black and white - Why is 18% grey considered to be in the middle for photography?

I heard someone (a photographer) say recently that 18% grey is half way between black and white, not 50%. This seemed a bit illogical to me, and when I asked her why, she said she didn't know. After reading a few online articles, I found that 18% is often referred to as middle grey, and considered to be half way perceptually. Is 18% for some reason half way between black and white, and if so why (maybe these percents work on a non-linear scale for whatever reason... ). If not, why do we think 18% is half way, not 50%. Do we see color non linearly?, do our cameras capture light non-linearly, or is this just a sort of relative brightness illusion.

After reading the question this is supposedly a duplicate of, I still don't see why Ansel Adams choose 18%, was it a visual thing?, or why it was so widely adopted. Is this number arbitrary? just what someone though looked correct... or does it have some valid claim to being the middle grey, due to perception (it appears our eyes see things linearly, do cameras do likewise?) or other technical reasons.