One of the main themes in King Lear is the relationship between appearance and reality. When Lear calls upon his daughters to profess their love for him publicly, he's much more concerned with the appearance of their love rather than the substance, the reality. Cordelia is the only one of his daughters who truly loves him, yet she is also the only one who refuses to play along with the whole charade. Lear mistakes Cordelia's reticence for lack of feeling and immediately banishes her. By the same token, Regan and Goneril are rewarded despite the fact that their hearts are full of loathing and contempt for their foolish father.
Even after Lear has divided up his kingdom, he still expects to be treated as a king. That he isn't is a major source of his subsequent descent into madness. Once again, Lear is fixated on appearances, completely ignoring the reality that he no longer enjoys any power. Gloucester's relations with his own children provide a notable parallel with Lear's. He also realizes much too late which one of his children really loves him. For much of the play, Gloucester is quick to believe the lies and slander spread by Edmond against Edgar.
In such a toxic, treacherous environment, where the boundaries between appearance and reality are so frequently blurred, it's no surprise that various characters are required to put on disguises in order to act nobly. The unfailingly loyal Kent pretends to be Caius to be able to serve his master; Edgar disguises himself as Poor Tom, a crazed, half-naked beggar, in order to move about freely and deal with the slanderous accusations made against him.
King Lear by William Shakespeare is focused on themes of love, family, and aging. King Lear expresses one of the major themes, that of the duty of love between parent and child and its betrayal by the child's ingratitude when he states:
"Ingratitude, thou marble-hearted fiend,
More hideous, when thou show'st thee in a child,
Than the sea-monster." -
King Lear, 1.4.283
One major thematic concern is how one distinguishes the true love of Cordelia for her father from the false love and flattery displayed by the other two sisters. The adultery sub-plot echoes the sister's ingratitude as it also emphasizes improper relationships and inconstancy. The issue of children's duty to parents leads to the marriage theme in another way in asking the proper degree for affections to shift from the birth family to the new family, and how conflicting obligations and priorities are handled