I said to one of the scientists: “It seems very evident that due to changes in the chemical process of the brain, many of our own subjective experiences like perception and sensation occur: Can one envision the reversal of this causal process? Can one postulate that pure thought itself could effect a change in the chemical process of the brain?” I was asking whether, conceptually at least, we could allow the possibility of both upward and downward causation.

The scientist’s response was quite surprising. He said that since all mental states arise from physical states, it is not possible for downward causation to occur. Although, out of politeness, I did not respond at the time, I thought then and still think that there is yet no scientific basis for such a categorical claim. The view that all mental processes are necessarily physical processes is a metaphysical assumption, not a scientific fact. I feel that, in the spirit of scientific inquiry, it is critical that we allow the question to remain open, and not conflate our assumptions with empirical fact.”

–From The Universe in a Single Atom