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...
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. Hmm. 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! Nothing more, will I teach you today. Clear your mind of questions.