William Shakespeare's Sonnet 116

Let me not to the marriage of true minds Admit impediments. Love is not love Which alters when it alteration finds, Or bends with the remover to remove. O no, it is an ever-fixèd mark That looks on tempests and is never shaken; It is the star to every wand'ring bark, Whose worth's unknown, although his height be taken. Love's not time's fool, though rosy lips and cheeks Within his bending …

…constant. As any logician could testify, however, these premises have no necessary relationship to their conclusion. The couplet is designed to shut down all opposition, to secure the thing (unchanging love) the poem has staked its heart on. It is sheer bravado, and of course it fails. What fails as logical proof, however, succeeds quite brilliantly as poetry. The sonnet has staged its own undoing and, doing so, has rendered an eloquent portrait of faith-under-pressure.