This is only true if your object system is based on the idea of "methods belong to classes" as opposed to "methods belong to generic functions", as seen in Comm

This is only true if your object system is based on the idea of "methods belong to classes" as opposed to "methods belong to generic functions", as seen in Common Lisp for example. If you define a generic function ATTACK with arguments ATTACKER and ATTACKED, you are free to create a method with specialization ((attacker knight) (attacked mage)) and write code that will be executed only when a knight attacks a mage.