Why would physical-mental laws be unusual in a unified scientific theory?

In the context of a unified scientific theory, physical-mental laws are considered unusual because they do not fit into the established network of scientific principles that govern the natural world.1

Scientific theories generally aim to relate simple, fundamental entities, such as subatomic particles. In contrast, a physical-mental law would necessarily involve highly complex processes, such as brain activity involving millions of neurons, each of which is itself an intricate entity. Because these laws would appear as isolated, anomalous additions to the body of scientific knowledge, philosopher Herbert Feigl famously termed them "nomological danglers."1

Furthermore, the existence of such laws would create a situation unlike the rest of science, where phenomena are typically explained through a hierarchy of physical constituents. Materialists argue that these "danglers" are problematic because they suggest that mental events are not identical to physical events, thereby challenging the goal of achieving a truly unified scientific account of reality. Consequently, one of the primary objectives of materialism is to eliminate the need for these laws by demonstrating that mental phenomena are ultimately physical.213