Sam Spade expresses it succinctly in the final chapter of The Maltese Falcon when he tells Brigid O'Shaughnessy he is sending her to San Quentin and gives her seven reasons for doing so, then asks:
"Now on the other side we've got what? All we've got is the fact that maybe you love me and maybe I love you."
"You know," she whispered, "whether you do or not."
"I don't. It's easy enough to be nuts about you." He looked hungrily from her hair to her feet and up to her eyes again. "But I don't know what that amounts to. Does anybody ever?"
Is there such a thing as true love? Or is it at best only a mutual delusion, a folie a deux?