At an early period, during the formation of the Upanişads and the rise of Buddhism and Jainism, we must envisage a common heritage of meditation and mental discipline practiced by renouncers with varying affiliations to non-orthodox (Veda-rejecting) and orthodox (Veda-accepting) traditions... These schools [such as Buddhism and Jainism] are understandably regarded as heterodox ("nāstika") by orthodox ("āstika") Brahmanism.