I love optical illusions. One reasons I love them is that many are evidence for the core argument in Hayek's The Sensory Order: the idea that our mind fills in patterns based on experience even when those patterns don't exist in reality. In other words, we see what our brain thinks we should see, based on accumulated learning. We live, as Hayek said, in a world of expectations.
I just came across a new one (HT: Eugene Volokh) that fits this well. This color illusion is fascinating because we see two different colors even though the two colors are in fact the same. And we see them differently because:
and we see a spiral pattern because:
Here's another optical illusion that illustrates the same idea of "filling in the pattern" even better.