Sometimes, though, a speaker or writer doesn't have enough of her own credibility to convince the audience. What should she do then? Well, this is usually when a rhetor borrows credibility from somebody else. That's one of the reasons it's important to know how to cite credible sources. The sources we use when we write give us some of their credibility. As the Yale University Writing Center encourages students, "Incorporating other people's ideas into your writing allows you to stand on their shoulders as you explore your topic." 
Think, for example, about the way I quoted Aristotle earlier in this discussion. I'm a teacher, so I have some authority. You're reading the information in an open textbook, and that also gives me some credibility, but those two things combined may not be enough to convince you that I'm an authority on the subject. So, I borrowed credibility from the man who first wrote about ethos. I used quotes from Aristotle's most important book on rhetoric, and those quotes help establish my own credibility on the subject at hand.
 Yale Writing Center. "Why Cite?" The Writing Center. Yale University. 2009. Web. 4 July 2010.