"You see what you expect to see"
"…mediocre, arrogant as his father, a determined rule-breaker, delighted to find himself famous, attention-seeking and impertinent…"
"You see what you expect to see, Severus," - said Dumbledore, without raising his eyes from a copy of Transfiguration Today. "Other teachers report that the boy is modest, likable, and reasonably talented. Personally, I find him an engaging child".