We like illusions because they create a deeply subjective sense of incongruity. That is, they “break” our brains. In this case, the circles appear to be moving, but they also appear stationary, which creates a feeling that can only be described as “Whoa…”
The psychologist Steven Pinker likes to share illusions on Twitter because of what they reveal about our brains. For example, we read in textbooks that the human brain is not a naive observer of the world but actively constructs what it observes. But none of us really believes that. Persistently throughout daily life, we act as if what we perceive is really “out there” and treat illusions like this, not as if they’re typical of observation, but as if they’re edge cases, special exemptions to the brain’s normally reliable function.
It’s important here to realize that what makes the illusion an illusion is not the illusion bit. It’s the juxtaposition of that with a benchmark which simultaneously demonstrates what we’re actively observing cannot be the case.
Certain drawn illusions depict two images in one — it’s either a duck or a rabbit, or it’s the face of an old woman or that of a young woman turning away. Our brain cannot see both at the same time. That’s really important, because it shows there is some extra layer that we do not perceive between the image and our sense of it. That is, we don’t see the image as it is. We see a drawing of a duck or a drawing of a rabbit — or a face in the electric socket.
That extra unperceived layer of perception that interprets those lines on a page doesn’t go away when we stop looking at optical illusions. It is always there. We’re normally not aware of it, not because it isn’t active, but because the juxtaposition present in the illusion that tells us it can’t be real is missing everywhere else, giving us the subjective sense that our everyday observations are stable and reliable, even when they may not be.
Thus, in a very real sense, illusions are closer to reality than our sense of reality because they reveal where our sense of reality is broken.