It isn’t. Like a writing desk. Not much, anyway. I guess there’s something there about them both being “inky”? Or how each is merely an immovable surface against which the psychological condition of the person engaging with them is pressed into words. (“Nevermore!”) There are other suggestions of how to make sense of Lewis Carroll’s open-ended riddle. But the fact that we can consider the comparison of raven and writing desk at all is a hallmark of the human condition.

Cognitive linguists George Lakoff and Mark Johnson, in Metaphors We Live By, argue that “metaphor is pervasive in everyday life, not just in language but in thought and action” (3). Our cultures, our actions, our worldviews, they say, are tuned by and to certain prevalent metaphors--Lakoff and Johnson highlight “argument is war,” “time is money,” and other metaphors dominant in their own experiences as American academics. Additional research has shown that a person’s emotions, legal judgments, even what they’re willing to do to prevent cancer can be influenced by a carefully chosen metaphor. (For more on this, check out this Quartz article with links to a bunch of that research.)
Our propensity to seek out metaphor is evident in the English language (and plenty of other languages, too, but I don’t want to overstep). In fact, once you get into etymology, you start to discover that an enormous proportion of the language is metaphorical rather than literal. Take the word “discover” (as used in the previous sentence) as an example. Are we literally “uncovering” something in our etymological quest? No. There aren’t Old French and Proto-Indo-European words hiding under throw pillows in my house or yours. That sounds dreamy, but no. It’s metaphorical.

Because of the ubiquity of metaphors in thought and language, many writers incorporate them into their writing intuitively and, on the whole, successfully. But there are still ways that metaphors can fail to do its job correctly, and it’s important to keep these in mind as you develop your writing.
Mixing Metaphors
The power of the metaphor is amplification. A single idea, image, or action appears doubly, both literally and then, through the metaphor, figuratively. In his first inaugural address, Franklin D. Roosevelt describes the American people as “a great army” engaged in “a disciplined attack” on their problems. The metaphor draws a double line under the idea of Americans as resilient people, capable of being trained to fight through the enormous challenges of the Depression. FDR was wise to stop with a single metaphor, though. Once you start mixing metaphors, amplification ceases to benefit the message.

Amplifying twice in the same space is a recipe for disaster. In a literal sense, a microphone interprets a person's voice into another form, and the speaker delivers an amplified version of that voice. Some nuances of the original voice get lost in that interpretation-amplification process. That’s fine because most of the time the ear can fill in the gaps--just as the mind easily interprets metaphors--and the message gets through. But try speaking into a microphone and then holding a megaphone up to the speaker to interpret and amplify twice. The final output will be garbled nonsense.
Imagine if FDR, in his next sentence, had likened the problems being “attacked” by his “army” of American citizens to a nasty winter that would soon pass into spring. There’s nothing inherently wrong with the idea that difficulties come and go in their season, but what can an army do about it? No one can take up arms against a force of nature. The effect is more absurd than anything.

The Overwrought Metaphor
Let’s say you take my advice and limit yourself to a single metaphor. That’s great, but that doesn’t mean you’re now free to wring that metaphor dry. Unless you are doing it to be funny, employ a light touch with your metaphors. How light? The good news is, you have options.
An extremely light touch may be nothing more than a suggestion of metaphor--a word choice that evokes a comparison without pointing to it directly:
Once the idea of starting a mutual aid fund in her own city was planted in her mind, she couldn’t stop thinking about it.
Here, “planted” is a (fairly common) metaphor comparing the idea to a seed. The metaphor is subtle and common enough that it might be overlooked entirely or, perhaps, taken in the wrong sense. The reader might imagine that the idea was “planted” like evidence is “planted”--surreptitiously.

The mere suggestion of a metaphor may not sufficiently amplify the image, concept, or meaning you have in mind. If you want to clarify or elevate your metaphor at all, expand it just a little. Use more explicit comparative language or touch the metaphor twice:
That seed of an idea--the thought that she could start a mutual aid fund in her own city--soon began to take root.
Now, the seed is fully visible to the reader. It has roots and movement. The metaphor paints the mutual aid idea as a small, dormant thing that, once awakened, draws strength from fertile soil. But it doesn’t say any of that. It trusts that the reader knows what a growing seed is like and will fill in those details. When used well, a metaphor saves you time and space in your writing.

It’s tempting to let the metaphor run away with you at this point. You can take this further, but things may get a bit flowery if you’re not careful:
Through long conversations with her collaborators, Cruz watered the idea. She fertilized it with extensive research. Working with lawyer Henry Brunt, Cruz even built a legal framework--a kind of fence or trellis to protect and support her project. Four months later, that burgeoning idea, so recently a little seed, was almost ready to bear fruit.
I mean, we get it. The metaphor isn’t really adding much anymore. In fact, it might be displacing some pertinent and substantive details that could add depth to the writing to balance out the frills.
It’s not that extended metaphors can’t work. But usually they work best when you need the metaphor to explain a complicated idea to your reader. For instance, you want to explain how an emergency financial freeze could help the economy, so you offer the metaphor of a medically-induced coma. At this point, we’re more or less talking about analogies, which are metaphorical in nature but deserve their own treatment. So I will leave it there.

It is impossible for you to write without metaphors. But it is possible for them to hijack your writing, so keep alert. Limit yourself to one metaphor at a time, and keep the touch light. Trust your reader to complete the comparison once you suggest it. It will be worth it in the end. Metaphors be with you!
Really interesting!
Dad