The common belief that Darwinian evolution provides a complete materialistic explanation of human beings has led to the idea of the soul being completely dismissed in our culture. A growing number of Christian intellectuals are even joining their secular colleagues in this rejection of the soul in favor of an anthropology of monism or physicalism. Insofar as many Christians still talk of a soul, it has been stripped of almost all of its content, to consist of little more than that which goes to heaven when we die. We have lost touch with the history of thought, according to the authors, because we have turned to scientists to be the leaders in culture, when their training is far too narrow to deal with the relevant issues. It is philosophy, not science, that as a knowledge discipline is equipped to properly frame and debate the issues. In this dense philosophical work, the authors seek to reaquaint Christians with what they consider to be the most reasonable, and the at the same time the most biblical, picture of what constitutes human personhood. The picture is one of Thomistic substance dualism.
The soul is an immaterial substance which grounds, forms, and animates the body. But it is not identical to the body and can survive in a temporary disembodied state after the body dies. While body and soul are joined, they have an intimate, dependent relationship so that the body is the means by which the soul expresses itself. If the body is not developed adequately so that the physical capacities move to a certain level, the capabilities of the soul cannot be expressed. By way of example, you may be a great pianist, but if your piano is out of tune, you're not going to be able to play a beautiful piece. Thus the body/soul relationship is one of functional holism while the soul is in the body, but not ontological holism. You are a soul, you have a body.
The soul is what you are aware of when you introspect. It contains thousands of capacities (or powers) which are grouped together with similar capacities into faculties. The mind is a faculty or compartment of the soul that contains your capacities to think, believe, and reason. The spirit is another faculty, and it contains the soul's ability to have a relationship with God. The human soul also contains five sensory faculties. The faculty of sight is not in your eye; it is contained in your soul. Your eye doesn't see, you do. There's two reasons why you can see: you've got the right kind of soul (as opposed to an earthworm's soul) and second, your body parts work. And while you are in the body, this faculty has to use a relevant body part before it can work. Now if your soul leaves your body, as with near-death experiences, you can see without needing eyes. By the same token, your brain doesn't think, you do. God doesn't have a brain but is able to think. Your brain is causally connected to your mind while you are in the body but is not identical to it.
The problem we run into today is that people conflate correlation, causal relationship, and functional dependence with identity, and that is a fundamental error. Just because it can be shown that fire causes smoke, that in no way proves that fire is the same thing as smoke. In the same way just because a neuroscientist can cause a patient to experience a memory by doing something to his brain, that does not prove that the memory is located in his brain. There are things true of mental states that are not true of brain states, and vice versa, and this proves that they are not the same thing. Thoughts, for example, have the property of being true or false, and no brain state has this property. Brain states can be described in physical terms but thoughts cannot. One other major difference is that brain states are third-person public - a scientist may well have better knowledge of your brain than you do; mental states, however, are first-person private - only you have access to them.
Identifying mental states with brain states and thus reducing human anthropology to nothing but physical language creates all kinds of counter-intuitive problems. One is that it is difficult to explain the continuity of the self over time when every atom in the body is replaced every seven years or so. And yet we instinctively know that we are the same person we were decades ago. Moral responsibility also becomes problematic. How can you punish someone for a crime committed ten years ago when, in physicalist terms, he is not the same person as the perpetrator of the crime? Free will is another problem if we are just physical beings, because physical things are governed by physical laws and cannot actually cause anything. Yet we know intuitively that we do make choices and with these choices we could have chosen otherwise.
The authors argue that where science is competent to enter into the discussion it favors dualism. Recent evidence from embryology indicates that a DNA-first, genocentric view of the organism is having to be replaced by an organocentric view: DNA is not what directs development, but presupposes the organism as a whole and is just a tool the organism uses in development. This is consistent with dualism, which claims that the soul acts as a unified principle of life that directs the teleological development of the organism.
The second half of the book spells out the ethical implications of one's anthropology and argues that the crisis in ethics we face in areas such as abortion and euthanasia is due to our having adopted a faulty view of the human person. If humans do not have souls and are just physical bodies, then there is nothing to ground equal rights and the notion of human personhood. According to substance dualism, personhood is a non-degreed property, which something either has or doesn't have, like numbers being either odd or even. This is because personhood resides in the essence of the soul. So there is no such thing as a human non-person. But if humans are just physical things without a soul, there is nothing metaphysical to ground personhood in, and we end up establishing artificial functional criteria such as self-awareness and ability to use language. Personhood becomes, then, a degreed property, one that can be had to a greater or lesser extent. Human non-persons become a possibility and equal human rights are really impossible to maintain.
Recovery of the soul restores an anthropology that both accords with our basic intuitions that we have about ourselves, and clarifies the ethical dilemmas that have arisen from the rise of scientism.