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1.
Pondering philosophical questions, which may be enjoyable, yet are so removed from reality, or experience that they have no practical use.

Someone that thinks of far out 'what if' questions

Note- Not all philosophy is mental masterbation.
"Today we spent three hours mentally masterbating to Kant's critique of pure reason"

"I got a degree in mental masterbation!" someone graduating with a degree in rhetoric, philosophy, anthropolgy, multicultural studies, etc
by Marcus L December 05, 2007