> What even is a soul? How can it be identified?
Our soul is the part of us that connects to God. Whether we are three parts (body/soul/spirit), two parts (body/soul), or one part (we are a soul), the soul is what connects with God.
> What leads you to believe that anything has a soul?
1. Evidence of our thoughts and feelings betrays that we are more than just material objects.
2. Our sense of self (and perception of self, not just in thought, but as an entity) gives evidence that we have a conception of an immaterial self.
3. Our perceptions of truth and falseness (necessary for scientific inquiry) betray a belief that among the random and chance happenings of evolution and naturalism, content (apart from natural phenomena that we can empirically experience) has arisen that we can trust to be reliably true. (The conditional probability that our cognitive faculties are reliable, given naturalism together with the proposition that we have come to be by way of evolution, is low.) The reliability of cognitive content in addition to self-perception gives evidence that something besides pure materialism is present in our beings.
4. J.P. Moreland makes the argument from Leibniz's law of the indiscernibility of identicals: For any X and Y, X is identical to Y if for any property P, P is true of X if and only if P is true of Y. It's a variation of A = A: for two things to be truly identical, they must be truly identical, in which case the argument can go like this:
[a]. You are a body and a soul, or you are just your body.
[b]. It is possible (by which I mean it is strongly conceivable, therefore I have good grounds to believe it is possible) for you to survive the death of your body (as evidenced by genuine "near death" experiences).
[c]. It is not possible for your body to survive the death of your body.
[d]. Therefore, you are not identical to your body.
5. Carl Jung, the famous Swiss psychiatrist and psychoanalyst, said "There are indications that at least a part of the psyche is not subject to the laws of space and time. Scientific proof of that has been provided by the well-known J.B. Rhine experiments. Along with numerous cases of spontaneous foreknowledge, non-spatial perception, and so on, ... these experiments prove that the psyche at times functions outside of the patio-temporal law of causality. This indicates that our conceptions of space and time, and therefore of causality also, are incomplete. A complete picture of the world would require the addition of still another dimension; only then could the totality of phenomena be given a unified explanation." (
Memories, Dreams, Reflections p. 304)
> Do all things have souls, or is it just living things?
As far as we know from Christianity, just humans have souls.
> If only some things have souls, then what differences cause some things to have souls but not others?
It's not a biological difference. Genesis 2.7 says that God created humans to be souls, and that we are unique among living organisms in that sense.
> And how have these determinations been made? That is to say, what method have you used that led to you knowing this information?
Scientific observation and logic have led me to believe that we are more than just materialistic organisms. I mean, sure, Picasso's "Guernica" is just paint and canvas, right? Not at all. It's a powerful work of art, far more than the sum of its naturalistic characteristics. Beethoven's 9th Symphony, even though it's just notes on a page and the physics of sound, is far more than just paper and physics. Materialism doesn't really begin to explain the true realities of art, music, literature, etc. In the same I believe, like art and music, we are far more than the sum of our parts. Physics doesn't explain music. Paint doesn't explain art. Materialism doesn't explain humanity. we have compelling affirmation, if not confirmation, that biology can't reach what truly makes us human: consciousness, love, wisdom, desire, grief, and discernment.