As the author of Stephen's Guide to the Logical Fallacies (firmly in the Copi tradition and very much in need of a new edition) I have had a long-standing interest in fallacies. I define a fallacy simply as 'a common error of reasoning' and eschew the much deeper debate over this definition that appears to be very much the preoccupation of this SEP article. I think we'll find that for each legitimate sort of reasoning there will be a set of accompanying fallacies. It's useful to know about fallacies because they provide a bridge into knowing how to reason well. Image: THE.