When I have fears that I may cease to be Before my pen has glean'd my teeming brain,
Wow. Keats sure doesn't pull his punches. This poem starts out by laying it on the line: what would happen if I died today? (Take a second and think about that. Chances are that it doesn't inspire all that many happy thoughts.)
Come to think of it, how often do you think about how (and when) you're going to die? Nowadays, we'd call that morbid. In fact, we might even send you to a counselor to make sure that you're not about to commit suicide. Death just isn't one of those topics that healthy people spend all that much time worrying about, right?
Well, that wasn't true for the Romantics. See, back in the early 19th century, everyone who was anyone (literarily speaking) did a good bit of thinking about their own mortality. It helped to put things into perspective: big, scary world vs. little, mortal human being. That's just how things were.
Acknowledging your mortality doesn't make you any less fearful, though. That's precisely the problem that sets this poem in motion. See, Keats knows that he's got quite a bit to say. In fact, his brain is "teeming" with the poems that he has yet to write.
The first two lines of this poem set up a hypothetical world. Keats isn't dead yet (although, eerily enough, he will be within a few years). His poem, however, is based on two certainties:
1. He WILL have lots of important stuff to write.
2. He WILL die before he has a chance to write it all.
Directing the reader towards two things that haven't yet happened (and which will, of course, cancel each other out) places us in a very weird situation. We're deep inside a very contradictory imagination – that of the poet himself.
Before high piled books, in charactery, Hold like rich garners the full-ripen'd grain;
Ah, metaphor. But what does it mean?
Here's the rough translation: If I die before I have written lots of books (that's the "high piled books," in case you were wondering) which hold my words like a grain silo (that's the "garner") holds ripe grain…
Keats had some seriously flowery language up his sleeves. If you're a fan, don't worry. Keats has lots more to come.
Why play around with all of this metaphorical language? Well, some interesting things happen when Keats starts to compare his poetry (or, more broadly, the products of his imagination) to other things. The image he chooses refers straight back to nature: his poems are like harvested wheat. They're the natural product of a fruitful earth (or, er… his brain). It's almost like Keats' mind becomes an natural element in this particular metaphor.
Look a little bit closer, though, and something strange starts to occur. Harvested grain is, well, dead. It's not the actual living plant. It's that dry, brittle husk containing the seeds of future growth. Published poems, to follow this metaphor, aren't alive in the same way that the poet's brain is. Just like grain, though, it's the published poems that bring in the bucks. (After all, who ever head of paying good money for a wheat plant?)