As in his plays, Shakespeare’s sonnet introduces several themes reflecting Renaissance thought. The most important of those here is the belief that everything under the moon was corrupted by Adam’s fall from grace. Thus, although the sun (the “eye of heaven”) moved in an uncorrupted sphere above the moon, the earthly influence upon its shining could make it either too hot (line 5) or too hazy (line 6). A corollary of this fall was the consequent mutability of the sublunary creation. For Shakespeare the change was not lateral; rather, it involved a progressive degeneration of beauty, created by chance or by the influence of time on nature (lines 7, 8).
In Shakespeare’s Sonnet 18, one may thus discern Renaissance beliefs about nature. One can also see remnants of medieval thinking. This combination appears most obviously in the poet’s treatment of the Ovidian tradition. The Middle Ages had interpreted Ovid (43 b.c.e.-17c.e.) as a moral poet whose Metamorphoses (c. 8 c.e.) contained a cosmology based on Greek and Roman myths. The Renaissance, on the other hand, saw him as an erotic poet whose Amores (c. 20 b.c.e.; Loves) and Ars Amatoria (c. 2 b.c.e.; Art of Love) provided the model for Petrarch and later sonneteers.
In Sonnet 18 one finds both the moral and erotic suggested in the words “lovely,” “darling,” and “fair.” Emphasis on the physical beauty of the person addressed is tempered by hints that this beauty outshines that of the natural universe itself; through the poet’s lines, it becomes one with Plato’s eternal forms. Missing from this sonnet, however, is that part of the Petrarchan tradition that sees the lover complaining of his mistress’s rejection and displaying his own despair or resolution resulting from it. In its place one finds the central theme of mutability, the imperfection and impermanence of the sublunary world, infusing the first eight lines and providing the foil for the rest of the poem.
In contrast to the mutability theme, the concluding sestet proclaims Shakespeare’s art as the antidote to time and change. The poet’s consciousness of his own genius, although placed here within a tradition maintained by several of his predecessors, transcends the limitations of the fallen world. Ars longa, vita breve (art is long, life is brief) becomes the underlying theme, arrayed in Shakespeare’s unique and comprehensive poetic language.