Fallacies are defects in an argument other than false premises which cause an argument to be invalid, unsound or weak. Fallacies can be separated into two general groups: formal and informal. A formal fallacy is a defect which can be identified merely be looking at the logical structure of an argument rather than any specific statements.
Formal fallacies are only found only in deductive arguments with identifiable forms. One of the things which makes them appear reasonable is the fact that they look like and mimic valid logical arguments, but are in fact invalid. Here is an example:
1. All humans are mammals. (premise)
2. All cats are mammals. (premise)
3. All humans are cats. (conclusion)
Both premises in this argument are true, but the conclusion is false. The defect is a formal fallacy, and can be demonstrated by reducing the argument to its bare structure:
1. All A are C
2. All B are C
3. All A are B
It does not really matter what A, B and C stand for we could replace them with wines, milk and beverages. The argument would still be invalid and for the exact same reason. Sometimes, therefore, it is helpful to reduce an argument to its structure and ignore content in order to see if it is valid.
Informal fallacies are defects which can be identified only through an analysis of the actual content of the argument rather than through its structure. Here is an example:
1. Geological events produce rock. (premise)
2. Rock is a type of music. (premise)
3. Geological events produce music. (conclusion)
The premises in this argument are true, but clearly the conclusion is false. Is the defect a formal fallacy or an informal fallacy? To see if this is actually a formal fallacy, we have to break it down to its basic structure:
1. A = B
2. B = C
3. A = C
As we can see, this structure is valid, therefore the defect cannot be a formal fallacy identifiable from the structure. Therefore, the defect must be an informal fallacy identifiable from the content. In fact, when we examine the content, we find that a key term, rock, is being used with two different definitions (the technical term for this sort of fallacy is Equivocation).
Informal fallacies can work in several ways. Some distract the reader from what is really going on. Some, like in the above example, make use of vagueness or ambiguity to cause confusion. Some appeal to emotions rather than logic and reason.
Categorizing fallacies can be done in a number of different methods. Aristotle was the first to try and systematically describe and categorize fallacies, identifying thirteen fallacies divided into two groups. Since then many more have been described and the categorization is more complicated. Thus, while the categorization used here should prove.