In Sonnet XVIII, written in the Petrarchan form of an octave followed by a sestet, Shakespeare incorporates Renaissance themes such as the imperfection of the universe because of the fall of man.
- The mutability of Nature
This theme of the imperfection of Nature is presented in the octave. In these first eight lines, the poet presents the argument that Spring and Summer "hath all too short a date," and, in addition to the fleeting nature of these seasons, at times summer it often too hot. Of course, the seasons change, as well. This last idea is the turn in thought, or volta.
- Art is immortal
In the sestet, then, the last six lines answer the argument presented in the first eight; namely, that Nature changes and, therefore, comparing the lover to "a summer's day" is an imperfect and temporal metaphor. But, putting his words of love into this poem will provide the lover with the perfect metaphor because in verse her "eternal summer shall not fade." That is, at least as long as mortal man lives so that "eyes can see" and read this verse, the woman's beauty will last. The concluding heroic couplet summarizes the poet's thoughts,
So long as men can breath, or eyes can see,
So long lives this, and this gives life to thee.