Although we “believe it when we see it with our own eyes”, there are many holes in both input and interpretation of our senses.
A favourite perceptual flaw of mine is inattentional blindness: the inability to see things right in front of our eyes, because we’re attending to something else.
There was a famous experiment on this subject by researchers at the Visual Cognition Lab. Students were asked to watch a video and count the number of times one team passed a basketball to another. After the video, the researcher asked the students how many times the ball was passed. Some said 12, many said 13, some said 14.
Next, the researcher asked if anyone noticed anything strange in the video. Some said boys passed it boys more than girls. Some said the girls didn’t move as much.
He asked them to watch the video again: this time not to count the passes, just watch. Almost nobody believed it was the same video. It all looked the same, but in this one, a gorilla walked to centre stage, beat his chest, waved his arms, and walked off again. The students had simply failed to notice this blatant gorilla play right in front of their eyes, because their focus was on counting the number of passes.