There're plenty of vague abstract ideas running around which we all understand but none of us can describe.
What's "intelligence"? Nobody knows. What's "mind"? Nobody knows. "Space"? "Time"? "Art"? "Left" and "right", "up" and "down"? We all have the ideas of it in our heads, but when it comes to putting them into words, nobody has any.
It's fascinating. Somehow, we understand those ideas without describing them, learning about them as we go, with nobody telling us about it - and yet, we can't describe any of those. We understand that space is the... space we exist in, where matters exists in, like, where we can walk, fly and stuff; and art is a process of creating something which has... some traits which attract us in a certain way - but we can barely touch their meaning with words.
It feels like, for most of those ideas - those that composite the vague ideas listed - we don't have words. Describing space using "space" is recursive and, therefore, useless in understanding of it. Same goes for time and the directions we're so used to. You can easily say while standing on the ground where "up" and "down" is, but as you leave the affect of terrestrial gravity, you lose those directions: where's up and where's down in a space which has no sky and no ground? It's the same for "left" and "right", and not only in the gravity-affected space.
What got me thinking about it is the concept of "fish". Stephen Jay Gould, after a lifetime of research, concluded that there's no such thing as a fish: that the sea creatures we call "fish" are as diverse and unrelated to each other as air-life is (with, for example, a salmon being closer to a camel than to a hagfish). Still, we feel pretty comfortable calling those sea creatures "fish" because they appear very similar to our view - and we can probably tell that a fish is a creature that lives underwater and breathes using gills, but that's a far as we can go, and ever here we've already hit a few bumps: dolphins and whales are mammals, which, ideally, puts them outside of the certain "seafood" category.
Still, we feel pretty confident calling those animals fish along with the rest of the small, legless crowd. I wonder if a fish, had it mind to comprehend it, coming out of the water for a time, see the earthly animals similar to each other, in the similar way to Europeans seeing Asians very similar to each other - or us, seeing fish...