Camus basically resolves this in the following way:
- The universe has no innate meaning, and if causality exists (which is still an honest debate in metaphysics) it is unknowable to us. Camus calls this condition the Absurd.
- In an absurd world, our choice is limited. We can make decisions with an outcome in mind, but that outcome is by no means guaranteed.
- Ultimately, a person’s purpose in life is created by the individual as a means of escaping existential despair (which Sartre calls ennui) as a response to an absurd world.
- But within this purpose, there are always choices. Sisyphus has limited choices in his situation: his purpose in life (or afterlife in this case) is to roll the boulder up the hill for eternity. He is free to choose not to roll the rock up the hill — but this would invalidate his purpose.
- If Sisyphus chooses to not role the rock up the hill, it is akin to suicide in that he has given up his purpose. If humans have any purpose in life (which isn’t certain in the case of a deterministic model of causality), it is to define our own purpose in life. This isn’t a choice we can avoid, and the meaning we assign to life ceases to have meaning as soon as we die.
Existentialism is an intensely personal philosophy. It makes no assumptions about other people or societies writ large; only that people are free act in accordance with their chosen (and some would say arbitrary) purpose. It was a reaction of Camus and Sartre’s experiences in Nazi-occupied France — a situation where choices were extremely curtailed, but required the individual to make them nonetheless.
TLDR: there is always hope, even in an absurd world. The world is largely arbitrary and meaningless, so your choices only matter insofar as they define your existence. Or as I like to define it, when you stare into the void of nihilism you see only your own reflection.