The standard objection to dualism was all too familiar to Descartes himself in the seventeenth century, and it is fair to say that neither he nor any subsequent dualist has ever overcome it convincingly. If mind and body are distinct things or substances, they nevertheless must interact; the bodily sense organs, via the brain, must inform the mind, must send to it or present it with perceptions or ideas or data of some sort, and then the mind, having thought things over, must direct the body in appropriate action (including speech). Hence the view is often called Cartesian interactionism or interactionist dualism. In Descartes’s formulation, the locus of interaction in the brain was the pineal gland, or epiphysis. [ … ]
How, precisely, does the information get transmitted from pineal gland to mind? Since we don’t have the faintest idea (yet) what properties mind stuff has, we can’t even guess (yet) how it might be affected by physical processes emanating somehow from the brain, so let’s ignore those upbound signals for the time being, and concentrate on the return signals, the directives from mind to brain. These, ex hypothesi, are not physical; they are not light waves or sound waves or cosmic rays or streams of subatomic particles. No physical energy or mass is associated with them.
How, then, do they get to make a difference to what happens in the brain cells they must affect, if the mind is to have any influence over the body? A fundamental principle of physics is that any change in the trajectory of any physical entity is an acceleration requiring the expenditure of energy, and where is this energy to come from? It is this principle of the conservation of energy that accounts for the physical impossibility of “perpetual motion machines,” and the same principle is apparently violated by dualism. This confrontation between quite standard physics and dualism has been endlessly discussed since Descartes’s own day, and is widely regarded as the inescapable and fatal flaw of dualism.
Daniel Dennett, Consciousness Explained (Backbay Books: 1991) pp. 33-35