Formal logic gives us standards by which we can distinguish good reasoning from poor reasoning. Most often, when someone reasons poorly, they are not making an error in formal reasoning, but rather sidetracking their hearers with an informal fallacy. Informal fallacies are less structured errors made in the everyday use of language.
The Introductory Logic text identifies eighteen different types of fallacies, but of course there are many more ways to go wrong than that. My new rhetoric text Fitting Words includes a few popular fallacies not included in Introductory logic. Let me summarize them.
It is fallacious to claim that a conclusion has been established when the premises support a different conclusion. When this is done by diverting the attention of the audience from the main point to a plausible but unrelated issue, this is called a “red herring,” a type of missing the point.
Example: “Our church should purchase a larger building. After all, Jesus said that many will come from the east and west to sit with the prophets in the Kingdom of God.”
Another fallacy of missing the point occurs when a person distorts his adversary’s argument with a caricature that is more vulnerable to attack (i.e. a straw man), and then attacks the caricature.
Example: “I don’t believe in heaven. Do you seriously believe that you will grow wings and sit on a cloud playing a harp for eternity?”
Another type of mischaracterization in refutation occurs when an alleged chain reaction (with dubious links) connects a relatively harmless first step to some disastrous conclusion, in order to argue against taking that first step.
Example: “You shouldn’t play cards. Next thing you know, you’ll be playing poker for chips, then for real money, then playing at the casino, until finally you lose everything!”
For an analogy to work, the conclusion must be based on a relevantly similar source. If the similarities between the source and the conclusion are irrelevant, or there is some other structural problem, you can make a false analogy.
Example: “The human body is a machine, and it’s not wrong to turn off a machine that’s been running a long time and not working well. So it is acceptable to end the life of the sick and aged.”
There are many other types of informal fallacies that are fun to learn and useful to know. I have included some of these in my Fitting Words text, which will be available the Spring of 2016. In addition, I would direct younger students to The Fallacy Detective by the Bluedorn brothers, and older students to The Amazing Dr. Ransom’s Bestiary of Adorable Fallacies by N.D. Wilson and Doug Wilson.