I have argued against Evan Thompson that philosophical texts are the proper source for philosophers, so let me now turn our discussion there: specifically to Śāntideva, whom both of us cite.
First let us be clear about two points on which I think Thompson and I agree. The first of these points is that Śāntideva himself believes in rebirth, and this concept deeply suffuses his philosophy; Thompson and I agree about that. The second is that Śāntideva is wrong in this belief: though Ian Stevenson’s kind of work does present a potential anomaly, the best evidence we have in psychology still shows us that human consciousness is tied ineluctably to human bodies, and when that body dies, the consciousness dies with it. As far as I can tell, Thompson accepts this latter proposition. If he does believe human consciousness is reborn at death, my apologies: in that case we are having a very different conversation, and I would be genuinely intrigued to hear his reasons for such a belief. Thompson has not said anything of the sort in the conversation to date, however, so I will proceed in the present discussion on the assumption that he does not.
The question then is how a contemporary Buddhist who accepts both of these points should read Śāntideva’s work. It was specifically in answer to this question that I first turned to a naturalized theory of karma: I did so because I wanted to take Śāntideva as seriously as possible. My dissertation was all about understanding Śāntideva’s reasoning at a deep level, so I looked in detail at the kinds of arguments and reasons Śāntideva offers for acting or feeling one way and not another – what Thompson calls their “warrant and motivation”. As the dissertation discusses, these reasons for action generally fell into three categories, not always separable from each other: the pleasant and unpleasant mental states the actions generate; metaphysical insight into the nature of things, especially their emptiness (which I explored in more detail in a later article; and good or bad karma. Only the last of these three is closely tied to rebirth. The latter terms “good and bad karma” are specifically my translations of puṇya and pāpa, terms ubiquitous in Śāntideva’s work; for him it is puṇya and pāpa, rather than karmaphala or karmavipāka, that most describe the process by which good and bad actions lead to good and bad results.