“The point of living, and of being an optimist, is to be foolish enough to believe the best is yet to come.”

Introduction, p. 1.
The Best of All Possible Worlds (2006)

This is actually James Branch Cabell from The Silver Stallion (1926)
Misattributed

1990s, Long Walk to Freedom (1995)
Context: I am fundamentally an optimist. Whether that comes from nature or nurture, I cannot say. Part of being optimistic is keeping one's head pointed toward the sun, one's feet moving forward. There were many dark moments when my faith in humanity was sorely tested, but I would not and could not give myself up to despair. That way lays defeat and death.

John Perry Barlow 2.0 (2004)
Context: It’s a perfect set of circumstances to give us the time Yeats foretold, with the best having lost all conviction and the worst full of passionate intensity. I’m an optimist. In order to be libertarian, you have to be an optimist. You have to have a benign view of human nature, to believe that human beings left to their own devices are basically good. But I’m not so sure about human institutions, and I think the real point of argument here is whether or not large corporations are human institutions or some other entity we need to be thinking about curtailing. Most libertarians are worried about government but not worried about business. I think we need to be worrying about business in exactly the same way we are worrying about government.

Coth, in Book Four : Coth at Porutsa, Ch. XXVI : The Realist in Defeat
Source: The Silver Stallion (1926)
Context: Yet creeds mean very little... The optimist proclaims that we live in the best of all possible worlds; and the pessimist fears this is true. So I elect for neither label.

As quoted in Serving "60 Years to Life", Newsweek Europe (12 December 2005)

Source: Vorkosigan Saga, Shards of Honor (1986), Chapter 7 (p. 101)

"The No Point", p. 256
Sun Ra : The Immeasurable Equation (2005)

The Best of Gene Wolfe (2009), afterword to "The Boy Who Hooked the Sun", p. 381
Nonfiction