With birth comes consequences and death. With death comes rebirth. The cycle, endless as it might be for most, has an internal logic: Karmic imprints from one life determine the circumstances of rebirth into the next. Karma, an impersonal force, has no agenda. While many religious traditions have personified deities according to their various roles relative to the turning of the karmic wheel of rebirth, the process is one largely left up to the being currently living a life. Karma is not only a prison in the past; it is also the key to freedom in the future.
Modes of desire (or sense-)-based existence in Samsara are sixfold (three “lower” and three “higher”): hell beings bring up the rear, hungry ghosts (pretas) are one step closer to the top, then animals, then humans, then two modes of gods (asuras and devas). Although, most Pali sources divide the deva (god) realms into a variety of divinities.
The desire realms possess symbolic value. The lowest realm (or least desirable, as the realms are commonly depicted as a wheel and not a ladder) is the hell realm—populated by what we call demons. These beings are characterized by their extreme anger. The next realm is the hungry ghost realm. These beings possess an insatiable hunger which can never be satisfied. Hungry ghosts are depicted with huge stomachs and tiny mouths. Next comes the animal realm. Animals, lovable and beautiful as they are, embody ignorance. They possess only instinct and no capacity for self reflection. The next realm is the human realm. The human realm is the only realm from which one can achieve Nirvana, because this realm contains just the right mixtures of delusion and wisdom, pain and pleasure, and anger and good will. The last two realms are the god realms. First are the “lower” gods; the lower gods are jealous gods. The last and “higher” gods are gods without jealousy who dwell in a constant state of bliss. The envy and bliss, respectively, are barriers to Nirvana for the two godly classes.
In this system, humans are the fortunate ones. Pure, blind bliss necessitates no change. Likewise for constant jealous striving. Same for ignorance, craving, and anger. However, the six sense realms are not the only realms of existence.
The pure form world resides “above” the six desire realms. The form world and its realms are attainable to those who practice the four jhanas of meditation. The importance of the practice of the four jhanas is best understood when taking into account that the first of four jhanas is said to be eliminated (along with the six “lower” sense realms) during periodic contractions of existence. Only the second through fourth jhanas remain, and, along with them, the corresponding form realms.
The form realms progress from a “gross to subtle” level, just like the six sense realms. The lowest level of the form realms, Brahma’s Retinue, maintains the vestiges of the lower realms of existence with all their trappings of categorization and attachment. The highest level, The Supreme, approaches the elimination of all distinctions.
The formless world awaits “above” the pure form world consists of four realms: infinite space, consciousness, nothingness, and neither consciousness or un-consciousness.
In the Buddha’s worldview, even the Brahmanical gods and goddesses are bound to the cycle of dependent origination and rebirth. He did not simply reject the gods like some of his peers. Instead, he brought the gods into his own system of cause and effect.
Many Buddhist texts describe the deeds in one life (gati) that will determine rebirth in the next. For example, an insatiable appetite for sense pleasures is associated with rebirth as a hungry ghost—beings with giant stomachs, long, thin necks, and small mouths incapable of eating enough food to satisfy their ravenous appetites.
The cosmological and cosmogonical systems of ancient India shaped Buddhism. Mahayana thinkers explain away the Buddha’s affirmations of Brahmanical beliefs by stating that the Buddha—indeed, any Buddha—suits the teachings to the circumstances. Regardless, the man who lived and walked the earth as Siddhartha Gautama spoke to his followers in Brahmanical terms. To any Brahmanical thinker, the escape from the cycle of Samsara is the ultimate goal. Buddha did pay occasional deference if Brahmanical theism, but did not emphasize the Atman/Brahman unity that so many of his predecessors and contemporaries favored. The ages of time, or the kalpas, were concepts of pre-Buddhist Brahmanical thought. Buddhism merely adopted the kalpas and samsara to suit its own needs.
The cycle of Samsara, a very real thing to Buddhists and like-minded thinkers, is symbolically represented by the Bhavacakra–the wheel which turns the cycle of rebirth of beings into, and out of, the three worlds.