" Harry learns that some types of spells require specific emotions or thoughts from the caster. As he learned from Snape, and will never forget, the emotions of dueling an enemy create different magical conditions than dueling a friend. Spikes of loyalty and courage conjure Fawkes and the Sword of Gryffindor for Harry. And to open the Chamber of Secrets, you don’t have to be the Heir of Slytherin. You just have to know how the Heir of Slytherin would feel—as Ginny does when she’s possessed and as Ron will, years later, after the Horcrux. This is one of Rowling’s most important themes: that empathy is the basis of powerful magic. "

― Lorrie Kim , Snape: A Definitive Reading