It’s a line drive headed straight for your glove. All you have to do is close your hand at the right moment. It takes only about fourteen one-hundredths of a second to close your hand around a baseball.
But in that time, a baseball going ninety miles an hour travels over eighteen feet. Closing your hand a little late or a little early could turn the final out into a home run.
But how do we know when the ball is exactly fourteen one-hundredths of a second away? We could use two variables–the ball’s speed and its distance–to calculate when it would arrive.
Luckily, our sub-conscious mind does it without calculating and with only one variable. That variable is the rate at which the image of the ball is expanding in relation to the size of the image at that moment.
Here’s how it works: a ball thrown from a distance grows gradually from a tiny speck until it reaches us. The speck grows slowly at first, but the closer it gets, the faster it grows.
A big ball far away may look the same size as a small ball up close, but if they’re traveling at the same speed, the closer ball grows faster.
Knowing how big the speck is and how fast it’s growing, you could calculate how long it will take to reach you, but our sub-conscious mind does it in a single glance.
By interpreting a single variable–namely how fast the ball appears to be growing relative to how big it appears at that moment–our mind knows just how long it will be before that speck turns into a baseball in our hand.