Instead of honoring this sacred obligation, America has given the Negro people a bad check, a check which has come back marked "insufficient funds."

from Martin Luther King, Jr – I Have A Dream on Rap Genius

Meaning

King continues using the check metaphor to describe the condition of blacks in America. While it appears that the Emancipation Proclamation gave them a legitimate “check” to cash, when they try to “cash in” on these rights they realize that the original check was not strong enough to guarantee these rights. This speaks to the inability of the Emancipation Proclamation to effectively end racism.

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