This is directly from an interview with the director of the movie:
"I should say this. All that stuff fascinates me on an intellectual level, but the origin of the idea really came from an emotional place. For me, I wanted to tell a story that looked at the fragility of the human mind with empathy. Our brains are fragile, and there may be people in your life that you love deeply, but you see the world completely different from them. It can be for any number of reasons. It could be political, it could be that their brain is deteriorating, it could be addiction. Just whatever it may be, perhaps there’s something not right with their mind, but they see the world so vastly different than you that there’s almost no common ground. Reaching out and connecting with them is hard to do. In that context, I thought, “Well, I really want to tell a story about someone.” In this case, it’s Emily, the daughter who despite the fact that it’s very hard and she doesn’t see the world that he sees, that rich detailed, strange landscape that’s all infused with his emotions, even though she doesn’t see it, she doesn’t give up on him and it makes a difference.