The third common theme of karma theories is the concept of reincarnation or rebirth (saṃsāra). Rebirth is a fundamental concept of Hinduism, Buddhism, Jainism and Sikhism. The concept has been intensely debated in ancient literature of India; with different schools of Indian religions considering the relevance of rebirth as either essential, or secondary, or unnecessary fiction.Karma is a basic concept, rebirth is a derivative concept, so suggests Creel;[35] Karma is a fact asserts Yamunacharya, while reincarnation is a hypothesis; in contrast, suggests Hiriyanna, rebirth is a necessary corollary of karma.