In Sonnet 116, Shakespeare asserts true love’s constancy and permanence through poetic devices including allusion, metaphor, and paradox. He opens with
Let me not to the marriage of true minds
The “marriage of true minds” is a metaphor for love as a genuine connection between souls. This metaphor emphasizes the deepness of love; it is not a dalliance or superficial relationship between two bodies, but a profound union between like-minded or similarly thinking beings. This “marriage of true minds” transcends mere romantic notions and physical couplings.
Shakespeare then uses allusion to stress true love’s steadfastness:
it is an ever-fixed 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.
The “ever-fixed mark” is the North Star, the anchored beacon by which sailors navigate. They use the North Star as a steady, unshakable guidepost for steering away from treacherous weather and waters (“tempests”). They determine their location and chart their course by measuring their position against the North Star (“his height be taken”).
Another allusion that emphasizes true love’s immutability is Father Time, who morphs into the Grim Reaper on Judgment Day. Shakespeare states:
Love's not Time's fool, though rosy lips and cheeks
Within his bending sickle's compass come;
Love alters not with his brief hours and weeks,
But bears it out even to the edge of doom.
Father Time causes time to pass to the point where people age and then the Grim Reaper appears with a scythe (“sickle”) to bring death (“doom”). Mixed within this allusion is the metaphor of “rosy lips and cheeks.” This image represents youth, beauty, and fertility. Over time (“hours and weeks”), though, these bright features mature and fade. In contrast, love remains unchanged (“alters not”).
Shakespeare closes Sonnet 116 with a paradox, a literary device where the writer states a contradictory and absurd idea that actually contains a bit of truth:
If this be error and upon me prov'd,
I never writ, nor no man ever lov'd.
Essentially, he states that if he is incorrect (i.e., that it is not true that true love is immutable), then he has never written anything. He will eat his words if he is wrong. However, the idea that he has never written anything is absurd; after all, we are reading his poetry! Therefore, by his logic, true love is just as he describes in earlier lines: constant. He is so sure that his idea of love is valid that if its reverse is true, then the impossible is true—that he never wrote and humans never gave or received love. Yet we see proof of his writing and man’s love. Thus, true love never changes.