as an example of a context clue, when driving you tend to use the size of a car as a proxy for its distance, instead of stereo vision depth perception. It's fairly common for people to lose acuity in one eye but can still drive for this reason.

The smaller peripheral eyes of spiders seem to be smaller and simpler then two (or four) "main" eyes. Processing data from more eyes requires more of the expensive brain tissue.

In my purely speculative mind it's because that's the number of eyes that is in harmony with the rest of the genes.

> (Also, I find it interesting that most of internal organs are not symmetrical)

And they can sometimes grow 'mirrored': https://en.wikipedia.org/wiki/Situs_inversus

Eh, similar answer, your basic body plan goes back really far in your genetic history and is a really early part of embryonic development. All of your relatives going back to a common ancestor ~500Ma have two eyes. It’s a quick internet answer not an essay, the exact correct common ancestor / taxonomy group is a bit difficult to pin down.

Bilateria is the reason the number is even… some common ancestor after you split with spiders is the “exactly two” reason, which i think is all vertebrates now, or at least close to that level.

'External symmetry is an adaption to environment' seems to be a promising starting point. You can see this at the micro level too: cell externals and internals.

>2 is not the only even number for this.

No, but it's a very common number in more advanced organisms, and the smallest possible that satisfies bilaterality/symmetry.

Having an eye and a fraction of an eye wouldn’t really make much sense. I don’t understand the point you’re trying to make here.