Warning: Massive spoilers below.
The key to the whole
movie is Marion Cotillard's Mal, the disturbed wife of lead dream
extractor/inceptor Dom Cobb. She is painted as a projection, trying to
trick Cobb into thinking his world isn't real, like the real Mal did
before killing herself.
Having Inception explained depends
on whether Mal is right. If Cobb's totem topples after the final cut to
black, then reality was reality the whole time. Yet if that totem kept
spinning, then Mal was right all along and Cobb was still dreaming. If
one believes that, they need to re-imagine who or what Mal was the whole
time.
If it is all just a dream, Mal could still just be a
projection of Cobb's mind, trying to keep punishing him for his past.
But in order to survive his guilt-ridden dream world, Cobb has to fend
off her memory once and for all. As such, the other characters are his
projections that help him do it, in particular architect Ariadne.
But
what if Mal isn't a projection? If Inception is explained as a
dream, then when Mal 'killed herself' she would have woken in the true
reality - proving her right in a different way. She could have gone back
under to get Cobb out, but since he was so lost in his dream reality,
nothing could wake him.
It could be that Inception is
explained as Mal performing inception on Cobb, just as he did on her.
She may have been trying to implant the idea that his world wasn't real,
so he could finally wake up. But when Cobb finally let her go, he lost
his only chance of returning to reality, for a happier but fake life.
