When the gang are together in the library, Oz thinks to himself "I am my thoughts. If they exist in her, Buffy contains everything that is me, and she becomes me. I cease to exist". This is a nod to French philosopher René Descartes, who came up with the Latin phrase "Cogito, ergo sum" which translates to "I think, therefore i am". Descartes used it to prove his own existence, claiming his ability to form thoughts made him a real, living, human being.
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