Metaphors are used in speech and writing when one thing is said to be another, as in, "He has a heart of stone," or "Her eyes were oceans, filled with watery light." The effects of metaphors on a reader include the creation of vivid imagery, such as the transfer of emotional content from something generally understood to something less understood, the intrusion of non-literal significance into consciousness and the revitalization of familiar words and phrases with new meanings.
The Creation of Vivid Imagery
The effectiveness of written and spoken communication often depends upon the ability of the speaker or writer to present the listener or reader with specific, detailed, memorable images. Metaphors accomplish this by combining ideas in unexpected ways. For example, Shakespeare's poetic phrase in Sonnet 18, "Shall I Compare Thee to a Summer's Day," he writes: "Sometime too hot the eye of heaven shines," evokes an image of the sun as a glaring eye.
Transfer of Emotional Content
Metaphors are effective partly because they borrow emotional content from something that is already well understood and lend it to something that the writer is trying to help a reader understand. This explains why metaphors often use commonly understood objects, such as the moon, stars and oceans. For example, an author might write, "When he saw her, his head burst with starlight," or, "Friendless he wandered, through a desert that knew no bounds."
Much of language is non-literal, and this portion of communication is often deeply significant. Non-literal communication may have the logic of a dream, associating two things on the basis of surface meanings or striving for deep symmetry between concepts not obviously connected. Metaphors operate on this non-literal level, deepening the reader or hearer's grasp of meanings below the surface of consciousness. For example, in Sonnet 18, the speaker compares the object of desire to a summer day, but then states that she is "more lovely and more temperate" than summer. This sets up the reader to understand that the beloved possesses qualities that extend beyond the literal, temporal world of nature’s seasons: "But thy eternal summer shall not fade," the speaker states. It is this transfer of the literal description of nature to how the woman possesses spiritual beauty that resides in a dimension apart from temporal beauty. This is a glimpse into the beauty and power of metaphor.
The Revitalization of the Familiar
The power of language to communicate ideas tends to dissipate through overuse and the passage of time. Phrases that once surprised hearers with their freshness, become trite. Metaphors reverse this process by revitalizing old words, giving them new meanings and applying them to new contexts. For instance, Vladimir Nabokov, in his novel Pnin, described a character's mouth as a seaside cave and his tongue as a "fat, sleek seal."