The poem known as Sonnet 116 is one of the most famous of William Shakespeare's sonnets. In it, the poet expresses the message that love is eternal and unchanging regardless of circumstances. It is not addressed to a particular lover, but rather offers thoughts on love in general.
The poet begins with the observation that there should be no "impediments," or obstacles, to two minds united in love. Love remains constant even when there are "alterations," or changes, in the surrounding circumstances.
After these thoughts, Shakespeare compares love to an "ever-fixed mark." In other words, it is something that does not move or shift even though it may be beset by tempests, or storms. It is like a constant "star" that guides ships ("barks") at sea. It is of such great value that its worth is immeasurable, although people might attempt to measure it, perhaps by its duration or the actions that people take on their lover's behalf.
Shakespeare goes on to say that true love is not made a fool by time. "Rosy lips and cheeks" are signs of youth, and a "bending sickle" is a symbol of time's ultimate victory of death. However, even though outward appearances may fade and lovers will eventually die, true love has no limitation of time and lasts forever, even beyond "the edge of doom," or the end of the world.
The last two lines in the sonnet are an exclamation by the poet of how much he believes what he has just written. He declares that if what he writes is not true, then he has never written anything and no one has ever loved.