Karl Popper, “The Necessity for Peace,” All Life is Problem Solving, (New York: Routledge, 1999), p.143:
The future is open. It is not fixed in advance. So no one can predict it — except by chance. The possibilities lying within the future, both good and bad, are boundless. When I say, ‘optimism is a duty’, this means not only that the future is open but that we all help to decide it through what we do. We are all jointly responsible for what is to come.