乌鸟之狡，虽善不亲。不重之结，虽固必解。
乌鸦聚集在一起，虽然看上去很和好，实际上并不亲密；不慎重地结交朋友，虽似牢固，终必破裂。
We teach a subject not to produce little living libraries on that subject, but rather to get a student to think mathematically for himself.
What other dungeon is so dark as one's own heart! What jailer so inexorable as one's self!