I was in the national gallery today, looking at various paintings, and something struck me as I was looking at Elijah fed by Ravens.
Elijah has bunions.
Obviously the subject that Guercino was working from had feet and so Guercino painted the prophet Elijah with the same feet as this old guy sitting around in 17th century Italy.
Guercino had no idea what the real Elijah looked like, skin colour, clothing etc. it's all guess work. So what's so interesting about the feet?
Well it reminded me of a couple of pictures I saw on the net recently. Below are the feet of a habitual shoe wearer.

Now look at a non-habitual foot wearer.

Looks quite odd doesn't it? Except that's the natural formation of the foot, toes splayed out to provide balance. Wearing shoes squashes your toes into little curls, like spent crepe swirls.
If I was obsessive about this, right about here is where I could invite you to pay sites depicting 'natural toes in all their healthiness'
Tip: don't search on google for 'natural feet' with safe search turned off. It's 90% porn. Actually don't search for anything on google with safe search turned off, it's still 90% porn.
I looked at other pictures, and they all had the same feet. Check out Perseus crashing a party, zoom in bottom left and check out the guys feet. Open toed sandals, fitted to feet that clearly live inside shoes. Or Adam and Eve, little shoe-toes (plus bellybuttons, but let's not discuss that philosophical quandary).
Now, I'm not sure that these ancient people didn't wear shoes. Apparently the shoe dates from around 8000 BC for definite, so it's not impossible that they had feet like this. And of course angels and cherubs are gonna be painted with human feet, how else can you paint them? It's pedantic and boorish to attempt to analyse biblical art in this way. Luckily that's not my point.
What struck me is how we take so much for granted, and these are your feet. This is an intrinsic part of your body, and you have no idea what it naturally looks like. When you see people with distended necks, or massive lip and ear deforming plates, there is a tendency to recoil, and say 'That's not normal'.
Every day you get up and strap foot deforming shapes onto your feet, which definitely do not improve your balance (stop you getting disease from the sharp and pointy city, though). And we have no idea that feet can look any different. Even when an artist sits down and tries to paint something fantastical, he still lovingly recreates the deformations created by modern footwear. All historical art and prose is filled with subtle and indetectable anachronisms.
An alien physician could study humans and deduce that these paintings were not painted in the time frame they represent, just by the shape of the feet.
So if something as seemingly concrete as the human form is shielded from our vision, then what other assumptions and veils are we working behind?