Specifically in regards to the hero's journey. Does the mentor in such a traditional story have to necessarilly be an actual mentor, or can the learning experience perhaps be within the protagonists head?
Yes I'm a plotter, though in general I don't tend to follow (at least a traditional) hero's journey within a long short story. The story is often about the emotional experience, and then may not have that traditional "resolution as hero's reward" type climax.
One person I know of calls the formula the tragedy, though I don't think only tragedies have to be plotted that way. (Like stories that seem to punish the hero, but turn out more rewarding than what the plot was originally setting up -- very difficult to do well.)
I have some thinkinf to do, as a lot of my short story tragedies are starting to look the same.
Well, if he is a voice he isn't an outside source of advice. That is the point of the mentor. You can't remove that without some very different things. Can't kill him, can't hide him, and so on.
In what way are they ending up similar?
Well, maybe go with less Ex Machina and more logic.
Does the mentor in such a traditional story have to necessarilly be an actual mentor, or can the learning experience perhaps be within the protagonists head?
I would say definitely. The point of the mentor isn't the learning experience, it's that the hero is learning from someone else and is often reluctant or combative about it.
The Mentor doesn't have to be an actual mentor or teacher figure -- in fact, the Mentor need not be a person at all. It could be a symbolic object, the character's conscience, or any number of things. It could even be the function of a character who, in the rest of the story, personifies a completely different archetype. The important thing is that the hero needs a little guidance, knowledge, and wisdom on his/her journey before handling the greatest tests on his/her own. Whatever provides this dons the Mentor mask. :)
It isn't that Geek and Hook are wrong, but I think they might be answering a different question then you asked. Those both seem like good answers to
"Do I need a mentor for my MC to learn and grow?"
Regarding what you asked
Specifically in regards to the hero's journey. Does the mentor in such a traditional story have to necessarilly be an actual mentor, or can the learning experience perhaps be within the protagonists head?
Like I said, it depends specifically why you don't want one. It does change things, but that isn't always bad. Which leads me to ask, why don't you want one/what are you worried about? Or what makes you conflicted?
Sarah, I'd highly recommend picking up a copy of The Writer's Journey, by Christopher Vogler. It distills everything Campbell taught about the Hero's Journey and all the associated archetypes into a guide specifically for writers.
Sticking to basic concepts- doesn't a mentor- by definition- have to be conscious of not "alive" per se? At least for the mentoring portion of the story?
Otherwise, the hero who has an object do the "mentoring" is really learning by him/herself, as no "mentoring" is actually going on.
@Sarah - So why do you want to put one in or why are you hesitant to leave one out? You seem like you've made up your mind. Which isn't bad, I just don't see your real life conflict.