So Alice just sent me a long email arguing that it doesn’t matter whether the ending of Inception is a dream or not. It was more persuasive and thoughtful than almost anything I'd read about the movie, but I still disagreed. Strongly. So I ended up writing her an equally long response, and I figured I might as well share it here too:
The film’s climax is Cobb’s confrontation with Mal in limbo. During their conversation, Cobb realizes two key things: 1) That to a certain extent, he can alleviate his grief and guilt with the fact that he and Mal actually did grow old together during their decades in limbo. 2) That the person he's confronting is just a projection of his own mind. She is not Mal.
So Cobb finds meaning and solace in his years with Mal, even though they occurred in a purely mental dreamscape, because he shared them with another human being. Conversely, he leaves his fantasies of Mal and his children at the end, even though the decision causes him insane heartache, because he will be alone if he stays. That’s not just a “possible” reading of the scene, it’s exactly what Cobb says to Mal when explaining why he won’t stay: “I wish. I wish more than anything. But I can’t imagine you with all your complexity, all your perfection, all your imperfection. Look at you. You are just a shade of my real wife. You’re the best I can do, but I’m sorry, you are just not good enough.”
Then there are two ways to look at Cobb's decision to walk away from the spinning top. It might be that Cobb doesn't care whether or not that he's dreaming. But this interpretation makes a complete hash out of the earlier scene, because it means Cobb has simply traded one solipsistic world for another.
I also think "it doesn't matter whether I'm dreaming" is just a really weird and disturbing way to look at life, even if you read “dream” as a stand-in for “movie.” Yes, there are movies that have incredible value to me even though they’re completely fictional. Ditto dreams I’ve had. That doesn’t mean we shouldn't distinguish between dreams/movies and real life. If Cobb doesn't care whether he's dreaming, that basically turns Inception into the anti-Truman Show or anti-Matrix, with a hero who chooses a happy fantasy over the unhappy reality. It also means that Cobb is a bad dad who decides, "Well, fuck, it doesn't matter whether or not my kids actually get to be with their father, as long as I can trick myself into thinking that they are."
The alternative interpretation, suggested to me by Devindra, is that Cobb walks away because he doesn't need the totem to tell him what's real. Frankly, I don't find that satisfying on a plot or character level either (I'm not a big fan of the ending, regardless of interpretation), but it does kind of resonate with what happens earlier.We know Mal is irreparably damaged by her time in limbo, because she can no longer tell the difference between dream and reality. We know Cobb is healed because he finally can tell the difference.