What strength do you value?

Some really interesting quotes on strength that I found on Quoteland.com. While most people easily dismiss mental strength, it is usually harder to achieve than physical strength, and this is what I looked for highlighting in these quotes. I love the first quote, it is simple and direct but incredibly intense.

To the world you might be one person, but to one person you might be the world.
-Anon.

"What does not kill me makes me stronger."
-Friedrich Nietzsche, Twilight of the Idols

We can let circumstances rule us, or we can take charge and rule our lives from within.
-Earl Nightingale

"Nothing is so strong as gentleness and nothing is so gentle as real strength."
-Ralph W. Sockman