
This may be an unanswerable question; after all, beauty is said to be "in the eye of the beholder". And a casual leafing through one of the current crop of home decor or fashion magazines will serve as illustration; better still, leaf through a 1970s edition. How many of those prize-winning designs would you give house room to? Somebody did, and pronounced it "fabulous!"
And I share with a few (too few, alas!) an appreciation for the creepy-crawlies that share my life. Dust mites and spiders, slime molds and fruit flies; that sort of thing. I think many of them are breathtaking, but most of the people in my life say things like "Ewwww! Ugh! Get that away from me!"

And this critter (above)? He's a Cuban Solenodon, and I ran into him on Endangered Ugly Things. A helpful little guy, an insect-eater about 18 inches long. Not beautiful. We agree on that, don't we?
So, what do we mean when we say "beautiful"? What makes a kitten cuter than a solenodon? How do we determine the difference?
Maybe simpler to answer; what feelings are aroused in us by the sight or sound of beauty? What causes them?
At one time, I looked to our past for an explanation. And came up with this: greenery suggests good food sources, good cover, a favourable climate. Water is essential to life. And real estate agents say that a home with a view of water -- any water -- outsells a similar one without. So we have learned to see scenery that would enhance our lifestyles as beautiful.
I was wrong. I saw that fairly soon. Because the sight of enormous bare rocks peeking through high mountain icecaps gives me the absolute shivers, they are so overwhelmingly beautiful. As does the curve of a sand-dune, the jagged, tumbled mass of lichen-covered scree, the dry, dry, dry, scratchy, bare hills of the Chilcotin. Or think of the images from Hubble. Not hospitable havens for humankind, by any stretch of even a sci-fi writer's imagination.
So what is it? I have no answers. Do you?
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Photos from Wikipedia and Mushroom Expert.com