Keller links optimism with the willingness to explore what is unknown. Her examples of studying the stars and travelling to uncharted places represent activities that require curiosity, effort and some confidence that discovery is possible. Pessimism, by contrast, is presented as an attitude that can discourage such attempts before they begin.

The statement is rhetorical rather than a literal historical claim that pessimistic people have never made discoveries. Its broader point is that exploration depends partly on a willingness to act under uncertainty. Scientific inquiry, geographical exploration and other ambitious undertakings often require people to invest effort without knowing whether they will succeed. Keller therefore treats optimism as a practical source of initiative: it helps make experimentation, risk and sustained investigation possible. The quote fits her wider argument that hopeful beliefs matter because they can encourage action and contribute to progress.