The two themes collide in this play in Act I scene 1, when Cordelia gives her truthful yet ill-fated answer to her father, in comparison with her sisters, whose responses to Lear show their flattery and deceit. Cordelia uses the word "Nothing" to respond to her father's question about the extent of her love for her father, and Lear responds with the following line:
Nothing will come of nothing. Speak again.
The word "nothing" here is incredibly important, because it relates to the theme of identity and the real nature of both Lear and Cordelia. It points towards the overarching theme of appearances vs. reality. Remember that Cordelia has just heard her two sisters give speeches that testify to their great love for their father whereas in reality she knows that they do not care for him. She, to emphasise her love all the more, chooses to let her love be made plain by her actions alone, proving her genuine and real love through deeds rather than mere words. Her identity as the most dutiful, honourable and loving daughter is therefore clearly indicated through the use of the word "nothing."
Lear, on the other hand, shows his very different character through how he uses the word "nothing." He, at the beginning of the play, fails to discern between appearances and reality, and in fact champions appearances over reality in insisting that he will apportion out his kingdom based on the quality of the words his daughters offer him alone. He interprets Cordelia's use of the word "nothing" and her inability to flatter him as being "untender," and as a result disinherits her. Notice his use of the word truth in his response:
Let it be so. Thy truth then be thy dower.
For by the sacred radiance of the sun,
The mysteries of Hecate and the night,
By all the operation of the orbs,
From whom we do exist and cease to be,
Here I disclaim all my paternal care...
Ironicaly, Lear uses the word "truth" when he is not able to see the "truth" and understand the depth of Cordelia's regard for him. Lear's identity as a foolish old man who is unable to differentiate between appearances and reality is revealed through his response to the word "nothing."