Various

Although the world is full of suffering, it is also full of the overcoming of it.
– Helen Keller

To know that we know what we know, and that we do not know what we do not know, that is true knowledge.
– Henry David Thoreau

What we play is life.
– Louis Armstrong

I know all one can know when one knows nothing.
– Marguerite Duras

Doubt is an uncomfortable condition, but certainty is a ridiculous one.
– Voltaire

Stop thinking, and end your problems.
– Lao-tzu