Yoda: "Yes, run! Yes, a Jedi's strength flows from the Force. But beware of the dark side. Anger, fear, aggression; the dark side of the Force are they. Easily they flow, quick to join you in a fight. If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan's apprentice."
Luke Skywalker: "Vader... is the dark side stronger?"
Yoda: "No, no, no. Quicker, easier, more seductive."
Luke Skywalker: "But how am I to know the good side from the bad?"
Yoda: "You will know... when you are calm, at peace, passive. A Jedi uses the Force for knowledge and defense, never for attack."
Luke Skywalker: "But tell me why I can't..."
Yoda: "No, no! There is no 'why'."