Perhaps the most identifying quote of the play "Othello" is spoken by Iago:
I am not what I am. (1.i.65)
Here readers, or watchers, of the play can recognize the fact that knowledge of self is very important. Here Iago is defining the difference between how people can portray themselves to others while hiding the reality of who they truly are.
This being said, another way of looking at this is through ones own interpretation of who others really are. What this means is that if a person can recognize the fact that they can hide, or not know, who they really are what keeps others from doing the same?
The fact that "ocular proof" is deemed as the end all for the characters of the play is simplistically ironic. Too many times what people see in others can be the public mask which they choose to show to the world.
In the end, the theme of trust and perception heighten the truth behind self knowledge and the knowledge of others. The statement always remains: "I am not what I am."