Back to the mysteries of consciousness. I argued here that subjectivity, surely one of the most basic and intuitively obvious aspects of consciousness—is a fundamental and often overlooked problem for physicalism, the notion that the universe is comprised solely of physical entities subject to physical laws. But why settle for physicalism in the first place? Why not believe in Cartesian (or substance dualism), the idea that we have separate souls that influence our minds, our thoughts, and our actions? The answers help shed light on what I find so puzzling and frustrating about subjectivity.
First, a clarification on what "physicalism" really entails. A physicalist would not say that the only things in the world are physical things—quite obviously, there are other things, like thoughts, ideas, and abstract concepts—but only that all non-physical things derive their properties, or are dependent on, physical things. For the philosopher of mind, the most interesting things that appear at first glance to be non-physical are mental properties or events, but the physicalist says that all mental properties depend on, or supervene on, physical properties.
The trendiest reason to reject dualism today come in the form of brain imaging studies. They show how physical events in the brain correlate clearly and consistently with mental events (though their claims are often overstated in the media!!) But there are other convincing philosophical reasons to reject dualism.
The idea that there is a separate mental substance, operating outside of physical laws, that governs our thoughts behavior runs into various problems relating to causation. If our minds are comprised of something non-physical, how do they cause demonstratively physical events, when the laws of physics already seem to have that pretty well covered? In other words, where and when does the non-physical soul intervene in the chain linking one physical event to the next, in making behavior a reality?
Since a soul is, by assumption, non-physical, it also can't occupy, or move along inside, a physical body. But this leaves one with the puzzling task of deciding which souls, all presumably operating in some other realm, interact with which bodies. In other words, dualism has a "pairing problem;" there's no reason to pair up one soul with one body and not another, since there can be no physical connection between any of them anyway.
Okay, coming soon: how this pairing problem relates to consciousness; then after that, since I'm so well-read now, back to mental causation and the various physicalist "theories" of mind....it's all really fascinating, i promise