I’ve always wondered why candidates have to “approve this message”; I mean, if President Obama is on camera talking about himself, then can’t I assume he approves the message? Why does he have to state that he approves it at the end?
There’s certainly a law that governs what must be said at the end of a political advertisement, or else President Obama wouldn’t say exactly the same thing as every other politician at the end of an ad, but there’s also an element of persuasion at work here. By appearing on camera saying that he approves the content, the President is giving the ad credibility. It’s about him, his work, and his beliefs, and by saying he has approved the ad, President Obama is saying, “You can trust this information about me.”
This appeal to credibility is known as “ethos.” Ethos is a method of persuasion in which the speaker or writer (the “rhetor”) attempts to persuade the audience by demonstrating his own credibility or authority. I think the best way to understand this kind of appeal to the credibility of the author is to look at the three most common ways a rhetor attempts to demonstrate authority on a topic.
Intrinsic authority is authority that comes from the rhetor herself. It might come from her work experience or college degrees or generally good morality, or it might come from how well she demonstrates that she can speak or write about her topic.
Aristotle, who coined the term “ethos,” said that “persuasion is achieved by the speaker’s personal character when the speech is so spoken as to make us think him credible.” This is true, he said, because an audience will “believe good men more fully and more readily than others.” For Aristotle, though, this kind of persuasion shouldn’t derive from who the person is exactly; rather, it “should be achieved by what the speaker says, not by what people think of his character before he begins to speak.”  For us, however, in an age in which Google is a click away, the speaker’s character and achievements have an impact on the way we think about what she says. It’s unavoidable. But the person’s ability to speak authoritatively on her topic is just as important.
Remember, though, that a rhetorical situation, in which a speaker or writer seeks to purposefully persuade her audience, is an artificial situation: an author shares only a part of herself with the audience. To put it another way, as M. Jimmie Killingsworth, a scholar of rhetoric from Texas A&M University, does in a 2005 article in Rhetoric Review: “The author’s position is not simply a personal account of himself or herself. The author is a complex individual who selectively reveals (or invents) aspects of character pertinent to the rhetorical work required at the moment” (251–52). 
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.
Beware of “Oh, that makes sense”: Ethos in Context
There is also, however, the credibility that comes from saying or writing something that the audience already believes or that reinforces the audience’s experience. We should treat this kind of ethos with a healthy dose of suspicion. Just because something sounds right to you or makes you feel good about what you believe does not mean that it is true.
Take, for example, the idea that the Founding Fathers of the United States were Christians. Many well-known commentators and politicians have made this claim, and it has been generally accepted as the truth. The statement has the authority that comes from being the conventional wisdom. The problem, however, is that the Founding Fathers weren’t all Christians. Some of the most prominent members of this group, including Thomas Jefferson and Benjamin Franklin, were actually Deists. They didn’t believe that Jesus was God, which is a central tenant of Christianity, and wrote extensively about their Deism. (Refer to Benjamin Franklin: Autobiography, Poor Richard, and Later Writings, edited by J. A. Leo Lemay, and David Holmes’s The Faiths of the Founding Fathers for information about this.) There is a lesson in this for both audience and speaker. It’s dangerous to accept something just because it sounds true. Credibility can’t be established just by saying what the audience wants to hear.
By now, you’ve hopefully gotten an idea of what ethos is: an attempt to persuade by appealing to authority or credibility. You might be wondering, though, what ethos looks like in writing or in speaking. Here are a few examples:
- References to work experience or life experience related to the topic. When an author writing about the stock market talks about his years working for an investment bank, that’s an appeal to credibility.
- References to college degrees or awards related to the topic. When your biology instructor makes clear in the syllabus that he has a PhD in biology and that you’ll be using the textbook he’s written for the class, he’s reminding you of his authority and credibility on the subject.
- References to the character of the writer. When a politician writes in a campaign brochure about his years of public service and the contributions he’s made to the community, he’s letting you know he’s trustworthy, a good person, and a credible source of information about the community and the issues that affect it.
- The use of supporting sources written by authorities on the subject. When a student writes a paper about why school hours should be changed and uses quotations from principals, teachers, and school board members (all of whom know something about the topic), he’s borrowing their credibility and authority to increase his own.
- References to symbols that represent authority. When a candidate gives a speech in front of an American flag, he or she is associating him- or herself with the symbol and borrowing the authority it represents.
- Ad Hominem (Argument to the Person): Attacking the person instead of the argument. For example, “You say I shouldn’t drink so much, but you drink every day.” The validity of the argument (drink less) can’t be based on the behavior of the person making the argument. Instead, the validity of the argument should be evaluated on its own terms—separate from the person making the claim.
- Argument from Authority: Claiming to be an expert and, on that basis, to be deserving of trust. It’s important to remember that there are different kinds and levels of expertise: My weekend cooking class doesn’t make me an authority on recipes, though I can honestly say I’ve studied cooking. So, I might be an authority on some elements of cooking, but not all of cooking. When faced with an argument from authority, it is important to investigate the credentials of the speaker or writer.
- Appeal to Authority: Using a statement taken out of context as authoritative support. For instance, it would be fallacious to use Malcolm X’s declaration “by any means necessary” to justify indiscriminate violence against protestors. Such an assertion ignores the context, audience, and purpose of Malcolm X’s statement.
- Argument from False Authority: Using an expert in a specific field as an expert in all related fields. For instance, if I am writing a paper about heart disease and I quote my chiropractor, Dr. Wallace, then I would be making an appeal to fallacious ethos; despite being a doctor, she is not an authority on heart disease.
- Appeal to Anonymous Authority: Using appeals to nonspecific groups (e.g., doctors, scientists, researchers, and so on). For example, “Research shows that all women are inferior to men.” Or, “Studies indicate that all college students binge drink.” Neither of these statements offers a specific credible source, so both claims lack authority.
- Inflation of Conflict: Using a conflict between two authorities as a reason to dismiss their arguments and knowledge. For instance, it would be fallacious to assert that global climate change does not exist because two scientists disagree about its effects.
 Aristotle. (2007). On rhetoric (W. Rhys Roberts, Trans.). The Internet classics archive. Web Atomic and Massachusetts Institute of Technology.
 Killingsworth, M. J. (2005). Rhetorical appeals: A revision. Rhetoric Review, 24(3). 249–63.
 Yale Writing Center. (2009). “Why cite?” The Writing Center. Yale University.