What is the main purpose of Sonnet 18 by William Shakespeare?
So long as men can breathe or eyes can see,
So long lives this and this gives life to thee.
The sonneteer's purpose is to make his love's beauty and, by implication, his love for her, eternal. In doing so, he takes a rather circuitous pathway by beginning with a comparison that does not describe her.
In the first two quatrains, he is essentially saying, "No, I cannot compare you to a summer's day, and here's why." The last two lines of the first quatrain,
Rough winds do shake the darling buds of May,
And summer's lease hath all too short a date:
along with the whole second quatrain (four lines of poetry with alternating rhyme) explain why she can not be compared to a summer's day. For instance, "rough winds" do not shake her beauty and her beauty is not sometimes dimmed as even the mighty sun's glory is sometimes dimmed:
[that] too hot the eye of heaven shines,
And often is his gold ... dimm'd;
The third quatrain, beginning with the contradictory conjunction "but," begins the explanation that leads up to the main purpose expressed in the ending couplet (two rhyming lines of poetry). In this final quatrain, the sonneteer says "But ..." your beauty shall be an "eternal summer" and shall not fade nor be dimmed so that even Death shall not rob your beauty.
Then comes the implied "because" of the couplet that explains that as long as Sonnet 18 can be read, then she will have beauty and she will have life eternal.
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