There’s a famous video on YouTube – blandly titled “Selective Attention Test” – of six college students passing basketballs to each other. Viewers are instructed to count the number of times the students wearing white shirts pass the ball.

The answer is 15, but that is not the real point. About 25 seconds into the video (plot spoiler) someone dressed in a gorilla outfit walks across the floor, turns toward the camera, pounds his chest and walks off.

Amazingly, about half the people who watch the video fail to notice the gorilla, says Daniel Simons, author of “The Invisible Gorilla: How Our Intuitions Deceive Us.” Professor Simons, who teaches cognitive science at the University of Illinois, says that “inattentional blindness” is to blame. In this case viewers are so focused on counting the number of passes that they fail to see the gorilla as it ambles by.

“This experiment reveals two things: that we are missing a lot of what goes on around us, and that we have no idea that we are missing so much,” says Prof. Simons.

There are a lot of psychology experiments online that make the same case. Another video posted by Prof. Simons on his “Invisible Gorilla” website shows two women talking about a surprise party (“Movie Perception Test”). Though their conversation lasts only 35 seconds, it contains nine “intentional editing mistakes.” I challenge anyone to spot more than two, though they seem obvious when they’re pointed out.

“What you actually experience is what your mind and your brain give you. It’s an alternative reality,” says Prof. Simons in a TEDx talk, “Seeing the World As It Isn’t.” He points out that we focus only on a small part of the visual field we take in – about the size of a thumb held at arm’s length. Everything else fades into the background.

This is an important insight into our daily lives. What it means, in effect, is that we look but we do not really see. Same with the other senses: we listen but we do not hear; we eat but we do not taste; we touch but we do not feel; we inhale but we do not smell. It is not that external stimuli fail us: they stream the same information all the time. It is our minds that fail us, because we are not paying anywhere near full attention.

This sad state of affairs affects us in two primary ways: It degrades the overall set of sensory inputs we are exposed to, and it robs us of specific inputs that could be vitally important. Imagine a world without color – that is how much sensory information is lost to us.

Of course, full attention and awareness are not possible: Our senses are not set up to apprehend every input and our brains are not equipped to process every stimulus.

But with understanding and effort, we could take in a great deal more than we do.