From too much liberty, my Lucio, liberty. 
As surfeit is the father of much fast, 
So every scope by the immoderate use 
Turns to restraint. Our natures do pursue, 
Like rats that raven down their proper bane, 
A thirsty evil, and when we drink, we die. (1.2.122–27)

Claudio speaks these lines in response to his friend Lucio, who has just asked him why he’s been arrested. Claudio insists that he’s being punished for enjoying “too much liberty.” Claudio laments his lack of self-restraint, which has forced others to restrain him. Just as overeating can lead a person to “fast,” so does “immoderate use” inevitably lead to “restraint.” Yet according to Claudio, such a situation isn’t simply a personal failing. Indeed, it’s a matter of “our natures.” To demonstrate his point, he likens humans to “rats that raven down their proper bane”—that is, rats that devour poison. In other words, we humans cause our own suffering by not having sufficient self-knowledge to know what’s harmful to us.

Heaven hath my empty words, 
Whilst my invention, hearing not my tongue, 
Anchors on Isabel. God in my mouth, 
As if I did but only chew His name, 
And in my heart the strong and swelling evil 
Of my conception. The state whereon I studied 
Is, like a good thing being often read, 
Grown sere and tedious. Yea, my gravity, 
Wherein—let no man hear me—I take pride, 
Could I with boot change for an idle plume 
Which the air beats for vain. O place, O form, 
How often dost thou with thy case, thy habit, 
Wrench awe from fools, and tie the wiser souls 
To thy false seeming! (2.4.2–15)

Angelo utters these words when he’s alone onstage, waiting for Isabella to arrive and respond to his proposition about having sex with him in exchange for her brother’s life. As his speech makes clear, he is all too aware of his own hypocrisy. He expresses this hypocrisy in religious terms, indicating that his commitment to moral purity is little more than “empty words,” and that he’s effectively put “God in my mouth” and “chew[ed] His name” up. Everything he’s taken personal pride in is, thus, a matter of “false seeming.” Angelo is clearly struggling to understand himself in the face of unfamiliar passions, which have led him to the brink of an identity crisis: “What does thou, or what art thou, Angelo?” (2.2.210).

Duke (as Friar): I pray you, sir, or what disposition was the Duke? 
Escalus: One that, above all other strifes, contended especially to know himself. (3.2.231–34)

The one character who seems to have achieved some degree of self-knowledge is the Duke. Escalus expresses this view in an exchange with the Duke, who’s still in disguise as a friar. Interested to know how his subjects view him, the “Friar” asks Escalus for his opinion of the Duke, and the judge replies that, more than anything else, the Duke “contend[s] especially to know himself.” This assessment of the Duke is high praise, particularly in a play where many characters fall short of meaningful self-knowledge. Importantly, we in the audience can confirm Escalus’s judgment about the Duke’s self-awareness. In an earlier scene with Friar Thomas, the Duke reflected critically on his own failures as leader. This recognition of his personal shortcomings is, arguably, a key reason he sets in motion an elaborate plan to restore the moral order of his realm.