The term “soul” has had a variety of meanings over the centuries since Plato defined it as “the essence of the person”. Western religious traditions generally consider the soul to be experientially synonymous with the conscious mind, but immaterial (ie. not identical to the physical brain). Since the soul is non-physical, it can survive the death of the body. In fact, as far as many religious traditions are concerned, that’s the whole point: the soul persists eternally in a state of bliss or torment, or is recycled into a better or worse incarnation, according to the way the deceased has led his or her life.
Surely, if the soul can survive biological death, it should also be immune to less severe insults to the body. This suggests a way of determining which, if any, aspects of our mental life are attributable to it, and not just to the activity of neurons and neurotransmitters.
Consider the single most obvious aspect of the mind: that stream of subjective awareness we call “consciousness”. Most people spend about one-third of every day in the unconscious state — in fact it is necessary to continue functioning and remain healthy that we do so. Unconsciousness occurs naturally in sleep, or can be induced by the ingestion of a variety of substances which interact with the nervous system in ways that are now well-understood. If the soul is the seat of consciousness, then where is it, and what is it doing, while we are asleep? If it is immaterial then why should it be affected by neuro-active chemicals? It certainly doesn’t look like consciousness has a non-physical cause.
Or consider reason and cognition: anyone who has ever had more than a few drinks (or a dose of certain drugs, either medicinally or for recreation) remembers the confused perceptions and thoughts that ensued. As with consciousness, reason is vulnerable to the action of neuro-active chemicals. The tragic stories of patients with mental illness, dementia, stroke or traumatic brain injury show us instances of long-term impairments of cognition. Across multiple case histories, damage to specific areas of the brain consistently results in the same specific impairments. If the seat of reason were non-physical, why should physical brain damage affect it, often devastatingly?
Similar patterns can be shown to occur with emotions, memory formation and retention, to any other aspect of the mind: they all seem to depend intimately on the functioning of the physical brain, with nothing left over requiring a non-physical explanation.
There is no evidence of a non-material component to our mental lives, nor of any conscious survival beyond death. The most reasonable conclusion is that there is no “ghost in the machine”, no soul associated with our brain.