I have two principles: 1. I never lie to people . 2. I never believe anything anyone says, including myself.
"Sir, I'd like to remind you that your second point contradicts your first." 
"How does it contradict me?" "Since you never lie, why don't you believe what you say?"
"Oh, sorry, I forgot to mention, I don't consider myself a person."
"?"
 ... Let me introduce myself. My name is Cheng Shi, an honest man who never lies. What, you haven't heard of me? That's alright, you just haven't been fooled by me yet. Soon, you will remember.

Chapter 1 - Chapter 1: The Doctor