One thing I have eschewed in all my fantasy fiction is the use of ‘spirits.’ By this I mean, all sorts of incorporeal beings, whether ghosts, demons, angels, human souls, elementals, what have you. As I do not believe in such entities in ‘real life’ it feels hypocritical to include them in the stories.
Rather, any and all beings of this sort are as solid as any other creature. They simply exist in other worlds. They are whole beings, not spirit joined to physical body — there are no souls moving from body to body or anything of that sort. I simply will not use that concept, nor will I base magic on any ‘spiritual’ force. Both allow way too much leeway and too many opportunities to fudge things.
As does all the paranormal stuff, telepathy, and so on that uses some sort of ill-defined ‘energy.’ All my magic is thoroughly mechanical and in some way relies on the ability to seek and/or travel through the infiniverse (and I do find the concept of infinite universes at least believable). Those are the boundaries I have set myself and I have stuck to them.