Ethos

No argument, Aristotle asserts, no matter how logical seeming, no matter how appealing emotionally, will succeed if the audience rejects the arguer's credibility, the writer's "ethical" qualities.  As members of the audience we need to believe that the arguer is a person of knowledge, honesty, and goodwill.

--Dorothy U. Seyler, Read Reason, Write: An Argument Text and Reader, p. 74

In order for a paper to succeed, the writer needs to come across as someone who is believable, reasonable, and knowledgeable.  If an author begins an essay by saying, "I really don't know much about _____________, but I'm going to tell you what I think about it anyway," most audience members will probably stop reading.  However, when an author comes across as someone who is informed and thoughtful, audience members are more likely to read the entire essay carefully. 

If you do not have a great deal of first-hand experience with a topic, you might develop a sense of ethos by researching the topic and referring to experts in the area.