How are the themes of knowledge of self and rhetorical power used in Shakespeare's Othello?

The themes of self-knowledge and rhetorical power can be
seen in many places throughout Shakespeare’s tragedy Othello, and
indeed Shakespeare’s treatments of these themes are so complex that they cannot easily
be discussed here. One passage, however, invites particular
attention.

In that passage, Othello has been summoned
before the aristocrats of Venice and has been accused by one of these men, Brabantio,
with the crime of using witchcraft to seduce and marry Brabantio’s daughter, Desdemona.
Othello answers these charges in one of the most famous and rhetorically powerful of his
speeches in the entire play. Othello begins by addressing the assembled
aristocrats:

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Most potent, grave, and reverend
signiors,

My very noble and approved good
masters,

That I have ta’en away this old man’s
daughter,

It is most true; true I have married her.
(1.3.76-79)

Othello speaks
with quiet authority and genuine eloquence, even though he later wrongly claims, “Rude
am I in my speech” (1.3.81). His rhetorical power derives from his strong
self-confidence and his strong sense of self-knowledge.  At this point in the play, he
knows who he is in particular and also the kind of person he wants to be. He knows his
values; he knows his history; he knows his strengths; and he knows that Desdemona loves
him. The speech that begins “Her father loved me” is extremely powerful rhetorically,
partly for all the reasons just mentioned (1.3.127-169). Indeed, the speech is so
powerful that when it concludes, the Venetian Duke comments, “I think this tale would
win my daughter too” (1.3.170).  In this scene, therefore, Othello demonstrates the
strong tie between self-knowledge and rhetorical
power.

Ironically, another character in the first act who
also demonstrates a strong tie between self-knowledge and rhetorical power is Iago. 
After cleverly using rhetoric to manipulate Roderigo, Iago congratulates himself on his
self-knowledge and worldly wisdom:

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Thus do I ever make my fool my
purse;

For I my own gained knowledge should
profane

If I would time expend with such a
snipe

But for my sport and profit.
(1.3.372-75)

Iago is such a
rhetorically powerful figure throughout the play because he knows himself so well and
can therefore manipulate others so easily. His absolute self-assurance and
self-knowledge allow him to use rhetoric to control others, especially those who are
less and less sure of themselves, as later becomes true, ironically, even (or
especially) of Othello.