Historical Rules · 25 August 2017
Rule 701.2a
Official source text from the 25 August 2017 Rules release
To activate an activated ability is to put it onto the stack and pay its costs, so that it will eventually resolve and have its effect. Only an object’s controller (or its owner, if it doesn’t have a controller) can activate its activated ability unless the object specifically says otherwise. A player may activate an ability if he or she has priority. See rule 602, “Activating Activated Abilities.”