In Shakespeare's tragedy, King Lear, both King Lear and the Earl of Gloucester do not see the reality of their family relationships. Both are blind to the world around them - Lear can not see the devotion of his beloved daughter Cordelia or the shallowness of his other daughters, while Gloucester is blinded by his son Edmond's treachery. This failure to see reality leads to Lear's psychological blindness, his insanity, and Gloucester's physical blindness, his trusting tendencies.
At the end, both Lear and Gloucester recognize the truth, demonstrating the need to have a clear vision in life.
I' th' last night's storm I such a fellow saw,
Which made me think a man a worm.
My son came then into my mind, and yet my mind
Was then scarce friends with him. I have heard more since.
As flies to wanton boys are we to th' gods,
They kill us for their sport.