Describe the evolution of thought in "True Love," Sonnet 116 by Shakespeare.

The evolution of thought in Shakespeare’s Sonnet 116
follows the logical expansion of an idea that defines the nature of love. Instead of
turning to a contrasting idea at the sonnet voltas (volta means
turn, as in a Petrarchan turn to a contrasting idea), Shakespeare
continues expanding on the nature of love but turns to other
metaphors for it; the metaphors are expressed in imagery of unified minds, ships
navigating in troubled seas, and "Time" as a reaper. The ending couplet presents a
paradoxical resolution that says, in paraphrase, "If I'm proven wrong about this that
I've said, then it must also come to be true that I never wrote anything and that no man
ever loved."

The first iambic pentameter quatrain, in the standard
Shakespearean rhyme scheme of abab,
metaphorically compares love to unity of mind, "marriage of true minds." He describes
this unity as one that does not alter in devotion due to changes (usually understood to
mean physical changes, as in old age) in the one loved. "Or bends with the remover to
remove" means that the unity of mind stays steadfast even if the one loved withdraws
their love.

The second quatrain, with a volta at line 5, turns to a
second metaphor that compares love to objects by which sailors navigate in tempestuous,
stormy seas and by which they return back on their true course when tossed off course.
The metaphor means that love can keep one from being discouraged by problems ("looks on
tempests and is never shaken") and can be the measure of one's course in life ("every
wandering bark, / Whose worth's unknown, although his height be taken.") A "bark" is a
small sailing ship, and "height" refers to altitude as measured by longitude. "Whose
worth's unknown" refers to one who is young in life and has not yet made a great impact
in life, therefore, their "worth" to society cannot yet be known.

The
third quatrain turns at the second volta of line 9 to a metaphor that contrasts love to
"Time" that is personified as a reaper of "hours and weeks" and who has a "bending
sickle." Love withstands the changes and threats produced by time and is eternal, "even
to the edge of doom." The resolution in the couplet of the evolution of thought offers a
reversed conditional that challenges the reader to prove "If this be error" and
concludes with the contradictory challenge that if the ideas expressed in the metaphors
can be proved erroneous, then he never wrote, including the poem being read at the
precise moment, and "no man ever loved."