“Always be consistent by keeping within you that spark of healthy madness, which makes your soul vital.”

Tolstoy's Diaries (1985) edited and translated by R. F. Christian. London: Athlone Press, Vol 2, p. 512
Context: People usually think that progress consists in the increase of knowledge, in the improvement of life, but that isn't so. Progress consists only in the greater clarification of answers to the basic questions of life. The truth is always accessible to a man. It can't be otherwise, because a man's soul is a divine spark, the truth itself. It's only a matter of removing from this divine spark (the truth) everything that obscures it. Progress consists, not in the increase of truth, but in freeing it from its wrappings. The truth is obtained like gold, not by letting it grow bigger, but by washing off from it everything that isn't gold.

Source: Heart of Darkness

2009, Nobel Prize acceptance speech (December 2009)
Context: We do not have to think that human nature is perfect for us to still believe that the human condition can be perfected. We do not have to live in an idealized world to still reach for those ideals that will make it a better place. The non-violence practiced by men like Gandhi and King may not have been practical or possible in every circumstance, but the love that they preached — their fundamental faith in human progress — that must always be the North Star that guides us on our journey.
For if we lose that faith — if we dismiss it as silly or naïve; if we divorce it from the decisions that we make on issues of war and peace — then we lose what's best about humanity. We lose our sense of possibility. We lose our moral compass.
Like generations have before us, we must reject that future. As Dr. King said at this occasion so many years ago, "I refuse to accept despair as the final response to the ambiguities of history. I refuse to accept the idea that the 'isness' of man's present condition makes him morally incapable of reaching up for the eternal 'oughtness' that forever confronts him."
Let us reach for the world that ought to be — that spark of the divine that still stirs within each of our souls.

A Night at the Roxy (1978)

Variant: It is the way of weakened minds to see everything through a black cloud. The soul forms its own horizons; your soul is darkened, and consequently the sky of the future appears stormy and unpromising
Source: The Count of Monte Cristo

Source: Life, the Truth, and Being Free (2010), p. 90