What characteristics essentially define an immaterial soul? I've heard philosophers define a soul as being an immaterial substance which possesses a range of mental capacities or dispositions, but they never really define its internal structure. Immateriality is merely a negative attribute, but I am looking for a positive characterization of the soul. Souls have the essential capacity to have consciousness (as souls can be unconscious or conscious), but what intrinsic feature(s) of the soul explains this?

The term "soul" is a sort of a place-holder for a certain kind of something-we-know-not-what that may well not exist. That's the reason why there's not much to be said. The "definition" you cite is really just a way of fleshing out what people have in mind when they use the word "soul." It's not a stab at a theory.

If there is anything fitting this "definition" of a soul, then what internal structure it might have is a further and puzzling question. Since souls are supposed to be immaterial, it's not clear what it would mean to say that they have internal structure. Internal in what sense? Structure in what sense?

If someone asked me what features souls have that explain their supposed capacity for consciousness, my answer would be "How the h*ll would I know?" By insisting that souls are immaterial and yet still have physical effects, we've put ourselves in a hard spot: we can't call on any of the resources we usually use when we try to explain the goings-on of things in the world. Physics, biology, neurology, chemistry... are all out. What's left is anyone's guess.

For pretty much these reasons, most philosophers don't talk much about "souls." Understanding consciousness is hard; positing a non-physical thing to do the explaining doesn't really help.

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