How we cite our quotes: All quotations are from Star Wars: The Empire Strikes Back.
Quote #4
LUKE: 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.
The distinction between the dark and light sides of the Force is a key tenet of Jedi spirituality. The light side of the Force represents patience and passivity; the dark side represents anger and fear. Although it's obvious that the light side of the Force is the better of the two, the movie suggests that the optimal state of these two powers is when they are in balance.
Quote #5
LUKE: But tell me why I can't.
YODA: No, there is no why.
Yoda's sayings often sound like kōans, Zen Buddhist parables that typically use illogical statements to illustrate spiritual truths. Here, Yoda is using a sort of circular logic to emphasize Luke's need for faith.
Quote #6
YODA: That place is strong with the dark side of the Force. A domain of evil it is. In you must go.
LUKE: What's in there?
YODA: Only what you take with you.
Luke's experience in the cave is an important point in his spiritual journey for many reasons. First, it shows him that he's still far from being the perfect Jedi that Yoda wishes him to be. Second—and perhaps most importantly—it shows him that he's "taking" quite a few negative feelings with him everywhere he goes, like his anger, fear, and over-emotionality.