What is the difference between 原则 and 原理？
原则 is principle of personal conduct


Example: 

做人要有(原则) = A man has to have (principle)

工作至上是我的(原则) = My (principle) is works always come first


  原理 is principle of physics 


Example: 

利用槓桿(原理) = Using the (principle) of leverage

利用虹吸(原理) = Using the (principle) of siphon


"I know the 原理 of something" means I know the reason why something works
"I know someone's 原则" means I know what someone would or would not do base on his principle (personal believe and conviction)


The 原则 (principle) of insurance companies is "to try their hardest to avoid paying their customers" (that's what they do as an unwritten rule)