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<p>A persuasive paper that does not address the opposing viewpoints won’t persuade anyone except those who already are persuaded.</p>
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<p>There is an excellent rhetorical model, the Toulmin model, that addresses this problem and requires that the writer acknowledge opposing viewpoints in order to refute them.</p>
<p>I have taught college English for 20 years. I do feel that writing “That’s a lie” on a student paper is unprofessional and inflammatory. It calls into question the morality of the writer and thus is personally insulting. It’s better to write another comment that accomplishes the same thing, i.e. “not factual” or “cited source is not reliable.”</p>