The theory further states that Nemo was merely a mental manifestation of Marlin so that he could come to grips with reality. Marlin used this mental manifestation so he could face the insurmountable pain of losing his family. ... On his journey, Marlin also meets Dory.

Is Nemo his dad's imagination?

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In a “Beautiful Mind”-like twist of fate, Marlin — Nemo's father, voiced by Albert Brooks — has just been imagining his son as a way to cope with the loss of his wife, Nemo's mother, and their kids. ... “The word 'nemo' in Latin means 'no one,' ” they explained.

Was Nemo dead the whole time?

They suggest that Nemo was actually dead from the beginning of the film, suggesting that Marlin's entire family, including Nemo's mom, Nemo, and all of their other children, were killed by the fish — Meaning there were no survivors.