One section of the book is devoted entirely to defending the idea if mind-brain physicalism. Carrier argues that what we think of the soul is simply the mind, and that the mind is totally dependent on, and cannot exist without, a functioning, material brain.
Theists believe that intelligence is possible without a brain (think of God, for example). Moreover, when we die, most religions teach that our souls or minds (containing our feelings, thoughts, impulses, memories, and sense of self) will live on, independent of the body.
Carrier makes the observation that mental powers of living animals are in direct correlation to the complexity of their brains. The human brain is possibly the most complex, containing a large cerebral cortex. He then argues on page 153:
It follows that a physically complex brain is necessary for a mind, and that a mind can only develop when the brain develops physically . . . If our minds were not dependent on the human brain, then there would be no plausible reason for us to have one. At best, all we would need is the minimal sort of hardware a comparable mammal had, though even that would be hard to explain the need of, since if the mind were independent enough to be able, for instance, to see, hear, think, and remember all on its own, the vast majority of the brain of even an ordinary mammal would be useless material to us, dead weight, a needless drain on our oxygen supply, of which our brains now take the lion’s share.
This is an interesting thought. If intelligence is possible without the body, then why did God give us such a large brain, an organ that is a physical disadvantage?