MIKE: Bulls***. They—they don't know you, your work, man. They know the guy in the bird suit who goes and tells coy, slightly vomitous stories on Letterman.
Riggan keeps insisting he doesn't exist, and in some ways he's right. When people think Riggan Thomson they don't think about him as an individual, they think about the guy who sits behind the bird mask in the movies. This play is Riggan's attempt to create an identity for himself apart from his superhero alterego.
MIKE: "A man becomes a critic when he cannot be a [sic] artist, in the same way that a man becomes an informer when he cannot be a soldier." Flaubert, right?
TABITHA: He's a Hollywood clown in a Lycra bird suit.
MIKE: Yes, he is. But tomorrow night, at eight o'clock, he's going out on that stage and risking everything.
There's a lot of labeling going on here, as Riggan will later point out. Tabitha calls him a "Hollywood clown" in an ad hominem insult that comes from her disdain of mainstream cinema, but her critique is lacking substance. She hasn't even seen the play. As for Mike's Flaubert quote, we see him taking a similar shot at Tabitha, telling her she's a has-been or a never-was. But his insult is also without substance, he's just using someone else's words.
SAM: […] let's face it dad, you are not doing this for the sake of art, you are doing this because you want to feel relevant again. Well, guess what, there is an entire world out there where people fight to be relevant every single day, and you act like it doesn't exist. Things are happening in a place that you ignore, a place that, by the way, has already forgotten about you. I mean, who the f*** are you?! You hate bloggers, you mock Twitter, you don't even have a Facebook page. You're the one who doesn't exist. You're doing this because you're scared to death, like the rest of us, that you don't matter and, you know what, you're right. You don't. It's not important. You're not important, okay. You're not important. Get used to it.
What does it mean to exist? Both Sam and Riggan define existence based on the knowledge of other people. If people don't know you, you're nobody. Most people aren't this passionate about social media, but Sam has a point. These days, if you don't have some kind of online presence, in a way your existence is diminished; you exist less than other people who have a virtual identity.
MEL, PLAYED BY MIKE: Okay, j-j-just put the gun down Ed. She just doesn't love you anymore.
ED, PLAYED BY RIGGAN: You don't, do you?
TERRI, PLAYED BY LESLEY: No.
ED, PLAYED BY RIGGAN: And you never will?
TERRI, PLAYED BY LESLEY: I'm sorry.
ED, PLAYED BY RIGGAN: I don't exist. I'm not even here. None of this even matters. I don't exist. I don't exist. [Slowly he lifts the gun to his head and, against the protests of Terri and Mel, he shoots himself].
This time it's not just Riggan who's struggling with an existential crisis, it's his character, Ed. We can, of course, compare the two, but it's more interesting to note the differences. What causes Ed to doubt his existence is, like Riggan's, his worth as a human being. But, whereas Riggan's worth is determined by respect and reputation, Ed's worth is determined by love.
SAM: If you weren't afraid, what would you want to do to me?
MIKE: I'd pull your eyes outta your head.
SAM: That's sweet.
MIKE: I'd put 'em in my own skull, and I'd look around so I could see the street the way I used to when I was your age.
This is Mike expressing his desire to redefine his perception, his identity. Normally we think of Riggan as the one dealing with mortality, growing old and losing his relevance. But Mike feels the aging too, and wants to regain the kind of youth that Sam has (while at the same time mocking her for wanting to play a game like a child).
RIGGAN: Stop saying we, there is no we. I'm Riggan f***ing Thomson.
BIRDMAN: No; you're Birdman. Because without me, all that's left is you: a sad, selfish, mediocre actor, grasping at the last vestiges of his career.
Despite what he wants us to believe, Birdman is most definitely a mental formation. He's the part of Riggan's ego telling him that he's not good enough. That he has lost all former glory and success and somehow ended up in a place that "smells like balls." Riggan's need for success outside of the bird costume is manifested in the berating words of Birdman.
SAM: These dashes represent the six billion years that the earth has been around, and so each dash represents a thousand years. And this [breaking off and handing Riggan a square perforated square of toilet paper] is how long humans have been around: 150,000 thousand years. I think they're trying to remind us that that's all our ego and self-obsession are worth. [In a moment, Riggan will forget about the significance of the toilet paper and use it to wipe his face.]
It doesn't get much more straightforward that this. Sam's exercise reminds her of the insignificance of all of humanity, and thus her own insignificance. It's an exercise meant for rehab patients to center their focus on something other than themselves. What to make of Riggan's dismissal of it is more ambiguous. Does he further the exercise by corroborating the insignificance of the human race? Or is this just another example of how selfish he is as he uses all of humanity to clean his face?