Helmholtz: All natural phenomena can be explained mechanistically. And anything that can be so-explained abides by the law of conservation of energy. This law states that energy does not just dissipate into thin air; it gets transferred from one operation to another. Nor can energy come from nowhere; ex nihilo nihil fit. So what of the mind? The mind, too, looks like it must abide by this universal physical principle. So the mind awaits a mechanistic explanation.

Descartes: It is here that we face our dilemma. Presumably, in your cold image, we cannot uphold our intuition that the mind is something distinct from the materials which comprise it. One is subjective and non-extended; the other is objective and extended. We have all the reason to think the two are distinct.

Helmholtz: Your conclusion seems to be that minds are not really natural phenomena at all. Things which defy scientific explanation, perhaps.

Descartes: That sounds about right.

Helmholtz: Explaining psychophysical causation will be difficult, you know.

Descartes: Why do we need to explain it?