What Hisou said just broke my entire way of thinking.... You can brake a man soul but you cannot break his logic...this man just did. I am gonna have to think about this now...
Mostly, it's the one that causes the other. There are a lot of parameters that can break a man's soul.
But the one having most impact is, simply, reality itself.
I, sincerely, want to know what kind of thought process you had.
Anyway, think of it this way: Humans became Titans and Titans became Walls. (as rightfully stated in this manga)