I thought I remembered witches referring to themselves as other than humans, but then I realized I was only certain of that memory from fanfic. Some quick googling did turn up some reference-type websites that specifically refer to witches as not human (<a href="http://hdm.wikia.com/wiki/Witches"here they're called 'human-like creatures'</a>, and <a href="http://www.bridgetothestars.net/wiki/index.php?title=Witches">here it refers to witches living longer than humans</a>). However, it has been quite a few years since I read the books, so I can't verify that this isn't just a case of general cluelessness that includes me.

OK, I think I see what you're trying to say a bit better. As I now understand it, your point about the TARDIS we know being female is part of why it squicks you, not a reason the concept couldn't happen. Fair enough. If something were to happen within canon (or within a fic) that erased a character's individual personhood and made them part of another character, then I would totally share the squick. Something semi-analogous happened in the Dollhouse finale and it creeped me out in a major way, even with the genders reversed. It doesn't bother me personally to ask the question "but what would it be like if there was a world where she'd always been part of him and he was part of her?", but I see why it squicks you.