When you write an essay, many of the claims you make will be drawn from our cultural common knowledge, which you share with your readers. But if your entire essay consists of common knowledge, it won't be a very interesting essay. You'll just be telling us what we already know, stating the obvious. Your thesis statement would be neither controversial nor informative (1.4.4.3). So if your essay is going to be interesting, if it is going to tell us something we don't already know, most of what you say will be claims that we are unsure about. Sometimes they are the kind of claims that we will accept on your authority--for example, a personal experience that illustrates your point. But unless your essay is entirely about your own experience, we probably won't accept your word for everything. (You can, of course, write a good essay just from your personal experience. But you probably can't write three. And you probably can't write one on any topic. It would require a topic that you have significant experience with.) So the major factor, often the major factor, determining whether your readers believe what you claim will be the quality of your supporting evidence.
