I recommend reading this page prior to the page on the thesis statement.
Metaphor is a keystone of literature, rhetoric, and any other type of intentional speech or writing. Why? Quite simply, metaphor makes people understand. Look at that sentence again:
Metaphor (this figurative device) makes (compel, cause to happen) people (your audience) understand (conceptualize, learn, visualize, persuade, even). Metaphor creates possibilities beyond the immediately perceptible.
First, let's define the term. Metaphor compares two unlike things ( a simile is a type of metaphor using like or as- I call it metaphor lite). One must be careful, though, as relationships of comparison may often be subtle.
Let's look at an example from Shakespeare's Hamlet:
"I will speak daggers to her but use none" (III.i).
How do we know this even is metaphor? I've bolded the literal issue. Unless you picture knives falling out of Hamlet's mouth, the writer invites a level of engagement with the text to further understand what exactly Hamlet means. It cannot mean he will stab "her," in this case his mother Queen Gertrude, as he said he will not use any. He's doing us a bit of a favor by promising he will not enact any literal usage of a dagger (to his mother, anyway).
Let's take a moment to think about how metaphors work. The etymology of the word suggests a meaning of "carrying over" or a "transfer." Well, this transfer of meaning from one thing to another is essentially a machine of two parts: the vehicle and the tenor. The tenor is the subject of the transfer that is being modified, and the vehicle is how this modification or transfer was delivered.
Tenor: Speak/Hamlet's speech
Vehicle: Daggers
But our work is not done. Consider how a dagger may modify our conception of Hamlet's speech, according to him. If one speaks daggers, one's speech may be cutting, harmful, destructive. One may emotionally and cognitively conduct damage LIKE (but not exactly) the way a dagger may do physical harm.
Metaphorical language is powerful because of the level of involvement and participation it invites from the reader, thereby creating a relationship/an exchange*. Popular music, especially found in spoken word and rap, employs metaphor.
For example, if a rapper proclaims they "spit fire" what does that mean? Which is the tenor? Which is the vehicle? What is the meaning?
What if this rapper wanted to further reward his or her listener by extending the metaphor? By turning it into a conceit? Well, perhaps we could expect further lines comparing bars and lyrical prowess to a wild fire, using ashes, cinders, and other fire-imagery to whatever purpose the lyricist would like?
The human (although there is research to suggest the capability of other animals to do so) has the tendency to communicate symbolically. I find this akin to object permanency- the understanding that appearance may not be reality. However, as you will see with my page on the thesis statement, metaphors function as a type of claim (or truism, perhaps). It is imperative that you, as the reader, are not only able to identify and analyze how the metaphor functions, but also evaluate whether____
What about our pal Hamlet?
*While deliberate and careful use of metaphor can be effective, there are examples that I personally find to be failures of metaphors. One example that I point to is a children's book that used the metaphor as a "story": What Would You do With a Chance. Without a narrative or character development beyond the main character as a placeholder for the reader, I fear young and developing minds are not being engaged enough. For metaphor to work, its contextual deployment matters.