The human soul makes a human animal a supernatural being as well as a natural being.
That which makes a human animal supernatural is its moral capacity to know the good and freely choose it. Knowing the good is bipartite. First, there is knowing the basic natural human goods, Second, there is knowing that which the basic human goods are good for. The natural goods are also bipartite. First, there are those conditions which make for human flourishing. Second, there is being the kind of person who freely chooses these conditions for human flourishing. Since basic human goods are goals as well as natural conditions, knowledge of goods as good give humans purposes. Purposes are goods which are intentionally sought. Knowing what basic human goods are good for gives humans a purpose for living itself. But purposes are not part of nature when we think of nature from the perspective of evolutionary theory as we are doing here. So, our having purposes makes us supernatural beings as well as natural beings even if most of our goods are natural conditions.
This capacity for knowing the good is a moral capacity because we can freely choose to act against attainment of what is good. But the fundamental law of morality is “Choose what is good!” With knowledge of what is good and free will comes obligation. We could say that it is having obligations which places us in both the natural and supernatural.
It must be emphasized that exercise of the capacity to know and pursue the good depends upon physiological states of an individual human but this moral capacity is not any physiological state or capacity. It is an additional feature that enables physiological states and capacities to be used in intentionally knowing and choosing what is good. Individuals with severe cognitive capacities still have this moral capacity although unable to exercise it. Individuals receive this moral capacity – the human soul – when they began to be human, which is at conception.
This moral capacity is essential to the human species even if it did not arise by natural selection. This means that in a thought experiment in which humans from the period when humanity began, off-spring of these ancient humans due to mating with contemporary humans would have all of the basic moral concepts we have now. See Natural and Supernatural Origin.
The soul of an individual human is that individual’s capacity to know and pursue what is good.
Why claim that the soul of a human is imperishable? Why claim that the soul of a human does not cease to exist at biological death. Why claim that the soul of a human does not cease to exist when there is no body to form into a moral agent? I give a Kantian answer.
A human being is morally perfect if that person becomes the kind of person who freely chooses the natural goods. Amongst these natural goods is being morally perfect.
Consider, now, these brief syllogisms. The justification for (1) is given above when it was pointed out that our natural goods are obligatory goods. Premiss (2) is an alleged truth of logic.
1. A person ought to be morally perfect.
2. What ought to be can be.
3. If a person’s capacity to be morally perfect ceased to be at biological death, that person cannot be morally perfect.
Hence: (4) A person’s capacity to be morally perfect cannot cease to be at biological death.
5. If a person’s capacity to be morally perfect were never embodied after biological death, that person’s capacity to be morally perfect could never be exercised after biological death
6. If a person’s capacity to be morally perfect could never be exercised after biological death, that person cannot be morally perfect.
Hence: (7) If a person’s capacity to be morally perfect were never embodied after biological death, that person cannot be morally perfect.
Using (1) and (2) above, we can conclude:
8. A person’s capacity to be morally perfect – a person’s soul- is embodied some time after biological death.
Note that if a person attained moral perfection that person cannot cease to be. For what ought to be cannot cease to be. If what ought to be ceased to be something which ought to be could no longer be.