An antithesis can be illustrated by juxtaposing two arguments which oppose each other. It can also be illustrated by a contrast of oppositional ideas such as "Give me liberty or give me death" (Patrick Henry, 1775). In describing the current state of inequality, despite the Emancipation Proclamation, King writes:
One hundred years later, the Negro lives on a lonely island of poverty in the midst of a vast ocean of material prosperity.
Island and ocean are not necessarily opposites but they are contrasting ideas. The significant antithesis in this sentence is the combination of poverty and prosperity.
King also uses imagery to convey the current plight of African-Americans as compared to a hopeful future. In these next two lines, darkness is the antithesis of the sun; and the quick sands are indicative of a hopeless sinking situation, the antithesis being a solid foundation of brotherhood and equality.
Now is the time to rise from the dark and desolate valley of segregation to the sunlit path of racial justice. Now is the time to lift our nation from the quick sands of racial injustice to the solid rock of brotherhood.
Two more examples of antithesis use imagery of the landscape and music:
With this faith we will be able to hew out of the mountain of despair a stone of hope. With this faith we will be able to transform the jangling discords of our nation into a beautiful symphony of brotherhood.
The antithesis of despair is hope. The antithesis of discord is harmony or a beautiful symphony. These two lines help set up the series of lines rising to the climax of the speech. In each of these lines, King declares "Let freedom ring" as the song of equality is sung throughout the landscape of America.
The "I Have a Dream" speech is full of antitheses because King is describing the current situation of racial inequality and the speech is bursting with hope for future racial equality.