In the first part of this series on the existence of the soul, “Man: Mind Over Matter or Mindful Matter?” I made the claim that “Some of the deep problems that plague physicalism cannot be solved by simply understanding the material brain better. They transcend neuroscience.” Remember, physicalism is the view that we are nothing more than our physical bodies and everything about us, from consciousness to higher rationality, can be explained by biochemical processes. In philosopher-speak: I am identical with my material body. In this installment, I will discuss one of the several major problems with this view.
Many physicalists (both theist and non-theist) point to the great strides the neurosciences have made in correlating brain states (neuron firing patterns) with mental states (conscious experiences). For instance, if stimulation of brain region Q in a subject who is anesthetized but awake results in the subject reporting an experience of tasting an orange, the physicalist conclusion is that the mental state (tasting an orange) simply is an event in brain region Q. They are one and the same thing, and there is no need to posit a soul to explain the conscious experience. This is known as mind-brain identity theory.
I believe this conclusion is unjustified, and my reasons have nothing to do with the hard sciences. In fact, the problems with mind-brain identity theory would not diminish even if neuroscience manages to one day have comprehensive knowledge of brain physiology. Here I will discuss just one of the problems.
We know from the principle of identity that in order for mental events (M) to simply be brain events (B), everything that is true of M must be true of B, or else they are not one and the same thing. Reflecting upon the properties of M and B, we can see that they do, in fact, have different properties. For one thing, mental events are self-presenting to the person having them, and cannot be accessed by an outside observer (such as the neuroscientist monitoring brain events). The subject experiences the taste of an orange, but the scientist only sees neurons firing in a region of the brain and must ask the subject to report the nature of the inner experience. J.P. Moreland and William Lane Craig have taken this fact to formulate an argument against mind-brain identity:
- No physical properties are self-presenting.
- At least some mental properties are self-presenting.
- Therefore, at least some mental properties are not physical properties. 
In other words, we can draw correlations between physical properties (brain states) and mental properties (first-person experience) all day long, but the fact remains that they cannot be one and the same thing. Stimulation of a brain region may cause neurons to fire in a certain pattern which in turn causes the orangey taste sensation, but there still must be a transcendent self having the conscious experience. Note that those who believe that immaterial souls exist do not deny the causal relationship between the brain and mental events; rather, they argue that the physical data cannot, by its nature, tell the whole story.
Gottfried Leibniz (1646-1716), famous German mathematician and philosopher, anticipated this kind of argument with his famous mill analogy (think of the old-fashioned mills that were run by flowing water turning a water-wheel connected to gears and pulleys which operated a grinding apparatus inside the mill house). Leibniz likens the mechanical mill to the physical human brain:
If we imagine that there is a machine whose structure makes it think, sense, and have perceptions, we could conceive it enlarged, keeping the same proportions, so that we could enter into it, as one enters into a mill. Assuming that, when inspecting its interior, we will only find parts that push one another, and we will never find anything to explain a perception. And so, we should seek perception in the simple substance and not in the composite or in the machine.
By “simple substance,” Leibniz is referring to the indivisible immaterial mind, which is the perceiver, the experiencer of sensations that are brought to it through sensory organs and brain events. Furthermore, he hints to the problem of consciousness, which continues to plague physicalist accounts of mind.
Ultimately, positing the soul is a metaphysical move, not a scientific one, and is a supplement to neuroscience in explaining our mental lives. Those who claim that science has made the soul superfluous are mistaken. Moreland says that
once we get clear on the central first and second order issues in philosophy of mind, it becomes evident that stating and resolving those issues is basically a (theological and) philosophical matter for which discoveries in the hard sciences are largely irrelevant. 
I love Charles Taliaferro and Stewart Goetz’s remark in their book, A Brief History of the Soul:
One cannot help but wonder if this alleged challenge from science against belief in the soul’s existence is much ado about nothing. 
Stay tuned for Part 3!
 For those keeping score, by “physicalism” I mean reductive physicalism; I think non-reductive versions are incoherent, but that’s a discussion I will save for another time.
 J.P. Moreland and William Lane Craig, Philosophical Foundations for a Christian Worldview (2003), 234.
 J.P. Moreland, Consciousness and the Existence of God (2008), 158.
 Charles Taliaferro and Steward Goetz, A Brief History of the Soul (2011), 152.