Ms. Skylark, (please forgive any misgendering)
You got my gender right.
I did not state nor imply that a soul is required to manipulate magical forces.
Sorry, I misunderstood.
However, secondarily and separately, I do postulate that a soul is required for a mortal to do so. The twin sides of Hellfire and Soulfire seem to imply that a soul can be used for magic. Heck dresden as a soul used himself to fuel his spells in GS, and we know he was nothing but soul.
It's possible, I suppose, but there are a few issues:
1) Is it possible to be a mortal and not have a soul? I had thought that Bob implied that if Harry burned up all of his soul, he would be destroyed. Also, Uriel said that he
was a soul, so...
2) Soulfire and Hellfire appear to be different power sources that interact with magic, but are not themselves magic. Bob describes them as rebar and concrete, respectively.
3) Harry appears to be using magic in Ghost Story in the same way that other ghosts do, and other ghosts don't have souls. It's a little ambiguous, because the only other "ghost" that we see using actual magic is Corpsetaker, and he/she
does have a soul, but Sir Stuart uses the same technique to make his bullets, and he
doesn't have a soul.
I would postulate that you need to have a spirit to use magic rather than a soul--Bob describes them as being a seed and soil, respectively, and I think he says that fae only have the former. The comparison of two things working together to become more is used both for spirit and soul and for magic and soulfire, so I think it fits. Also, angels are
only soul, and they don't appear to be able to use magic (I don't think. I think that Bob says something about them using soulfire rather than magic, but I'm not sure).
On the other hand, evidence for a soul being necessary for mortal magic is that at the end of White Night Bob says that you use bits of your soul in magic. This appears to be retconned in Small Favor, where it's revealed that using your soul as energy is a different thing altogether, namely soulfire, but it could have been a case of Bob not getting it, since he's explicitly not good at angel stuff.