When determining what kinds of evidence you will need to support your reasons (and thus your claim), you may find it helpful to start with the three classic rhetorical appeals: ethos, logos, and pathos. Each appeal contributes unique support to an argument, and rarely are they used in isolation; most often at least two, if not all three, of the appeals are used together.
The first appeal—ethos, known as the ethical appeal—pertains to establishing your credibility and authority on the subject you are arguing about. In order for your audience to be persuaded to accept your claim, they must believe that you are a trustworthy person who can knowledgeably speak or write on the matter.
Imagine, for example, that you were seeking help with writing a paper for a class, and one of your friends offered to assist you, claiming she was an excellent writer and could discuss with you some brainstorming strategies for how to begin. On the one hand, you appreciate your friend’s offer, but on the other, you know this friend tends to procrastinate on projects, leaving little time to go through all of the stages of the writing process herself (particularly planning, revising, and editing) to ensure a successful product. Your friend tends to get stressed working on papers at the last minute, and even if she scores well on an assignment, there is little enjoyment throughout the process or enthusiasm for the finished work. Thinking through what you know about your friend’s approach to writing, then, you may question the claim that she is an excellent writer, one you would want to trust (at least in comparison to other people, such as a Writing Center consultant) to help you. In this example, your friend’s ethos is not sufficient for you to be persuaded of the claim.
Logos, the logical appeal, focuses on persuading the audience on the basis of sound reasoning/logic. It depends on connecting to the audience as rational individuals who respond to an argument grounded in facts. We often associate logos with data, such as the use of statistics, to show why the claim has merit. To use the earlier example, if your friend who offered to help you shows you samples of her writing that demonstrate she is a strong writer, then you are more likely to be persuaded to accept the assistance. Your friend has provided you with evidence that she is an excellent writer, and presumably then she can offer you strategies to help you with your own writing. If the writing samples your friend shares also include high grades, then that data is likely to further persuade you beyond your own assessment of the quality of the writing.
The final appeal, pathos, is known as the emotional appeal because it focuses on persuading the audience on the basis of their emotions. When using this appeal, the intent is to connect to the audience’s feelings—whether fear, anger, happiness, sympathy, etc.—in order to support the argument. Again, using the example of your friend offering to help, she may point out to you that the assignment deadline is just a few hours away and the Writing Center is closed, but she has the time to assist you. In this way, your friend may be appealing to your sense of desperation, using as evidence the circumstances under which you are working, and that may be all that is needed to persuade you.
As mentioned above, although the three appeals are discrete in their methods of persuasion, rarely is only one used in an argument. More often, at least two, if not all three, will be needed to make the best case possible. Even with our example of a friend offering to help you with an assignment, you are most likely to be persuaded if she demonstrates the credibility to do so, provides data that shows she is a strong writer, and calls attention to the impending deadline (which may trigger fear) and lack of options for help (desperation).