“With the destructive power of today's weapons, keeping the peace is not just a goal; it's a sacred obligation. But maintaining peace requires more than sincerity and idealism—more than optimism and good will. As you know well, peace is a product of hard, strenuous labor by those dedicated to its preservation. It requires realism, not wishful thinking.”

1963, American University speech
Context: World peace, like community peace, does not require that each man love his neighbor — it requires only that they live together in mutual tolerance, submitting their disputes to a just and peaceful settlement. And history teaches us that enmities between nations, as between individuals, do not last forever. However fixed our likes and dislikes may seem, the tide of time and events will often bring surprising changes in the relations between nations and neighbors.

Quoted on BBC News http://news.bbc.co.uk/onthisday/hi/dates/stories/november/21/newsid_2549000/2549809.stm

The Four Faces of Peace (1957)

Statement of President Obama on the death of Vaclav Havel (18 December 2011) http://www.whitehouse.gov/the-press-office/2011/12/18/statement-president-obama-death-vaclav-havel
2011
Context: Having encountered many setbacks, Havel lived with a spirit of hope, which he defined as “the ability to work for something because it is good, not just because it stands a chance to succeed.” His peaceful resistance shook the foundations of an empire, exposed the emptiness of a repressive ideology, and proved that moral leadership is more powerful than any weapon. He played a seminal role in the Velvet Revolution that won his people their freedom and inspired generations to reach for self-determination and dignity in all parts of the world.

Source: What Got You Here Won't Get You There, 2008, p. 189 (in 2010 edition}

The Pursuit of Simplicity (1981), p. 151
Variant: Total security has never been available to anyone. To expect it is unrealistic; to imagine that it can exist is to invite disaster. I believe the most important aim for humanity at present is to avoid war, dictatorship, and their awful consequences.
Better a Shield Than A Sword : Perspectives On Defense And Technology (1987), p. 241
Context: The preservation of peace and the improvement of the lot of all people require us to have faith in the rationality of humans. If we have this faith and if we pursue understanding, we have not the promise but at least the possibility of success. We should not be misled by promises. Humanity in all its history has repeatedly escaped disaster by a hair's breadth. Total security has never been available to anyone. To expect it is unrealistic; to imagine that it can exist is to invite disaster. What we do have in our technological capacities is an opportunity to use our inventiveness, our creativity, our wisdom and our understanding of our fellow beings to create a future world that is a little better than the one in which we live today.

"I'm starting to think we have a problematic issue of personal philosophy here."
Jordan Kyle and Jace Herondale, pg. 27-29
The Mortal Instruments, City of Heavenly Fire (2014)

Source: An Alarm to the Unconverted aka A Sure Guide to Heaven (first published 1671), P. 143.

On Marriage (1530)