Luke's brief foray into the cave on Dagobah isn't just an important scene in The Empire Strikes Back. It's one of the trippiest scenes in the entire Star Wars series.

Spelunking

Before we get too deep into analysis, let's take a look at the scene itself. Luke crawls down into a dark cave, hears a splash in the distance, and sees Vader emerge. They have a duel, which Luke handily wins: He decapitates Vader in about fifteen seconds flat. To his (and our) horror, however, Vader's helmet explodes to reveal Luke's own face underneath.

Freaky stuff, huh?

On one level, the symbolism of this scene is pretty straightforward: it foreshadows the revelation that Luke and Vader are related. There's something a bit deeper going on here, though—you just have to listen closely to Yoda's warnings to Luke before he enters the cave to understand it.

Anger Management

The first is Yoda's statement that the cave contains "only what you take with you." This establishes that the manifestation of Darth Vader within the cave isn't caused by an outside force—it's something that exists within Luke himself. What's more, this shows us that Luke has all of the same emotionality, passion, and—yes—anger that caused Anakin to fall prey to the dark side of the Force.

The second is Yoda's warning to Luke to leave his weapons behind—a warning Luke ignores. Contrast this with Obi-Wan in A New Hope, who allows Vader to kill him rather than give in to anger towards his former apprentice. Luke, on the other hand, won't stop swinging even when he knows that Vader is imaginary.

Skywalking the Line

This, along with Yoda's warning that Luke's early departure will lead him to the dark side, shows that our hero is swimming in dangerous waters. He's a good man, true, but so was Anakin—even good men can fall prey to the dark side. Regardless, Luke's experience in the cave shows him that he'll need to better control his emotions to meet his destiny of becoming a too-legit-to-quit Jedi Knight.