I think that there are a couple of elements brought out in this idea. The first would be the issue of sight and blindness. Lear, possessed with physical sight at the start of the play yet possessing emotional blindness, acts in a manner that prevents him from seeing his children's true essences. He is blind to Cordelia's love and to the treachery of Regan and Goneril. At the same time, I think that his composure of being sane is challenged when he recognizes his own folly, made out of a supposedly sound mind. In a moment when the madman leads the blind, it is a stunning moment of epiphany for Lear because he is both. He understands his true nature in the middle of the storm, acknowledging his own blindness and his own madness. This is in stark contrast to him at the start of the drama when he believed himself to have both sight and sanity. Both elements are repudiated through his own error and the deviousness of others, culminating in a moment where Lear understands his true essence.