I came across this astonishing illusion the other day (via a Jerry Weinberg twitter). The blue spirals and the green spirals are the same colour. Really. Jerry said he'd looked at the pixels in Photoshop to confirm it. It's not that I don't trust Jerry but I looked too (in Gimp) because it's just so counter-intuitive. It is true. It's amazing isn't it.
The way we perceive the world is based partly on what information it's sending us but mainly on how we interpret that information. If we consciously saw everything we just couldn't cope with the information overload so our minds filter it for us. That's abstraction. We don't see what's there and we see what's not there.
