Ah, metaphysics. The fundamental, essential underpinnings of your world-building. The principles on which your worlds and wonders, your monsters and magic rely.
My advice to all writers is to shove all matters of metaphysics as far off-stage as possible. So far that if your characters are moved to argue about metaphysics, none of them come across as complete fools. (If they are not the type to argue about metaphysics, you have more leeway, but you should consider whether some things are metaphysically compatible.)
Ideally, metaphysics is the armature on which the world rests, and which never actually shows. In less than ideal situations, they should merely be a partial view, and as mysterious as possible. Ambiguity helps.
Your story should not look like it takes place on a limited stage. It should suggest a broader world out there that hasn't made it into the tale. Making it obvious that it's a stage setting that ends with what is shown weakens the story even when it's the countries not visited or the characters who have only bit parts.
Bringing metaphysical principles onboard -- still more bringing them entirely onboard so that all is known -- shrinks the entire world into a scope that can be brought entirely into a work of fiction. Nothing makes it look more flimsy.
On the other hand, "as possible" is an important caveat there.
A very important consideration is whether the viewpoint characters care.
If your viewpoint character forges through a multiverse and meets up with ghosts and does not care what either of those imply, you have more leeway. Even if the only reason is that the character is so hard pressed for time that there is no time to think.
But your leeway is finite.
I have run across a view in fiction -- and worse, in philosophy -- that you can have a living, breathing, moving, reasoning human being without a soul. One wonders what they think a soul is.
They should partake of works with more philosophical sophistication. I recommend Looney Tunes cartoons. With all their lunacy, whenever a white form identical in appearance to a character appears over the character's body, the form is the soul, leaving the body, and the body is dead, with x’s for eyes. Or, if you prefer, the body died and so the soul leaves it. As those really are the same thing.
(There are fairy tales in which you are told that someone keeps his soul separate from his body, and you must track it down to kill him. Then, the same tales term it a heart or a life as if it makes no difference. The concept is not philosophically clear.)
I've also run across the view that by "soul" must be meant the same thing, uniformly, so that if a soul is that which gives life, a man, a goldfish, and an oak all must have the same sort of soul.
That view, at least, has been philosophically held. As Shakespeare used for a quip:
Clown: What is the opinion of Pythagoras concerning wild fowl?
Malvolio: That the soul of our grandam might happily inhabit a bird.
Clown: What thinkest thou of his opinion?
Malvolio: I think nobly of the soul, and no way approve his opinion.
Clown: Fare thee well. Remain thou still in darkness: thou shalt hold the opinion of Pythagoras ere I will allow of thy wits, and fear to kill a woodcock, lest thou dispossess the soul of thy grandam. Fare thee well.
But then, so has another, that holds that an oak has only a vegetative soul; a goldfish, a sensitive soul (including all the faculties of a vegetative soul); and the man, a rational soul, the only immortal one (including all the faculties of the sensitive and vegetative soul)
Poul Anderson managed the rational vs other soul well in Three Hearts And Three Lions. His hero meets with a water spirit. She recounts how the tribes offer her human sacrifice, a young man and a young woman, every year, which is a waste because she's not a cannibal, though the costumes are pretty. He meets her gaze and realizes she has no soul. At least, no rational soul, and therefore no conscience.
Piers Anthony at first handled it well in Xanth. Good Magician Humphrey was able to assure a manticore that he had a soul because his desire to know stemmed from a faculty of his soul; if he had not had one, he would not have wished for one. Later, souls in Xanth were treated as a kind of detachable trinket.
If you use souls in your fantasy, if your wizard can draw the souls of people into jewels, you are wading into metaphysics. Still more if the body keeps on moving and acting. Has the wizard imported a new animating principle into the body to replace the soul? Or has the soul been broken, so that its rational parts are prisoner while its sensitive and vegetative parts keep working on?
Incurious characters can, of course, help. All the more if they only want to stop and undo the wizard's work. But some care is needed to avoid contradictions, such souls that let you reason but which you reason just fine without. And more may be needed when the characters discuss how to get the souls back into the bodies, and whether it can be done.
And above all else, if you bring the metaphysics on stage like that, don't have your characters argue about it like complete fools. They have seen what they have seen, and they may reason about it differently, but the sort of person who keeps denying the evidence of his senses had better be irrational and foolish -- and generally does not add to the story.
(More is coming. Metaphysics in fantasy is a deep topic. But I have to think deeply on the subtopics.)
> His hero meets with a water spirit. She recounts how the tribes offer her human sacrifice, a young man and a young woman, every year, which is a waste because she's not a cannibal, though the costumes are pretty. He meets her gaze and realizes she has no soul. At least, no rational soul, and therefore no conscience.
But apparently she still has an aesthetic sense.
With your three definitions of soul -- vegetative, sensitive, and rational -- I wonder, how would one determine if an obviously nonhuman being that was capable of speech and of sapient thought (as the nixie from 'Three Hearts and Three Lions' was) possessed a rational soul or just a sensitive one?
While it's not quite the same thing, this relates to a story idea I've been toying with set in late 1930's America where a sapient nonhuman that appears like an anthropomorphic animal gets rewarded with 'honorary human' status after saving the lives of some human children. Since otherwise, she wouldn't have any rights at all.