John F. Kennedy was one of America's best speechwriters, and his secret weapon was a rhetorical trick called Chiasmus. Some examples: 1) "Ask not what your country can do for you; ask what you can do for your country." 2) "Mankind must put an end to war, or war will put an end to mankind." 3) "Let us never negotiate out of fear, but let us never fear to negotiate." All of these follow the same pattern. The lesson is that humans love symmetry. Both in buildings like the Taj Mahal or St. Paul's Cathedral, and with language when we have symmetrical sentences like the ones JFK used to use so often.