More sympathetic philosophers have responded by noting that our inability to imagine how two substances such as an immaterial mind and a material brain can interact is not necessarily a reason for thinking that they don't. After all, it's very difficult to imagine how matter can bend empty space or gravity can pull objects or electromagnetism can attract and repel material substances, yet even before any possible explanations for these phenomena were advanced everyone believed they happened.
Apart from such examples, everyone knows that banging one's thumb with a hammer causes the sensation of pain but the sensation of pain is immaterial while the cause, hitting one's thumb, is physical. We haven't the slightest idea how stimulating certain nerve fibers produces the sensation of pain, but anyone who doubts that it does is welcome to try the experiment for himself.
Despite such counterarguments, however, the interaction problem persists. Neurosurgeon Michael Egnor at Mind Matters offers another interesting counterargument based on the distinction between a material object and its form (the properties of a thing which make it what it is).
He cites a property chemists call chirality. Chirality is the property of some molecules to exist in two different forms which are mirror images of each other, much like a man's right hand is a mirror image of his left.
Egnor writes that,
The interaction between an immaterial entity and matter is not as problematic as materialists would have us believe.If the matter of two molecules is identical but the properties are different then something other than the matter is responsible for the properties. In the case of chiral molecules, Egnor argues, the immaterial form that the molecule takes determines what the physical properties of the molecule will be even though both forms of the molecule are materially identical.
It is the property of certain things to exist as mirror images of each other; my right and left hand are an obvious example. From the standpoint of matter, my hands are identical. They have the same bones, muscles and nerves. What makes them different is their form. My right hand is a mirror image of my left hand. That is, it is the form alone — the organization of my hand — that makes the difference, rather than the matter of which my hand (right or left) is composed.
Chirality is a property of asymmetry. The same chirality exists in many molecules and chirality plays an essential role in biochemistry. Most biomolecules are chiral, and the function of biomolecules is tightly linked to their chirality. “Right-handed” glucose is metabolized by the body. “Left handed” glucose is not.
The difference between the smell of oranges and of lemons is due to chirality. The molecule — limonene — smells like lemon or orange according to its chirality alone — to its form, not to its matter.
There are more troubling examples. One chiral form of thalidomide is harmless to humans. The other chiral form of thalidomide is highly toxic to developing embryos (thalidomide babies). All amino acids (the building blocks of proteins) in living things are L-amino acids (‘left-handed’). D-amino acids (right-handed) are not found naturally in proteins, but are found in some bacterial cell walls.
The relevance of chirality to substance dualism and the interaction problem — how can something immaterial interact with something material — is that chirality is a beautiful example of how form alone—immateriality — can determine the properties of matter. Chiral substances are materially identical but formally different — different only in their organization, not in the matter of which they are composed.
Immaterial properties can completely determine the functional powers of matter.
Perhaps the interaction between mind and brain is something like that.