The ending of Christopher Nolan's action film 'Inception' has been the subject of several theories since its 2010 release. The final scene features Cobb (Leonardo Dicaprio) spinning a tiny top on a dining table before being reunited with his two young children. Just as the top, which indicates whether the character is still in the dream state, begins to wobble, the scene fades to black.

Nolan has kept mum about the ambiguous ending, until his recent speech at a graduation ceremony at Princeton University.

The director began by stating that unlike most great speeches, he will not advise the class to chase their dreams. He instead wants the students to pursue reality.

"I feel that over time, we started to view reality as the poor cousin to our dreams, in a sense," Nolan said. "I want to make the case to you that our dreams, our virtual realities, these abstractions that we enjoy and surround ourselves with - they are subsets of reality."

The 'Inception' director then cited his movie's ending, saying that Cobb decided not to keep watching the spinning top because he was happy to be with his children in his own subjective reality. Nolan said that the scene makes a statement, that "all levels of reality are valid."

Nolan also confirmed that the ending cuts out for a reason, and that he steps out of the theater right before it happens.

"The point is, objectively, it matters to the audience in absolute terms: even though when I'm watching, it's fiction, a sort of virtual reality," the director said. "But the question of whether that's a dream or whether it's real is the question I've been asked most about any of the films I've made. It matters to people because that's the point about reality. Reality matters."

The director ended his speech with a reference to Batman. He stated that Bruce Wayne attended Princeton University but didn't graduate. He then congratulates the graduates for being better than Batman.

Christopher Nolan is the executive producer of the upcoming film 'Batman v Superman: Dawn of Justice.' It is schedule for release on March 25, 2016.