Zemhjzd has the right context.

'La formule est paradoxale et même, semble-t-il, contradictoire puisqu’elle identifie le sujet, le moi, c’est à dire le pôle d’identité de la personne avec son contraire « un autre », indéfini, et étranger.'

Loosely translated:'The formula is a paradox and even, it seems, contradictory because she (it) identifies the subject, the me , which means the antithesis of the person, with his opposite << someone else >>, non defined, and a stranger.'

Je est un autre then will be : I is someone else . (Split personality? )

Still pretty heavy philosophic, but then:

'When he who hears doesn't understand him who speaks, and when he who speaks doesn't understand what he himself means - that is philosophy.
-Voltaire