The 'six realms of existence', sometimes also translated as the 'six states of samsara' or the 'six paths of rebirth', are the six main types of existence that sentient beings may be born into according to Buddhist cosmology. Karma drives this process - our actions and states of awareness from previous lives. According to Buddhist teachings, we will endlessly incarnate within these realms until we receive and practice the teachings, and through them break free of the karmas that pull us into the six realms, thereby attaining enlightenment. A human birth, which is one of the six realms, is the easiest realm in which to achieve this liberation, and is therefore the most auspicious birth.
The six-realm system is often depicted as a 'wheel of life' (shown to the right), especially in Tibetan Mahayana Buddhism. Theravada teachings typically define only 5 realms. Various schools of Buddhism also define other sub-realms within these, and other types of realms that are not desire-based, as these six are. In the Mahayana tradition, boddhisattvas may choose to incarnate in any realm, in order to aid the beings of that realm, as per the boddhisattva vow to continue to incarnate until all beings are liberated.
The realms are not only relevant for understanding rebirth, but also for understanding the karma associated with different states of awareness while we are alive in this life. Each realm has a certain mind-state that drives it, and the teachings on the six realms can help us to understand the risks and challenges of each mind-state.
The six realms are:
Deva or 'God' Realm - Defined by bliss and pleasurable states of all types, this realm is reminiscent of Greek myths about the realm of the gods. However, within Buddhism, this realm is not an immortal state, and also not the ideal one for attaining liberation, because we can become addicted to pleasure, including meditative bliss. If this happens, we become trapped, forgetting to work towards liberation, and falling into lower realms because of this forgetfulness and self-absorption.
Asura or 'Demi-God' Realm - Also pleasurable, this realm is nevertheless defined by jealousy and competitiveness. Although a birth here does offer more opportunities for pleasure than a human birth, here we are prone to coveting the pleasures of the Devas, which we can see (just as animals and humans can see each other.) In this state, we are prone to envy and/or a sense of victimhood - that we are not getting our fair share - and become fixated on evening the score. Theravada teachings generally do not recognize this as a realm separate from the Deva realm.
Human Realm - A middle realm, our human existence is defined by our ability and free will to experience any state, from blissful to hellish. It is therefore perfect for attaining enlightenment, because there is just enough suffering to motivate us to seek liberation (unlike in the god realms, where we are easily distracted by pleasure) but not so much that we cannot hear and practice the teachings (unlike in the lower realms, where we are so consumed by our suffering that we cannot practice.) From a human birth, we can cultivate the compassion and wisdom necessary to free ourselves from the entire wheel of samsara. In this realm we also have the most control over our future births, because we can influence our karma through our choices, whereas in the other realms we generally do not move into another birth until the karma that has brought us there has run its course.
Animal Realm - Within Buddhist cosmology, the animal realm is defined by ignorance, and an inability to think for oneself. Life is one-dimensional and survival-oriented, with little free will or choice. Therefore, as animals we do not have the capacity to hear or practice the teachings, although we may show signs of past practice in our temperament, i.e. compassion or intelligence.
Preta or 'Hungry Ghost' Realm - This realm is defined by constant desire and greed. In this realm, we are so overcome by our desire for more, more, more - whether food, drink, sex, wealth, or even certain emotional states - that we are consumed by it, and cannot focus on anything else. It is analogous to the state of an addict, in which getting the next 'fix' trumps all other concerns. In this state, we cannot practice the teachings because we cannot focus on anything other than our wants.
Naraka or 'Hell' Realm This realm is defined by hatred and rage, and by defining all other beings as enemies. Within this realm, there is no opportunity for compassion or desire for the teachings to arise, as all our momentum goes toward fighting others, and suffering the consequences. Depictions of this realm in various Buddhist schools is very similar to those found in other religions, with fiery torments. But within Buddhist cosmology this state, like all the others, is not permanent. Instead, when the negative karma that brought us here has run out, we will be reborn into another realm, with the possibility of working towards a human birth again.
According to the teachings, we may experience glimpses of each of these realms in our daily awareness, and through understanding their true nature and liberating ourselves from them through spiritual practice, halt the momentum of karmas that could cause us to incarnate into one of them. In this way, the teachings on the six realms are not simply teachings on rebirth, but teachings on awareness, its transience, and the dynamics of karma as it relates to our moment by moment awareness.
To explore karma and rebirth in more depth as taught within Tibetan Buddhism, consider: