At first glance, it looks like a misunderstanding. "I would never die for my beliefs" is unambiguous, and the "because I might be wrong" is merely a bit of explanation in case you're wondering why he'd take that stance. So obviously, Russell would not be willing to die for "2+2=4".

Russell, while a Philosopher of any sort, is perhaps best known for his contributions to math and logic. He is the sort of person who would have insisted that he can't be wrong that 2+2=4.

In the case that "X because Y", it is generally a... (read more)