Raison d'être is a French phrase meaning "reason for existence". In the universe of Ergo Proxy, it is an important motif through-out the series and a important part of the character development. It represents the purpose for which both AutoReivs and humans are created: a specific role to fulfill in society, often a single task. Failing to fulfill this task could entail harsh action from the Domed City's Regent.

Multiple characters suffer throughout the series as a result of the loss of their Raison d'être. We can see this as Iggy loses his ability to take care of Re-l. We see this again when Daedalus loses his ability to protect Re-l and his research in the Monad proxy.