“Our elders are always optimistic in their views of the present, pessimistic in their views of the future; youth is pessimistic toward the present and gloriously hopeful for the future. And it is this hope which is the lever of progress—one might say, the only lever of progress.”

On his hopes for mankind in “In the Author’s Universe: Interview with Sci-Fi Author Cixin Liu” https://vocal.media/futurism/in-the-authors-universe-interview-with-sci-fi-author-cixin-liu in Vocal (2016)

This quote has been attributed to Hesiod on the internet, and even published with citation as a dubious attribution, but there are no known occurrences of it in his writings.
Misattributed

Often quoted as "Traditionalists are pessimists about the future and optimists about the past.", e.g, Peter's Quotations : Ideas for Our Time (1979) by Laurence J. Peter, p. 112.
Faith for Living (1940)

Quoted in Women Know Everything!: 3,241 Quips, Quotes, and Brilliant Remarks By Karen Weekes, p. 41

As quoted in FPA Book of Quotations : A New Collection of Famous Sayings (1952) by Franklin Pierce Adams

Page 352-353.
Stepping Westward (1965)

Humor in Psychotherapy (2007)

"Heroic Reason", as translated by H. R. Hays, in Selected Writings of Juan Ramon Jimenez (1957) edited by Eugenio Florit, p. 231.
Context: A permanent state of transition is man's most noble condition. When we say an artist is in a state of transition, many believe that we are belittling. In my opinion when people speak of an art of transition this indicates a better art and the best that art can give. Transition is a complete present which unites the past and the future in a momentary progressive ecstasy, a progressive eternity, a true eternity of eternities, eternal moments. Progressive ecstasy is above all dynamic; movement is what sustains life and true death is nothing but lack of movement, be the corpse upright or supine. Without movement life is annihilated, within and without, for lack of dynamic cohesion. But the dynamism should be principally of the spirit, of the idea, it should be a moral dynamic ecstasy, dynamic in relation to progress, ecstatic in relation to permanence.