Camus isn't simply saying optimism survives hardship—he's describing something more subtle: the discovery that our resilience doesn't arrive from outside, doesn't depend on circumstances improving, but emerges from within as an innate fact about ourselves we didn't know we possessed. The "invincible" summer matters precisely because it cannot be conquered or extinguished, which means it has nothing to do with feeling cheerful during dark times. Someone working through months of depression might recognize themselves here—not because they suddenly feel better, but because they notice one morning that some essential part of them has stayed intact, untouched by the weight they've been carrying.