“Forgiveness is the key that unlocks the door of resentment and the handcuffs of hatred. It is a power that breaks the chains of bitterness and the shackles of selfishness.”

Without Forgiveness There Is No Future (1998)

Their Morals and Ours (1938)
Context: (On the American Civil War) "History has different yardsticks for the cruelty of the Northerners and the cruelty of the Southerners in the Civil War. A slave-owner who through cunning and violence shackles a slave in chains, and a slave who through cunning or violence breaks the chains – let not the contemptible eunuchs tell us that they are equals before a court of morality!"

The Rubaiyat (1120)
Context: The Vine had struck a fibre: which about
If clings my being — let the Dervish flout;
Of my Base metal may be filed a Key,
That shall unlock the Door he howls without.

Source: Little Women

Quantum Mechanics, The Key to Understanding Magnetism, Nobel Lecture http://nobelprize.org/nobel_prizes/physics/laureates/1977/vleck-lecture.pdf (December 8, 1977)

Source: The Key to Theosophy (1889)

Source: Think Big (1996), p. 216
Source: Think Big: Unleashing Your Potential for Excellence