Fodor (1981)
@The chief drawback of dualism is its failure to account adequately for mental causation. If the mind is nonphysical, it has no position in physical space. How, then, can a mental cause give rise to a behavioral effect that has a position in space? To put it another way, how can the nonphysical give rise to the physical without violating the laws of the conservation of mass, of energy and of momentum?@
Flanagan (1990) p.20-1.
@Descartes' mind-body dualism provides an elegant philosophical grounding for this portion of religious discourse which deals with personal immortality, with life after physical death. If humans are just exotic physical machines, as materialists believe, then the hypothesis that there is life after physical death is implausible since everything we know about bodies indicates that they die, decay, and disperse. If, however, our essence is nonphysical, then the possibility of immortality remains, since we have no evidence that nonphysical things die, decay, and disperse in the same way physical things do.@
On the other hand, and this to many people's way of thinking is the main disadvantage of Cartesian dualism, we have no evidence whatsoever that there are any nonphysical things. Furthermore, when we assume that there are nonphysical things we have to make some very implausible assumptions and give up some of our most cherished scientific principles, for example the principle, which Descartes espoused that ex nihilo nihil fit, that something cannot come from nothing. Just such a principle holds a central place among modern scientific principles under the guise of the principle of conservation of energy.
Now, the principle of conservation of energy requires that the total amount of energy in the universe remain constant, even as it is continually transferred and transformed in and among the myriad systems of causal relations. If Descartes is right that a nonphysical mind can cause the body to move, for example, we decide to go to a concert and go, then physical energy must increase in and around our body, since we get up and go to the concert. In order, however, for physical energy to increase in any system, it has to have been transferred from some other physical system. But the mind, according to Descartes, is not a physical system and therefore it does not have any energy to transfer. The mind cannot account for the fact that our body ends up at the concert.
If we accept the principle of the conservation of energy we seem committed either to denying that the nonphysical mind exists, or to denying that it could cause anything to happen, or to making some very implausible ad hoc adjustments in our physics. For example, we could maintain that the principle of the conservation of energy holds, but that every time a mind introduces new energy into the world—thanks to some mysterious capacity it has—an equal amount of energy departs from the physical universe—thanks to some perfectly orchestrated mysterious capacity the universe has. Unfortunately, such an assumption is totally unwarranted except as a way of saving Cartesian dualism, and, therefore, utterly begs the question.
@Conservation of energy dictates only that the energy in a closed physical system is constant, not also how that energy is distributed within the system. Since mental events could effect bodily changes by altering that distribution of energy, the conservation principle does not preclude minds' having bodily effects.@