Why does Kenneth Monnitoff say he can't continue the conversation with Donnie about time travel?
From Monitoff’s reaction, he was aware of Donnie’s history of mental illness and it was a diplomatic way of ending the conversation.
Donnie was becoming very agitated and his question started becoming a little insane. Other theories include:
Teachers cannot discuss religion/religious theories in school
The school in the film required their students to wear uniforms, which may be a sign that it was a Catholic school. If so, Monnitoff might be concerned that discussing scientific theories which would dispute the existence of God or conflict with Catholic doctrine might not be allowed, thereby putting his job at risk.
Monnitoff is an atheistic and does not feel comfortable discussing matters on science and destiny in their current setting
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