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