One of the major objections to the notion that we possess an immaterial mind that somehow works in concert with our material brain is that interaction between two radically different substances is incomprehensible. We can comprehend how one material thing can interact with another because we see it all the time, the materialist says, but how can an immaterial thing like mind cause an effect in a material thing like a brain? Since the dualist cannot explain how this is possible their belief in mind/brain interactionism is thereby believed to be discredited.
As physicist Stephen Barr points out in a book review in the January First Things (subscription only), however, few who pose this objection to dualism stop to ask how it is that anything interacts with anything else. The materialist (one who believes that matter is all there is) assumes that matter can interact with matter, but if you ask a materialist to actually explain the interaction you draw a blank.
The same explanatory problem confronting the dualist confronts the materialist whenever he tries to understand how two material objects interact or how a force like magnetism interacts with matter, or how matter interacts with space. We simply don't know how these phenomena take place.
Barr writes:
Material bodies are made up of electrically charged particles that interact with each other through the mediation of electromagnetic fields: The charged particles affect the fields and the fields affect the particles. By what "means" or "mechanism" this happens physics does not say. It simply says that when electromagnetic fields are present, the charges are, in fact, affected as described by a certain equation; and when the charges are present the fields are affected as described by another equation. In other words, physics posits two types of entities and mathematically describes, but does not otherwise explain, their influence on each other.
Gravity poses the same problem. We have no idea how gravity "pulls" objects toward a massive body, or, for that matter, how a massive body produces gravity in the first place, or even what gravity is. If we fall back on the Einsteinian explanation that gravity is simply the bending of space around a material object then we've simply pushed the problem back a step. How, it can be asked, do material bodies bend space? If space is essentially "nothing" how is it affected by a "something" in its midst?
The materialist has no answer, but that doesn't stop him from believing it nor from disparaging the dualist for thinking that mind and brains interact. Pretty ironic, don't you think?
RLC