I always wanted to have this conversation, but I never found “intelligent design" or “irreducible complexity” to be objective arguments. So I decided to loosely give them operational definitions, which aren’t perfect, but get the point across in an objective way.

1. Design – Anything requiring unique intervention to be achieved. For example, an anthill shows design because dirt does not gather into intricate mounds on its own, it requires ants. (Notice this is extremely broad; but contains no subjective notions about what is and isn't good design).

2. Irreducibility – The inability of something to be reduced or have properties removed without losing important functions.

Almost all animals have eyes. Not only that, but the underlying genetics for the development of photoreceptors is shared by everything from vertebrates to arthropods . To evolutionists, this means the genes were present before speciation occurred. Yet we have no info on what the first eyes were. We can only look at the simple, yet updated eyes of modern species. For example, phototropic bacteria exhibits complex behaviors in response to light. Planarians have “simple” eyes spots, but a more complex nervous systems than jellyfish.

Human vision exhibits a high level of design. By that I mean that even with our designed technology, we haven’t fully been able to replicate the entire system. This complexity means that all mechanisms have to work together, in perfect balance, for the entire process to work correctly. Aka, its irreducible.

Start with the extraocular muscles. We need them to not only move the eyes, but move in unison. Strabismus occurs when these muscles are not aligned, and it comes at the cost of depth perception. These types of misalignments can lead to amblyopia, in which signals from one eye are turned off. You also need your iris and ciliary muscles that adjust the lens to work right. Tear ducts, eyelids, eyelashes and eye brows all play important roles.

Your eyes also need to communicate with the vestibular system. Try to film with a camera while running, all you’ll see is a blur of shaky images. But you don’t have that problem with your eyes because we have a VOR reflex. It means information from our vestibular system travels through our brain and into the extraocular muscle to stabilize the image. Automatic and voluntary systems then work in unison to give us a steady image. If you have eyes but no vestibular system, your vision is useless.

The overall shape of your eye has to be perfectly balanced. Light has a narrow range at which it can focus on the retina. If your optic axis is too short or too long, you’ll have myopia or hyperopia, and be unable to see clearly. Your cornea also has to have a specific shape, any irregularities and you’ll end up with astigmatism. Your lens needs to be elastic enough, otherwise you’ll be unable to focus light, and have presbyopia. If the pressure within the fluid-filled chambers is too high, you'll have glaucoma and may damage your retina.

It goes without saying that photoreceptors are important. Diseases like macular degeneration and retinitis pigmentosa, in which photoreceptors slowly degrade over the lifespan show their irreducibility. Monochromacy occurs when a person lacks either rods or cones. In each case there are clear deficits. If the person lacks cones, then not only will the world appear black and white, but they will be unable to see during the brightness of day.

I could go on, because the real magic occurs not in the eyes, but in the brain. Not only that, but vision is important for many, many, many behaviours. From sleep, to eating, to falling in love. I'll save these for the comments.

So, seeing how the medical literature is extensive and clear about how much can go wrong if even the slightest thing is off, how exactly did it evolve via small random increments? Even the extremely rare mutations that give some women tetrachromacy (four cones) don't matter much because the brain isn't built for four cone types.

Vision isn't a single thing, its many things working together. That's true of even phototropic bacteria.