Metaphors are not equations
Argument by metaphor is always logically perilous, but a particularly severe error can be made when a metaphor is treated as an equation, equating the referent with the metaphorical expression. When Longfellow compare’s the skipper’s daughter’s eyes to Linum catharticum or when Burns describes his love as a red, red rose, they are talking about humans and emotion, not about botany. They are not equating eyes or love to plants. One would not seek gardening tips from their poems.
In a highly-commented on post below, BLT co-blogger Suzanne comments on N. T. Wright’s use of a metaphor – that creation was a marriage of heaven and earth. There is plenty of discussion about the biases that N. T. Wright brings to the table, but first and foremost, we need to remember that a metaphor is not an equation. A metaphor comparing creation to the marriage of heaven and earth may shed light on the nature of creation, but it is hardly is useful in understanding marriage.
BLT 
Well, what a co-incidence, Theophrastus! You link to “The Wreck of the Hesperus” in your post today, and I’ve discussed the Star named Hesperus in Greek literature (and also in the Greek translation, the Psalms) in a post today.
Well, that Venus sure gets around, doesn’t she!
Here is an example of the problems you can get into (I stole this example):
(1) Einstein is the “Babe Ruth” of physics.
Or putting it symmetrically
(2) Babe Ruth is the “Einstein” of baseball.
Clearly by substitution
(3) Einstein is the “‘Einstein’ of baseball” of physics.
But since the example of “baseball” was arbitrary, without loss of generality we can assert
(4) Einstein is the “‘Einstein’ of poetry” of physics.
Combining (4) with (1) we have
(5) The “Babe Ruth” of physics is the “‘Einstein’ of poetry” of physics
Cancelling the “the … of physics,” we get
(6) Babe Ruth is the Einstein of poetry
A surprising but inescapable conclusion, and another “home run of destiny” for the Bambino. Next, let’s try to figure out who the Spinoza of dental hygienics is….
That made me laugh out loud.
For something to read with a straight face, there’s Sara. J. Newman’s wonderful article
“Aristotle’s Definition of Rhetoric in the Rhetoric: The Metaphors and Their Message”
That last link isn’t working right, Kurk. Can you fix it? I’d very much like to read it.
Sorry about that, Kristen. The link is to the abstract of Newman’s article, which is in the journal Written Communication, January 2001, volume 18, pages 3-25. Unfortunately, it’s not fully available online for free. I hope your library has it, and if not do send me an email.
Excellent!