Quotations:

“We must remember that the future is neither wholly ours nor wholly not ours, so that neither must we count upon it as quite certain to come nor despair of it as quite certain not to come.” — Epicurus

This comes from Epicurus’ Letter to Menoceus, a letter from Epicurus himself, preserved in Lives and Opinions of Eminent Philosophers by Diogenes Laertius. In it Epicurus lays out a summary of ancient epicurean beliefs, one of which is to live in the present moment fully, since the future is uncertain.