Underlying many speculations about the origins of beliefs in rebirth or reincarnation is an assumption about the relation between metaphysics and ethics. Roughly speaking, the assumption is that ethical outlooks are grounded in metaphysical beliefs or theories, and hence that, in any given case, a metaphysical conception of the world is prior, logically and chronologically, to the ethical outlook. I call this the assumption of metaphysical priority. Among the theories in which we see this assumption at work is that developed by the anthropologist Gananath Obeyesekere in several publications since the late 1960s, which concerns how conceptions of rebirth evolved from ‘non-ethicised’ to ‘ethicised’ forms. Equating ‘ethicised’ with ‘karmic’, Obeyesekere posits a transition from ‘rebirth eschatologies’ to ‘karmic eschatologies’. According to this view, rebirth eschatologies are conceptions of what happens after death that involve rebirth but lack a distinctive ethical dimension, whereas karmic eschatologies incorporate into their conception of what happens after death the idea that rebirth is conditioned or determined by the ethical quality of one's actions in the preceding life. Although his thesis applies primarily to traditions deriving from South Asia – most notably Hindu, Buddhist and Jain traditions – Obeyesekere acknowledges similarities with conceptions of transmigration among certain Greek thinkers, noting for example that Pythagorean theory ‘perhaps’ constitutes a karmic eschatology.

The assumption of metaphysical priority is, however, questionable. It has been poignantly challenged by Catherine Osborne, who argues that certain metaphysical theories of transmigration might in fact have emerged from the ethical commitments that their proponents held prior to devising the metaphysical the ories. Although Osborne's claim is made with specific reference to ancient Greek philosophers and is not advanced as a direct response to Obeyesekere, the challenge it represents has wider implications. It forces us to question whether metaphysical conceptions must precede and undergird ethical values and practices, and thereby obliges us, when clear historical evidence is lacking, to refrain from simply assuming that they do have this undergirding role.