Consider a light, an L.E.D for example, turning on and off once per second. For humans, we will look at it and think "clearly on, clearly off, clearly on, clearly off" for each 'state'. Our view of whether it is on or off will continue like this if its 'blink frequency' is increased up to a certain point. Once it is blinking at a certain speed, we wont be able to tell if it is turning off at all and will seem 'on' to us 100% of the time. But why does it average out as 'on' to us and not 'off'?