As he passed by, he saw a man blind from birth. And his disciples asked him, “Rabbi, who sinned, this man or his parents, that he was born blind?” Jesus answered, “It was not that this man sinned, or his parents, but that the works of God might be displayed in him. We must work the works of him who sent me while it is day; night is coming, when no one can work. As long as I am in the world, I am the light of the world.” Having said these things, he spit on the ground and made mud with the saliva. Then he anointed the man's eyes with the mud and said to him, “Go, wash in the pool of Siloam” (which means Sent). So he went and washed and came back seeing. (John 9:1-7, ESV)
This miracle actually happened but this was also a practical illustration for what Jesus has been saying about being the light of the world. Having blind eyes open and “seeing the light” is an image for what happens to someone when they become a Christian.
We are blind to our own nature and the character of God. We are like a pet fish arrogantly assuming that it knows everything about the world and it's owner from gazing through the side of the bowl for a few hours a day. It is like Daniel Radcliffe thinking he knows more than JK Rowling about what makes Harry Potter tick. We think we can see ourselves more clearly than God who made us. We think we know more about what's best for our life than the Author of Life! We need our eyes opening.