In the movie, they mention that he went into a catatonic state and only woke up a year later and he wasn't Sagan anymore, he was Parry. Meaning, he woke up a completely different person, as if he reborn. He doesn't remember his former life, only in bits and pieces, because when Jack mentions to him he was a hunter teacher, robin williams went into a frenzy, because its like that image of being a teacher is familiar but he doesn't exactly remember being that person and living that kind of life. When he leaves Lydia's apartment, he is reminded of his wife, but only in fragments, like a half-remembered dream or something. He doesn't know who Sagan is, or who his wife was, or his former life. He is now parry. He always will be Parry from now on.
I actually went to Hunter College. It was funny hearing that name again throughout the movie. Very, very New York kind of movie by the way... I liked that about the film.