Materialism is a very popular thesis among philosophers of mind that says that only physical things can causally interact. The philosophical term is: “the physical is causally closed”. That means that from everything we know about causality and physics today indicates that only physical, measurable things can provide changes in other physical and measurable things, and no other things can provide these changes. Usually (at least in a Godless Europe) this is accompanied by a belief that none of these immeasurable immaterial things exists at all.

The more technical description of the problem goes as follows:

1. The mental and the physical are distinct things.
2. The mental and the physical has causal interaction.
3. The physical world is causally closed.

For most philosophers of mind, this leads to the conclusion that dualism is wrong, and that body and mind are made of the same physical matter.

For those inclined towards dualism this has usually been attempted solved by showing that the mental and the physical can interact, which implies that 3) is not true, or by showing that 1) is not true. Of course, for a philosopher, this leads to all sorts of different problems which I may or might say something more about at some point in time.