As the title suggests, the poem argues that true love will not be impeded, changed or stopped by transitory circumstances. True love, implicitly opposed to lust or some shallower form of affection, is a fixed form: the narrator describes it as "an ever-fixed mark." The poem sets it in opposition to all that is changeable.
If the love between two people is real, it will rise above whatever temporary material conditions might get in its way. If trouble comes, which the poet likens to "tempests" or storms, true lovers are not "shaken" by this. If time ravages the looks of the beloved, wreaking damage on rosy cheeks and lips, that doesn't matter either: love transcends physical appearance. The poet likens true love to a star that stays unchanging in the sky. It is eternal. The poem argues that true love is a perfect, Platonic form based on the union of minds, impervious to what can happen to the body.
If we understand this sonnet as the voice of a lover speaking to his beloved, it mounts a strong and fervent argument in favor of the depth and sincerity of the poet's love, reflecting the heart of a person who even promises to stay with the beloved to "the edge of doom."
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Shakepare's Sonnet No. 116, "Let Me not to the Marriage of True Minds" is a fervent expression of the theme of constancy in love. Love which is the "marriage" or sacred union between two minds is "true" only when it is constant.
He defines true love both negatively and positively. True love will not change when any type of change occurs in the life of the lovers or when the lovers are separated - "removed."
It is, Shakespeare asserts, "an ever-fixed mark" that can weather any storm. It is the fixed star by which sailors steer their wandering ships. Even though the lovers grow old and their youthful beauty fades with the passage of time their love for one another will remain constant and firm till death separates the true and constant lovers.
Shakespeare swears that all what he has said in the quatorzain is true and that if he is proved wrong then neither has any person truly been in love nor has he ever written. However paradoxically, since the poem has been written it means that all what Shakespeare has written about true love is indeed true.