Most nonnarrative writing, whether scientific or popular, is intended to serve two purposes: to communicate ideas and to convince others of the validity of those ideas. These two goals give structure to the text and determine the style used by the writer. If the content is controversial or the audience is hostile, for example, the author will pay particular attention to the argument structure. He or she must prove every point. If a government source were to compose a text for the scientific community, he or she would need to write more if arguing that scientific funding should be decreased than if arguing for an increase. When we know our work will be opposed, we need to support every assumption and conclusion. Much of the text itself has to be devoted to the argument and proportionally less to the content. If the material is generally accepted or the audience is receptive, less attention need be given to the argument structure and more to description. Authors can take assumptions for granted and be confident of their conclusions. Similarly, the argument structure and the views of readers

determine how the material will be processed. If readers are suspicious, they will pay attention to the argument. When a passage is consonant with readers' beliefs, the conclusion is already accepted and fine-grained analysis is not needed. When the passage is in conflict, the validity of the argument, for the reader, is part of what will determine attitude change. Judgments of validity are based on the content and the form of the argument. This chapter presents a model of argument generation and comprehension that simulates how people create and evaluate arguments.