Sartre’s point is “we must act without hope”. What he means by this is that we must act without making any assumption that what we are working for will be achieved. Maybe it will, maybe it won’t. We don’t know and never can know, but it doesn’t matter: “One need not hope in order to undertake one’s work.” What matters is action, not a mere idea of what one hopes that action will lead to: “It is only reality that counts, not dreams, expectations or hopes.” Sartre’s ambiguity therefore seems to rest on an embrace of a kind of hope without hope: hope without any illusions that we will prevail.
…Indeed, Sam Harris suggested to me that without hope we might be more at peace. “Hope and fear are completely natural responses to uncertainty. But they are two sides of the same coin: if we would be free of fear, we must let go of hope. Easier said than done, of course. But it is possible. And being without hope is by no means synonymous with despair. Rather, it is tranquility.”