For most of my life I viewed any notion of the "soul" a fanciful religious invention. I agreed with the view of the late Nobel Laureate Francis Crick who in his book The Astonishing Hypothesis claimed "A modern neurobiologist sees no need for the religious concept of a soul to explain the behavior of humans and other animals." But is the idea of a soul really so crazy and beyond the limits of scientific reason?
From the standpoint of neuroscience, it is easy to make the claim that Descartes is simply wrong about the separateness of brain and mind. The plain fact is that there is no scientific evidence that a self, an individual mind, or a soul could exist without a physical brain. However, there are persisting reasons why the self and the mind do not appear to be identical with, or entirely reducible to, the brain.
For example, in spite of the claims of Massachusetts physician Dr. Duncan MacDougall, who estimated through his experiments on dying humans that approximately 21 grams of matter — the presumed weight of the human soul — was lost upon death (The New York Times "Soul Has Weight, Physician Thinks" March 11, 1907), unlike the brain, the mind cannot be objectively observed, but only subjectively experienced. The subject that represents the "I" in the statement "I think therefore I am" cannot be directly observed, weighed, or measured. And the experiences of that self, its pains and pleasures, sights and sounds possess an objective reality only to the one who experiences them. In other words, as the philosopher John Searle puts it, the mind is "irreducibly first-person."
On the other hand, although there are many perplexing properties about the brain, mind, and the self that remain to be scientifically explained — subjectivity among them — this does not mean that there must be an immaterial entity at work that explains these mysterious features. Nonetheless, I have come to believe that an individual consciousness represents an entity that is so personal and ontologically unique that it qualifies as something that we might as well call "a soul."
I am not suggesting that anything like a soul survives the death of the brain. Indeed, the link between the life of the brain and the life of the mind is irreducible, the one completely dependant upon the other. Indeed the danger of capturing the beauty and mystery of a personal consciousness and identity with the somewhat metaphorical designation "soul" is the tendency for the grandiose metaphor to obscure the actual accomplishments of the brain. The soul is not a "thing" independent of the living brain; it is part and parcel of it, its most remarkable feature, but nonetheless inextricably bound to its life and death.