Paradox:
a statement that appears to be self-contradictory or opposed to common sense but upon closer inspection contains an acceptable and often profound meaning. Often used for emphasis or to attract attention.

Example:
"We have men-stealers for ministers, women-whippers for missionaries, and cradle-plunderers for church members."
-Frederick Douglass from An American Slave

Explanation:
This is a good example of how Douglass uses a paradox to help get his purpose across. His purpose is to show the reader the hypocritical people during that time period. The two opposing ideas stand out to the reader and emphasize Douglass's view on people that are slave holders and claim to be Christians. This makes the reader side with Douglass, and agree with his view on "slave-holding Christians", which is obviously his purpose.