what is the significance of king's Lear madness?

The theme of madness is explored deeply in Act III as we encounter at least three different forms of madness in at least three different characters. King Lear most notably goes, or is driven, to a madness he had predicted in this Act, but he is accompanied by two others whom are meant to be playing fools or madmen but to whom he grants the greatest sincerity. This evil leads Lear to his belief that madness on a large scale can only result from the betrayal of daughters. He has sincerely been led astray in his trust and loyalty and thus plunges into a darkness and a madness which the storm, the hovel, and the night quite literally and symbolically portray. Vividly Shakespeare portrays the transformation of man into storm and storm into man as Lear goes mad. Personifying the storm with himself and the children he has begotten, Lear wails, "Rumble thy bellyful. Spit, fire. Spout, rain./ Nor rain, wind, thunder, fire are my daughters" (III.2.14-15). The storm is given a belly and the elements are compared to daughters. Note even the sound effects are called for at key points in the dialogue to echo Lear's mutation. "Storm still " is included by Shakespeare, for example, between poor Tom's continuing rants and Lear's conclusion that his madness must be the result of the betrayal of his daughters (III.4.59-61).

Lear's madness shows us just what a mess he and his kingdom are. His mind has gone insane, as has his desire to have love proven from his daughters, the worst of whom actually lie. The storm in which he finds himself is a kind of madness (of the universe); such a storm could reduce anyone to his/her most basic state. Once Lear is reduced to the humblest of beggars (because of the storm), he begins to understand what is true (in daughters like Cordelia) and who is indeed honest, even the fool who is more insightful. If read closely, many of the lines spoken by the "mad" characters include real wisdom if it were only listened to.