An error in reasoning similar to making an unstated or invalid assumption occurs when a writer employs an unsupported assertion in defense of her central idea. An assertion is a declaration stated positively, but with no support or attempt at proof. When a writer makes an unsupported assertion, a reader has no way knowing whether the point is valid. No matter how well-reasoned the rest of the argument may be, an unsupported assertion casts doubt on its validity. The reader then has reason to question whether the author is thoroughly dedicated to investigating his central claim.
Writers can make similar errors when they suppress or provide incomplete evidence.
Suppose, for example, a student is writing an essay about Hamlet's ambiguous status as a hero, and she constructs an argument that makes reference to events in three scenes, but omits any mention of a fourth scene that obviously contradicts her main idea. By suppressing her knowledge of a possible counter-argument, this author weakens her paper. When you write an academic essay, it is generally a good idea to assume that the reader is familiar with your topic. You should suppose, therefore, that a reader, especially a professor or preceptor, will notice your omission and interpret it as a significant flaw in your argument, or even as deception. At one time or another, most writers have the unfortunate experience of discovering a counter-argument when they are almost finished writing a paper.