As a cyclical process, a corollary of karma doctrine, although the idea probably had independent origins, based on the observation of natural phenomena. It first appears in textual form in the Bṛhadāraṇyaka Upaniṣad (Chs. 4, 6) and Chāndogya Upaniṣad (Chs. 4, 5). Under the influence of renouncer ideologies, rebirth comes to be equated with suffering, and both suffering and rebirth (the cycle of saṃsāra) will continue unless the individual can intervene in the process. Rebirth thus provides a negative contrast to the renouncer's ultimate goal of liberation (mokṣa), which brings a permanent end to death, rebirth, and (in many cases) embodiment (i.e. reincarnation). With the development of elaborate cosmologies, the refinement of karma doctrine, and the need to accommodate householder aspirations, some rebirth states are portrayed as relatively better than others. It therefore becomes possible to aspire to a better, or a ‘good’ rebirth—even a rebirth in one of the heavens—on the understanding that this state will not last forever, and that only mokṣa can bring permanent release. With the rise of bhakti movements, this idea is further modified, so that, for some, only rebirth as an inhabitant of the deity's heaven (perhaps through the grace of the deity itself) will bring a permanent end to the cycle. A common signifier of an individual's yogic or spiritual power is the ability to remember his or her previous births, an attainment not normally given to ordinary individuals. See also karma; devayāna; pitṛyāna.