Persistence of Vision
Persistence of vision is a commonly-accepted although somewhat controversial theory which states that the human eye always retains images for a fraction of a second (around 0.04 second). This means that everything we see is a subtle blend of what is happening now and what happened a fraction of a second ago.
In film and video, this phenomena is often claimed to account for our ability to perceive a sequence of frames as a continuous moving picture. However this idea was debunked in 1912 and there is no scientific evidence to suggest that persistence of vision works in this way. Rather, it is thought that the illusion of continuous motion is caused by unrelated phenomena such as beta movement (the brain assuming movement between two static images when shown in quick succession).
Despite this, persistence of vision continues to be incorrectly taught in schools as the physiological mechanism behind video's illusion of movement.