“You see what you expect to see, Severus.”
“Remember, you see in any situation what you expect to see.”
“Do you become in visible?''No. I'm there, if you know how to look. I stand between the place you look at and the place you see. Behind what you expect to see. If you expect to see me, you do. I listen in places where no one expects me to be.”
“And what will you give me in return, Severus?’ ‘In – in return?’ Snape gaped at Dumbledore, and Harry expected him to protest, but after a long moment he said, ‘Anything...”
“All human eyes are useless. You see only what you expect to see, and nothing more; and what is the use of sight like that?”
“See first, think later, then test. But always see first. Otherwise you will only see what you were expecting. Most scientists forget that.”