A friend told me on Shabbat that one philosopher- I think it was Wittgenstein shattered the category theory of language by pointing to the word game. There is no trait, he argued, that all things defined by that word share; instead, they form a general family of meanings, with certain traits running through some of them, which in turn share traits with others, etc.
I think that an equally good example of this is poetry. What is poetry, exactly? Can you come up with a definition that includes both limericks and free-verse, Shel Silverstein and e.e. cummings, Shakespeare and Mother Goose? One definition that I have heard of poetry is that it contains layers of meaning beneath the literal. Bah. How, exactly, does the parsha poem that my high school passed around every Friday ("In this parsha, we have the flood/ Where Hashem turned all the land to mud"- not an actual quote, if only because it has a meter) contain more layers of deepness than Dosteovsky?
The inspiration for this musing is a certain...piece of my own. In the beginning, I called it a poem- it had a very strong meter, no rhymes, and was written in stanzas. Somehow, during the editing and re-writing process, it metamorphized into a story with a lot of cadence, written in paragraphs. In neither form does it rely solely on full sentences, and in neither form is the cadence perfect. (This was intentional- the mood of the...whatever-it-is was a bit too confused and unhappy for it to be perfect iambic.) It hasn't really got much in terms of metaphors or symbolism, I suppose, but it certainly is meant to create a mood through its use of images. So...which is it, really? Poem or prose?