DOCTOR: Question. Why is there no such thing as perfect hiding? Answer. How would you know? Logically, if evolution were to perfect a creature whose primary skill were to hide from view, how could you know it existed?

DOCTOR: You've probably just forgotten. Have you seen the size of human brains? They're hilarious.

DOCTOR: Let me tell you about scared. Your heart is beating so hard, I can feel it through your hands. There's so much blood and oxygen pumping through your brain, it's like rocket fuel. Right now, you could run faster and you could fight harder, you could jump higher than ever in your life. And you are so alert, it's like you can slow down time. What's wrong with scared? Scared is a superpower. It's your superpower. There is danger in this room and guess what? It's you.

DOCTOR: Imagine a thing that must never be seen. What would it do if you saw it?

CLARA: You're always going to be afraid, even if you learn to hide it. Fear is like a companion. A constant companion, always there. But that's okay, because fear can bring us together. Fear can bring you home. I'm going to leave you something, just so you'll always remember, fear makes companions of us all.