Metaphor compares two different things by speaking of one in terms of the other. Unlike a simile or analogy, metaphor asserts that one thing is another thing, not just that one is like another. Very frequently a metaphor is invoked by the to be verb:

Affliction then is ours; / We are the trees whom shaking fastens more. --George Herbert

• Then Jesus declared, "I am the bread of life." --John 6:35 [And compare the use of metaphor in 6:32-63]
• Thus a mind that is free from passion is a very citadel; man has no stronger fortress in which to seek shelter and defy every assault. Failure to perceive this is ignorance; but to perceive it, and still not to seek its refuge, is misfortune indeed. --Marcus Aurelius
• The mind is but a barren soil; a soil which is soon exhausted and will produce no crop, or only one, unless it be continually fertilized and enriched with foreign matter. --Joshua Reynolds

Just as frequently, though, the comparison is clear enough that the a-is-b form is not necessary:

• The fountain of knowledge will dry up unless it is continuously replenished by streams of new learning.
• This first beam of hope that had ever darted into his mind rekindled youth in his cheeks and doubled the lustre of his eyes. --Samuel Johnson
• I wonder when motor mouth is going to run out of gas.
• When it comes to midterms, it's kill or be killed. Let's go in and slay this test.
• What sort of a monster then is man? What a novelty, what a portent, what a chaos, what a mass of contradictions, what a prodigy! Judge of all things, a ridiculous earthworm who is the repository of truth, a sink of uncertainty and error; the glory and the scum of the world. --Blaise Pascal
• The most learned philosopher knew little more. He had partially unveiled the face of Nature, but her immortal lineaments were still a wonder and a mystery. . . . I had gazed upon the fortifications and impediments that seemed to keep human beings from entering the citadel of nature, and rashly and ignorantly I had repined. --Mary Shelley
• The furnace of affliction had softened his heart and purified his soul.

Compare the different degrees of direct identification between tenor and vehicle. There is fully expressed:

• Your eye is the lamp of your body; when your eye is sound, your whole body is full of light; but when it is not sound, your body is full of darkness. --Luke 11:34 (RSV)

There is semi-implied:

• And he said to them, "Go and tell that fox, 'Behold, I cast out demons and perform cures today and tomorrow, and the third day I finish my course."' --Luke 13:32 (RSV)

There is implied:

• . . . For thou hast been my help, and in the shadow of thy wings I sing for joy. --Psalm 63:7 (RSV)

And there is very implied:

• For if men do these things when the tree is green what will happen when it is dry? --Luke 23:31 (NIV)

Like simile and analogy, metaphor is a profoundly important and useful device. Aristotle says in his Rhetoric, "It is metaphor above all else that gives clearness, charm, and distinction to the style." And Joseph Addison says of it:

• By these allusions a truth in the understanding is as it were reflected by the imagination; we are able to see something like color and shape in a notion, and to discover a scheme of thoughts traced out upon matter. And here the mind receives a great deal of satisfaction, and has two of its faculties gratified at the same time, while the fancy is busy in copying after the understanding, and transcribing ideas out of the intellectual world into the material.

So a metaphor not only explains by making the abstract or unknown concrete and familiar, but it also enlivens by touching the reader's imagination. Further, it affirms one more interconnection in the unity of all things by showing a relationship between things seemingly alien to each other.

And the fact that two very unlike things can be equated or referred to in terms of one another comments upon them both. No metaphor is "just a metaphor." All have significant implications, and they must be chosen carefully, especially in regard to the connotations the vehicle (image) will transfer to the tenor. Consider, for example, the differences in meaning conveyed by these statements:

• That club is spreading like wildfire.
• That club is spreading like cancer.
• That club is really blossoming now.
• That club, in its amoebic motions, is engulfing the campus.

And do you see any reason that one of these metaphors was chosen over the others?

• The harvest is plentiful, but the laborers are few. --Luke 10:2
• The pile of dirt is high, but we do not have many shovels.
• The diamonds cover the ground, but we need more people to pick them up.

So bold and striking is metaphor that it is sometimes taken literally rather than as a comparison. (Jesus' disciples sometimes failed here--see John 4:32ff and John 6:46-60; a few religious groups like the Jehovah's Witnesses interpret such passages as Psalm 75:8 and 118:15 literally and thus see God as anthropomorphic; and even today a lot of controversy surrounds the interpretation of Matthew 26:26.) Always be careful in your own writing, therefore, to avoid possible confusion between metaphor and reality. In practice this is usually not very difficult.