An excerpt from Thinking, Fast and Slow by Daniel Kahneman:
Everyone has some awareness of the limited capacity of attention, and our social behavior makes allowances for these limitations. When the driver of a car is overtaking a truck on a narrow road, for example, adult passengers quite sensibly stop talking. They know that distracting the driver is not a good idea, and they also suspect that he is temporarily deaf and will not hear what they say.
Intense focusing on a task can make people effectively blind, even to stimuli that normally attract attention. The most dramatic demonstration was offered by Christopher Chabris and Daniel Simons in their book The Invisible Gorilla. They constructed a short film of two teams passing basketballs; one team wearing white shirts, the other wearing black. The viewers of the film are instructed to count the number of passes made by the white team, ignoring the black players. This task is difficult and completely absorbing. Halfway through the video, a woman wearing a gorilla suit appears, crosses the court, thumps her chest, and moves on. The gorilla is in view for 9 seconds.
Many thousands of people have seen the video, and about half of them do not notice anything unusual. It is the counting task – and especially the instruction to ignore one of the teams – that causes the blindness. No one who watches the video without that task would miss the gorilla. Seeing and orienting are automatic functions of System 1, but they depend on the allocation of some attention to the relevant stimulus. The authors note that the most remarkable observation of their study is that people find its results very surprising. Indeed, the viewers who fail to see the gorilla are initially sure that it was not there – they cannot imagine missing such a striking event. The gorilla study illustrates two important facts about our minds: we can be blind to the obvious, and we are also blind to our blindness.
I have a sneaking suspicion this phenomenon can show up, not only in relation to what we physically see with our eyes, but what we intellectually see with our minds. I suspect that, in almost any pursuit of knowledge – whether scientific research, Scriptural study, political pontificating, or philosophical reasoning – we can quickly contract tunnel vision.
Truth is...as a species, we humans tend to see what we're looking for.