Inception's ambiguous ending and plot has garnered an endless number of interpretations and debates. Cobb has systems and methods of discovering when he's in a dream and who's dream he's in at any given time, but these methods ultimately fail both him and us. How do we know we're in the "real reality" and not in a dreamlike state, in someone else's dream, in Tommy Westphall's snow globe, in The Matrix, a brain in a vat, in Plato's Cave, on The Truman Show or creations in The Sims 12? As Nolan demonstrates in Inception, we can't.
Last month, director Errol Morris wrote a New York Times series on 'The Anosognosic's Dilemma.' It's about Rumsfeld's "unknown unknowns." We simply don't know when we don't know the limits of our own knowledge, either individually or collectively.
We can't not even know if our senses are interacting with reality or not - we can't be completely certain about anything.
An extreme example I enjoy is from dystopian literature. Dostoevsky's underground man can't make 2 + 2 = 5. Winston Smith manages to believe it after torture, but I can't wrap my head around it. However, simply because I can't conceive of a universe where 2 + 2 = 5, that doesn't mean I can't conceive of the possibility of a universe where it does. Perhaps 2 + 2 has always = 5, and we humans just have faulty, pitiful brains that trick us into making basic mathematical errors, and we can't even realize that we're wrong.
Most simply, the fallibility of human methods of knowledge can be cast as the Münchhausen Trilemma. When we attempt to prove something, we fall into one of three patterns:
The circular argument, in which theory and proof support each other (e.g. we repeat ourselves at some point)
The regressive argument, in which each proof requires a further proof (e.g. we just keep giving proofs, presumably forever)
The axiomatic argument, which rests on accepted precepts (e.g. we reach some bedrock assumption or certainty)
Let's try it out with Inception:
1. The thimble tells Cobb he's in his own dream because it continues to spin when he dreams. He knows it stops spinning when he's in reality, because each time he's been in reality, it's stopped spinning. He knows he was in reality each time it stopped spinning because when Cobb is in his own dream, it continues to spin.
2. The thimble tells Cobb he's in his own dream because it continues to spin when he dreams. He knows it stops spinning when he's in reality, because the laws of physics function in reality whereas they do not always do so in his dreams. He knows the laws of physics function in reality because of decades of observation of reality. He knows his observations of reality were accurate because... (continue ad infinitum)
3. The thimble tells Cobb he's in his own dream because the thimble system is a properly basic belief and just self-evident, dammit.
We're simply not able to explain beliefs without referencing other beliefs. It's like trying to write a dictionary without using words.
It seems perfectly reasonable to go along with the game and act as if we're in reality and not a dream, and it seems rather difficult not to use human reasoning on at least an occasional basis. But the fallibility Inception reminds us of might lead to some epistemic humility.