First things first: what the hey is a motif, and how is it different from a symbol? We got you: it's a recurring symbol or metaphorical concept that stands in for a larger idea.

In "I Have a Dream," the repeating concept of the dream stands in for an overall vision of racial progress. So MLK's "dreams" are symbols for real-life changes.

That's just the tip of the iceberg (boom: that was a metaphor right there). Nearly every paragraph of "I Have a Dream" contains a metaphor. Searing chains, crippling manacles, promissory notes, banks of justice, storms of persecution, mountains of despair, stones of hope. Sometimes, these metaphors don't perfectly map onto reality.

What exactly is a "stone of hope" (18.3)? Would that be like a huge diamond? A hunk of nice granite? A lump of shale?

It doesn't matter. The point of a metaphor is to capture the listener's attention with an image. By definition, a metaphor compares unlike things. The power comes from the fact that it isn't literal…but is evocative of real-deal things.