The effect of the formal division of the Shakespearean sonnet, the four quatrains and closing couplet, is to pile up examples of a single idea—that the beloved’s beauty is really not comparable to the productions of nature and human art—so that by line 12, the reader wonders if there is anything at all about the woman that can be seen objectively as beautiful. The last two lines then provide a memorable explication of that idea: Objectivity and actual beauty are really no concern of the lover. While lines 11 and 12 dismiss comparisons to heavenly beauty as meaningless—mortals have no experience of the metaphysical world on which to base such similes—Shakespeare uses the mild expletive “by heaven” in line 13 to suggest in contrast that the impassioned subjectivity of the lover is itself metaphysical in origin, a kind of grace.

The speaker’s attitude in this poem is strikingly antimetaphoric, and lines 3 and 4 subject two conventional metaphors to examination by deductive logic. Line 3 begins with a premise, “If snow be white,” and concludes that the woman’s breasts are “dun.” In technical terms, the rhetorical device employed here is an “enthymeme,” a syllogism in which one of the terms is left out and must be inferred by the reader. One may reconstruct the full syllogism thus: Snow is white; my lover’s breasts are dull gray; therefore, my lover’s breasts are not like snow. Since snow is in fact white, one can concur with the conclusion’s logic and deny the validity of the simile “women’s breasts are white like snow.” Line 4 offers another enthymeme beginning with the premise “If hairs be wires” and concluding that the woman’s hair is black, or tarnished, wire. The full syllogism here would read: Hairs are golden wires; my lover’s hairs are black; therefore, my lover’s hairs must be tarnished.

The conclusion follows logically, but the metaphoric premise is untrue: Hairs are not wires, and if the woman is judged on the basis of this premise, one can only conclude by denigrating the woman’s physical characteristics as sullied examples of an ideal: tarnished gold. This is what Shakespeare means by “false compare”—unjust comparisons that not only ignore the possibility that the woman may be beautiful in her own right, but also miss the value of the beloved in the eyes of her lover: To him, she is, if not golden, at least as “rare.” That the poet has his persona subject love and beauty to deductive logic at all tells the reader something important about the lover’s attitude and about the overall meaning of the poem.