A formal fallacy represents a fundamental error in the logical structure of an argument. Unlike informal fallacies, which might have valid logic but false premises, a formal fallacy is inherently flawed in its reasoning pattern, making the argument unsound regardless of the truthfulness of its individual statements. In everyday discussions, the term "logical fallacy" often refers specifically to these formal errors. Recognizing formal fallacies is crucial
for evaluating the validity of deductive arguments, as they indicate that the conclusion does not necessarily follow from the premises, even if those premises were true.
Distinguishing Formal from Informal Fallacies
The primary distinction between formal and informal fallacies lies in where the error occurs. A formal fallacy is characterized by an invalid logical form, meaning the way the premises are connected to the conclusion is structurally unsound. This makes the argument deductively invalid. In contrast, an informal fallacy might possess a valid logical form, but its unsoundness stems from one or more of its premises being false or irrelevant. An argument can, in fact, exhibit characteristics of both a formal and an informal fallacy simultaneously, complicating its analysis.
For instance, a formal fallacy ensures that an argument is unsound because its logical structure is broken. If an argument is deductively invalid, it is considered a formal fallacy. However, an informal fallacy can have a valid logical form, yet still be unsound because its content—the truth or falsity of its premises—is flawed. This highlights that while formal logic focuses on the structure, informal logic often considers the content and context of an argument.
Common Examples of Formal Fallacies
One common example of a formal fallacy involves incorrect application of a valid logical principle or the application of a nonexistent principle. Consider the reasoning: "All birds have wings. That creature has wings. Therefore, that creature is a bird." This is fallacious. While it's true that all birds have wings, having wings does not exclusively define a bird. Other creatures, like bats or insects, also have wings. The error here is in reversing a premise incorrectly.
Another illustrative example is a syllogism that appears logical but is not: "All birds have beaks. That creature has a beak. Therefore, that creature is a bird." This argument is invalid because the conclusion does not necessarily follow from the premises. While birds do have beaks, other animals, such as turtles, also possess beaks. The mistake often arises because people mistakenly convert the premise "All birds have beaks" into "All beaked creatures are birds." This reversed premise might seem plausible because many people are unaware of non-bird creatures with beaks, but it is not the original premise given. Such deductive fallacies are formed by points that individually might seem logical, but when combined, reveal an incorrect conclusion.
The "Non Sequitur" as a General Formal Fallacy
The term "non sequitur," Latin for "it does not follow," is often used to describe a general formal fallacy. It signifies an argument where the conclusion does not logically follow from its premises. While "the logical argument is a non sequitur" is synonymous with "the logical argument is invalid," the term non sequitur typically refers to types of invalid arguments that don't fit neatly into more specific named fallacies. It can also apply to deductively invalid inferences that are very weak when assessed by inductive standards.
For example, stating, "Nuclear disarmament is a risk, but everything in life involves a risk. Every time you drive in a car you are taking a risk. If you’re willing to drive in a car, you should be willing to have disarmament," is a non sequitur. The conclusion about disarmament does not logically follow from the general statements about risk. In contrast, an argument like, "If she committed the murder, then there’d be his blood stains on her hands. His blood stains are on her hands. So, she committed the murder," is deductively invalid due to the Fallacy of Affirming the Consequent, but it isn't typically labeled a non sequitur because it possesses significant inductive strength, even if formally flawed.
