Illusions have a long history, going as far back as the ancient Greeks.

In 350BC, Aristotle noted that “our senses can be trusted but they can be easily fooled”.

He noticed that if you watch a waterfall and shift your gaze to static rocks, the rocks appear to move in the opposite direction of the flow of water, an effect we now call “motion aftereffect” or the waterfall illusion.

Tracking the flow of the water seems to “wear out” certain neurons in the brain as they adapt to the motion. When you then shift your gaze to the rocks, other competing neurons over-compensate, causing the illusion of movement in the other direction.