So there are two general ways of looking at the soul in the Buffyverse. One of them, which I'll call the Standard Theory, is that the soul is the real person and the whole real person, full stop, end of story. The other is that the soul is only a piece of a person; a very important part, but only a part. It's the equivalent of a conscience, and either allows or enhances the ability to feel emotions like guilt, empathy, and altruism. A person's physical memories and feelings and experiences, on the other hand, are what make them an individual, make them them.
The Standard Theory is put forth by Giles in the first season, and is referenced again, directly or indirectly, a number of times in later season of BtVS and AtS. The Alternate Theory is only put forth once, by the emissary of the Powers which visits Connor in AtS Season 4, trying to stop him from committing murder. It appears to Connor in the form of his mother Darla, who was soulless when she committed suicide to give birth to him. It is not clear what the nature of this being was, but when Connor asks if it is his mother, it replies that it has her memories and feelings, and is that not what makes a person what they are?
The Standard Theory is exceptionally useful for the situation that the show's writers created it for: providing a reason why it was totally OK for Buffy to be in love with Angel, despite the fact that he was a mass murderer. If the soul is the real person, then nothing that Angelus does is Angel's fault. Angel may remember it and feel guilty about it, but he did not commit those acts, and he cannot be held responsible for them. The same applies, of course, to any other vampire, much to the annoyance of Bangel fans.
However, there is an issue with the Standard Theory which the writers who originally conceived it could not possibly have anticipated. It's not a worldbuilding problem, but a storytelling problem, and that problem's name is Spike. Spike did not start out as one of the heroes, as Angel did. He started out as a villain, and as a soulless vampire, gradually changed his ways, and eventually sought and won his soul back.
You see the problem here, of course. If the soul is the real person, and the whole real person, then Spike's story is not the story of a bad man who fought and succeeded in becoming a better person. It's the story of a bad man who committed suicide and was replaced by another man who was already good. If soulled Spike cannot be held responsible for the bad unsoulled Spike did, he cannot get credit for the good unsouled Spike did, either.
There's nothing wrong with this on a worldbuilding level. But it makes absolute hash of Spike's character arc. It's one of the things that annoys me about the current storyline in the comics. On the show, the Standard Theory got blurred in the direction of the Alternate Theory over the years, and in S7 of BtVS and S5 of AtS, Spike always behaves as if he is the same person as the one who fought to get his soul back. That's why he can tell Angel in Destiny that they are nothing alike; that he is the one who fought for his soul. He may not want to be soulless again, but he can acknowledge the things he did while soulless that weren't evil, and even be proud of them. Nor does he ever suggest that he didn't love her while soulless.
Whereas in the comics, in order to remind the readers that it's totally OK for Buffy to (like? love? lust? Some four letter word beginning with an L, anyway) Spike, even though he tried to rape her before he got a soul, the writers have shifted the emphasis back in the direction of soulled Spike being a completely different person than soulless Spike (who didn't really love her after all). S10 Spike does not want to see any continuity between him and his former self, who in his eyes was capable of nothing good. He urges Buffy to kill him immediately should he ever lose his soul, and why not? It won't really be him she's killing.
I realize that they are still maintaining a degree of fuzziness here, and yes, you can interpret all this as metaphorical. (And I'll cling to that interpretation as long as I reasonably can.) But it's a much smaller degree of fuzziness than the show ended up with. It's no wonder Spike can't come up with any differences between himself and Angel any longer. Under the Standard Theory, there aren't any.
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