Embracing Physicalism in Buddhism (from Buddhism and Techno-physicalism: Is the Eightfold Path a Program?, by Mark Siderits, in Philosophy East and West, Honolulu 2001)
If embracing physicalism [i.e. that all that exists is physical in nature] means leaving open for the present whether or not to accept the doctrine of karma and rebirth, then we must ask how crucial this doctrine really is to Buddhism. What I would suggest is that while this doctrine has played an important role in many Buddhist cultures, it is not crucial to the central project of Buddhism. Indeed, if I take myself to live only one life instead of the indefinitely many lives promised by rebirth, then the fact of my own mortality takes on even greater significance, for I cannot then defer seeking a solution to the problem of suffering to some future life. Now within many Buddhist cultures it has been thought that some persons are unable to seek and attain Nirvana in this life. The doctrine of karma and rebirth holds out the promise to such people that if they perform karmically meritorious acts in this life, they will be reborn in more auspicious circumstances in which the attainment of Nirvana will be easier. So if karma and rebirth were rejected, then since Nirvana would not be open to all, this might make the Buddhist path seem less appealing. (Of course this would not show that the Buddhist analysis is itself false.) But we must ask why Nirvana is thought to be unattainable for some individuals in this lifetime. If this is simply because they find the path too difficult compared to the attractions of mundane life, then perhaps Buddhist need to redouble their efforts to convince these people of the truth of suffering. If, on the other hand, Nirvana is unattainable for some due to such life circumstances as extreme poverty and degradation, then it would seem incumbent on Buddhists to work to eliminate such social evils and thus make Nirvana genuinely available to all.
One sometimes hears it said that in the absence of the doctrine of karma and rebirth (or some other doctrine promising ultimate retribution for immorality), people would have no reason to obey the dictates of conventional morality. But even if this were true, it is not clear why this would constitute a reason for Buddhists to espouse the doctrine. And in fact, Buddhists have good reason to reject this claim. On the basis of the doctrine of nonself it is possible to construct an argument for a general obligation to seek to prevent pain regardless of where it occurs. That is, the doctrine that is central to the Buddhist project may itself be used to support a basic duty of beneficence, arguably the core of all forms of conventional morality. So if it is essential for a spiritual path to provide some support to conventional morality, Buddhism can do so without reliance on the doctrine of karma and rebirth.
So far we have been discussing the central project of Buddhism as taught in early Buddhism and Abhidharma. I said earlier that the Mahayana teaching of the essenceless of the elements might complicate matters. In Madhyamaka this doctrine is taken to mean that the very notion of how things ultimately are is empty. So there is no ultimate fact of the matter as to whether reality is wholly physical, both physical and mental, or only mental in nature. According to Madhyamaka we should, however, embrace at the conventional level whatever account of the world best accords with successful practice. So if physicalism should turn out to cohere better with our going theories, then Madhyamaka would grant it the status of conventional truth.
It is with Yogacara [Vijñanavada] that real difficulties arise. For this school the doctrine of the essenceless of elements is taken to indicate their ultimate nature, specifically their ineffability. And while it would of course be a mistake to say that ineffable elements are mental in nature, Yogacara does claim that it would be nearer the truth to say that they are mental than that they are physical in nature. (Note: This is because for Yogacara the path to the realization of the ineffability of the real goes through the doctrine of impressions-only as a key stage: one first realizes that there could only be inner impressions and not external objects, then sees that the notion of the mental relies crucially on the distinction between “inner” and “outer”, and thus one abandons any attempt at characterizing the reals.) So this school’s views are incompatible with physicalism. And Yogacarins claim that their idealist teaching of impressions-only represents the most effective way of realizing the truth of nonself. If this is correct, then the Buddhist project is indeed incompatible with physicalism. But Abhidharmikas and Madhyamikas deny that embracing an idealist metaphysics is required in order to attain the fruit of the Buddha’s teachings. And there are interesting and complex arguments developed on all sides in this dispute.