The Wheel of Becoming
"The Bhavacakra, or Wheel of Becoming, is a dynamic depiction of samsara, the endless round of existence. At the hub of this wheel are the three qualities that create suffering in all states of existence, also known as the three poisons--greed, hatred, and confusion. The six divisions in the spokes of the wheel represent the six realms of existence, divided into an upper half and a lower half. The uppermost realm is the sphere of the long-lived devas, or gods; to one side is the human realm, and to the other the realm of the asuras, or "jealous gods." The lower divisions represent existence in the realms of the animals, pretas, and the hells.
Buddhism teaches that enlightenment is only possible in the human realm. Surrounded by beauty and pleasure, and unfamiliar with suffering, the gods lack motivation for enlightenment. Obsessed with passion and striving, the jealous gods waste their opportunities. Opportunities for enlightenment also diminish in the lower realms of animals, preoccupied with survival and bound to emotionality; in the realm of the hungry ghosts, perpetually needy, unable to obtain satisfaction; and in the realms of the hell beings, continually burning with pain or frozen in the depths of despair.
Around the rim of the wheel are the twelve nidanas, the links in the chain of dependent origination known as pratityasamutpada. Beginning with ignorance and ending with birth, old age, and death, sentient beings reenact with each moment this interconnected chain of causation. Each stage leads inexorably to the next, ensuring the continual turning of the wheel of becoming. Thus sentient beings are continually in motion; now in the heavens, then in the hells; now torn with passions, then open and receptive. Holding the wheel firmly in his grasp is Yama, lord of death, the end of all beings, even the gods. The Buddha, the fully enlightened one, stands completely outside the wheel; he teaches beings how to break the chain of causation and escape this endlessly turning wheel of becoming." -back description