How can I explain to an agnostic the existence af a soul?
What is the difference between a dead person and a live one? Even if you get all the right matter together, it does not make a person. There is something that “animates” that matter and makes it a person. This is what we call a soul (in Latin anima, from which we get animates).
Soul is life.
The word for soul in Latin is anima.
A living body moves, if only the nose or the ribcage, so moving pictures are referred to as “animation.” A lifeless form is referred to as inanimate.
One argument for the soul is based on the difference between a corpse and a living body. In the morning a man is alive, but by the evening he may be a corpse. Now what is the difference between the two? It’s not a difference of material. There is no chemical you could add to the corpse that would make it alive again, nothing material could do that. But there is a difference. Therefore, there is a nonmaterial element without which the body is a corpse. Catholics call it the soul.
A second argument for the soul is based on justice. If there is no life after death, there is no justice. Good and evil are not balanced in this life. Good people often suffer, evil people often do well. Also, if there is no future life, there is no true morality. Rob, lie, murder – only be careful! The only way true justice can be served is if we have a soul that lives on after death, where a sentence of judgment awaits. But if there is no soul, then there is ultimately no justice and no morality either.
A third argument for the soul is based on the law of nature. Human beings naturally believe in life after death and in many cases look forward to it. Fr. Rumble’s book “Radio Replies Volume 3” uses this as an argument that the afterlife is real: “Could anyone conceive that God would form that most delicate organ of hearing, the ear, so wonderfully adapted to every kind of vibration, yet endow no objects with the power of causing sound? The whole tendency of the ear would be to hear, yet it would never do so because its complementary object would be wanting. Every natural tendency implies and has an object.” Therefore, something exists which corresponds to our natural expectation of an afterlife.
“… if a person grasps a certain concept, and if that concept is an object, then the person grasps an object. Since this is a mental act, his mind must take hold of something, and if it takes hold of a thing then that thing must make a kind of contact with it—which means, since there is no other plausible way of understanding it, that the concept must somehow be in its possessor’s mind. But if the concept is not the sort of thing that can be physically inside the possessor’s brain, his mind cannot be his brain, and moreover must be immaterial since only an immaterial thing can be suited to laying hold of the concept.” - David Oderberg
Hylemorphic dualism is the way to go
The presence of a spiritual soul is confirmed by the rational abilities in human nature.
Dualism the soul and body is the only system of thought that philosophy has devised for the state we find ourselves. There really are no ready alternatives. To accept other systems of philosophy of the human condition you must be ready to completely dispose of free will and with it morals and many other positive conclusions that we mostly take for granted.
The concluding paragraph from: newadvent.org/cathen/14153a.htm
Such is the Catholic doctrine on the nature, unity, substantiality, spirituality, and origin of the soul. It is the only system consistent with Christian faith, and, we may add, morals, for both Materialism and Monism logically cut away the foundations of these. The foregoing historical sketch will have served also to show another advantage it possesses — namely, that it is by far the most comprehensive, and at the same time discriminating, synthesis of whatever is best in rival systems. It recognizes the physical conditions of the soul’s activity with the Materialist, and its spiritual aspect with the Idealist, while with the Monist it insists on the vital unity of human life. It enshrines the principles of ancient speculation, and is ready to receive and assimilate the fruits of modern research.