(Credibility), or ethical appeal, means convincing by the character of the author. We tend to believe people whom we respect. One of the central problems of argumentation is to project an impression to the reader that you are someone worth listening to, in other words making yourself as author into an authority on the subject of the paper, as well as someone who is likable and worthy of respect
EX: My Dear Fellow Clergymen:
While confined here in Birmingham city jail, I came across your recent statement calling my present activities "unwise and untimely."...Since I feel that you are men of genuine good will and that your criticisms are sincerely set forth, I want to try to answer your statement in what I hope will be patient and reasonable in terms.
I think I should indicate why I am here in Birmingham, since you have been influenced by the view which argues against "outsiders coming in."...I, along with several members of my staff, am here because I was invited here. I am here because I have organizational ties here.
But more basically, I am in Birmingham because injustice is here. Just as the prophets of the eighth century B.C. left their villages and carried their "thus saith the Lord" far beyond the boundaries of their home towns, and just as the Apostle Paul left his village of Tarsus and carried the gospel of Jesus Christ to the far corners of the Greco-Roman world, so am I compelled to carry the gospel of freedom beyond my own home town. Like Paul, I must constantly respond to the Macedonian call for aid.
Martin Luther King, Jr. "Letter from Birmingham Jail"