoid sinking to a lower level of existence.