Christopher Chabris and Daniel Simons conducted an experiment at Harvard University more than a decade ago that became infamous in psychology circles. Their book The Invisible Gorilla popularized it. And you may be one of the millions of viewers who made their Selective Attention Test one of YouTube’s most-watched videos.

The two researchers filmed students passing basketballs while moving in a circular fashion. In the middle of the short film, a woman dressed in a gorilla suit walks into the frame, beats her chest, and walks out of the frame. The sequence takes nine seconds in the minute-long video. Viewers are given specific instructions: “Count the number of passes by players wearing white shirts.” Of course, the researchers were not interested in their pass-counting ability. They wanted to see if the viewers would notice something they weren’t looking for, something as obvious as a gorilla. Amazingly, half of the test group did not.

How is that even possible?

How do you miss the gorilla in the room?

The short answer is inattentional blindness.

Inattentional blindness is the failure to notice something in your field of vision because you are focused on something else, in this case people in white shirts passing basketballs.

But the first-century Pharisees make an even better case study. They were so focused on Sabbath law that they couldn’t see the miracles happening right in front of their eyes. Jesus healed an invalid who hadn’t walked in thirty-eight years, gave sight to a man born blind, and restored a man’s withered arm. But the Pharisees missed the miracle, and missed the Messiah, because they were blinded by their legalism. They couldn’t see past their religious assumptions.

Inattentional blindness can be as intentional as turning a blind eye to something you don’t want to see, like the Pharisees did. It can also be as unintentional as fading awareness of the constants in your life that you take for granted over time. Either way, it’s one of the greatest threats to spiritual vitality. One of the truest tests of spiritual maturity is seeing the miraculous in the monotonous.