Reasons
A strong, debatable thesis needs to be supported and developed with sound reasoning. You can think of an argument as a dialogue between the writer and readers. A writer states a debatable thesis and one reader wonders, "Why do you believe that?" Another reader wants to know, "But what about this factor?" A writer needs to anticipate questions such as these and answer them by presenting claims that are substantiated with evidence and by refuting opposing views.
Usually, a well-developed argument paper includes more than one type of claim and one kind of evidence. Besides generalizations based on empirical data or statistics, it often includes authoritative claims based on the opinions of experts and ethical claims based on the application of principle. In his proposal about reducing injuries in professional football, for example, Nicholas Buglione presents facts about the number of injuries in the previous and current season to establish the seriousness of the problem and includes quotes from an expert in football safety to explain the coach's role in promoting--or failing to promote--team safety. As you conduct research for your argument, note evidence--facts, examples, and expert testimony--that can be used to support each argument for or against your position.