For Hare Krishna devotees, human nature is rooted in the immaterial self, the spiritual being that only temporarily inhabits the physical body. ISKCON calls this immaterial being the atman (eternal self) or jiva (being), two terms taken from classical Hinduism. As an immaterial being, the atman ultimately cannot feel pain or experience suffering, but because human beings confuse their material bodies with their immaterial selves, they do experience these negative states.
In addition to its immateriality, the self possesses another central characteristic: it is eternal. While the individual material bodies through which the atman passes always change -- being born, growing older, and eventually dying -- the atman exists throughout time, passing between such bodies. ISKCON calls this process transmigration or reincarnation. Because of the doctrine of transmigration, human beings have lived innumerable previous lives, and will continue to do so in the future. These include lives spent as humans, animals, plants, and even as beings living on distant planetary systems far from earth.
As an eternal spiritual being, the atman shares these characteristics with its creator, Krishna. ISKCON teaches that Krishna created all life, all atmans, at the moment of the origin of the cosmos, through a process of emanation. Much as a star emanates light and heat, Krishna emanated life from himself. Krishna gave of himself and created all beings out of his own essence, but did not diminish himself in doing so. All living beings therefore share the essence of Krishna, though as emanations, they lack the same potency as their creator. They also lack his personality and identity, since each soul is unique and individual. For these reasons, ISKCON explains that the human soul is both identical and different from Krishna. Some Hare Krishna scholars have called this position "nondualistic dualism," since it distinguishes God and humanity as separate beings (dualism), but does so in a way that they share essential characteristics (non-dualistic).
According to ISKCON philosophy, the individual soul's separation from Krishna represents a supreme disconnect, and one that humans must work to overcome. In the parlance of the movement, people must seek to go "back to Godhead," to return to the creator from whence they came. Functionally equivalent to what other Hindus call moksha, this return to Krishna represents the bliss of rejoining one's creator, and an end to the cycle of transmigration. However, the distinct nature of each soul means that the return to Godhead does not entail complete merger with the divine. ISKCON explains this concept anthropomorphically as the devotee living forever in the realm of Krishna, engaging in eternal service to and pastimes with the Lord.
ISKCON teaches that a person may attain such a state through the yogic system of bhakti (devotion), what members of the movement also call Krishna Consciousness. As a yoga -- a discipline -- bhakti requires a person to engage in certain acts according to prescribed methods. Hare Krishna founder A.C. Bhaktvedanta Swami formulated a specific bhakti yoga system drawn heavily from the Gaudiya Vaishnava tradition, but formulated for Westerners to understand. He called this system Krishna Consciousness, since it aimed to bring about a state of constant awareness and service to Krishna. Bhaktivedanta's books, notably The Science of Self-Realization, Chant and Be Happy, The Nectar of Devotion, and Perfection of Yoga detail the specifics of ISKCON's bhakti yoga system. These include chanting the name of Krishna, singing songs of worship, ritually venerating images of the divine, performing acts of devotional service to those images, and preaching about Krishna and Krishna Consciousness.
ISKCON, and in fact the entire Gaudiya Vaishnava lineage, recognizes five modes of bhakti that represent five means of forming a devotional relationship with Krishna. Called rasas, these five modes offer a progression through which an individual devotee can build their connection to Krishna. Shanta rasa, the rasa of reverence, emphasizes beholding the magnificence and majesty of the divine. The rasa of service, dasya rasa, offers a hierarchical means of offering devotion to Krishna, as one might serve a master or lord. The next two rasas both emphasize an increasingly loving relationship, that of friendship for God (sakya rasa) and parenthood of God (vatsalya rasa). Finally, the rasa system culminates in the shringara rasa, the relationship of intimate erotic love.
While the Gaudiya Vaishnava school holds the fifth of these rasas as most efficacious, in practice ISKCON combines them into a single system of bhakti. A Hare Krishna devotee might engage in shanta rasa through darshan -- the viewing of the divine -- at a temple in the morning, chant songs of devotion after that (dasya rasa), and then feed, bathe, and clothe a small image of the divine at his or her home (sakya and/or vatsalya rasas).