On what does Juliet finally allow Romeo to swear his love?

Shakespeare underscores the idea that lovers must abandon their selfishness by having Romeo and Juliet swear to themselves, rather than to other bodies. For instance, when Romeo tries to swear by the moon, Juliet remarks that the moon waxes and wanes, and is too variable. Instead, she says, "Or if thou wilt, swear by thy gracious self" (2.1.155).