The following quatrain begins with a declamatory "O no", the narrator makes an almost legalistic argument for the eternal passion of love. The sonnet's major metaphor compares love to a priceless lighthouse that can withstand any storm, or like the Northern star guides lost ships. The North Star is used in navigation because of its fixed quality, similarly love is "ever-fixed" it does not bend nor does it alter. A "mark" is a beacon designed to warn mariners of dangerous rocks, love in this case is compared to a lighthouse that can withstand the stormiest "tempest" and "is never shaken". Although we may be able to measure love to some degree it's "worth's unknown", the actual value is neither fully understood or appreciated. .
In the third quatrain, there is reaffirmation of the perfect nature of love being unshakeable throughout time and remains so "ev"n to the edge of doom." or death. "Love's not Time's fool", this personification of Love states that true love is not subject to change in the passage of time, although changes occur in appearance "rosy lips and cheeks" becoming victims of "his bending sickle". Time is also personified into the grim reaper that destroys youth and physical beauty that fades with age. Sibilance in "sickle's compass" emphasises the swishing sound of a sickle as it cuts away the "brief hours and weeks" of human existence. However true love can withstand all and it "bears it out ". The last two lines of an English sonnet should rhyme together, however in this case, "proved" and "loved" do not rhyme although they appear to visually. This emphasises the word "error" on that line and could indicate that if this was a mistake like the poem then he Shakespeare could "never writ". The couplet announces the speaker's certainty that he believes beyond all doubt his definition of true love. He declares that, if he is mistaken about the constant, unmovable nature of perfect love, then he must take back all his writing on love, truth and faith.