Introduction to Fallacies


In order to understand what a fallacy is, one must understand what an argument is. Very briefly an argument consists of one or more premises and one conclusion. A premise is a statement (a sentence that is either true or false) that is offered in support of the claim being made, which is the conclusion (which is also a sentence that is either true or false).

There are two main types of arguments: deductive and inductive. A deductive argument is an argument such that the premises provide (or appear to provide) complete support for the conclusion. An inductive argument is an argument such that the premises provide (or appear to provide) some degree of support (but less than complete support) for the conclusion. If the premises actually provide the required degree of support for the conclusion, then the argument is a good one. A good deductive argument is known as a valid argument and is such that if all its premises are true, then its conclusion must be true. If all the argument is valid and actually has all true premises, then it is known as a sound argument. If it is invalid or has one or more false premises, it will be unsound. A good inductive argument is known as a strong (or “cogent”) inductive argument. It is such that if the premises are true, the conclusion is likely to be true.

A fallacy is, very generally, an error in reasoning. This differs from a factual error, which is simply being wrong about the facts. To be more specific, a fallacy is an “argument” in which the premises given for the conclusion do not provide the needed degree of support. A deductive fallacy is a deductive argument that is invalid (it is such that it could have all true premises and still have a false conclusion). An inductive fallacy is less formal than a deductive fallacy. They are simply “arguments” which appear to be inductive arguments, but the premises do not provided enough support for the conclusion. In such cases, even if the premises were true, the conclusion would not be more likely to be true.


Example of a Deductive Argument

Premise 1: If Bill is a cat, then Bill is a mammal.
Premise 2: Bill is a cat.
Conclusion: Bill is a mammal.

Example of an Inductive Argument

Premise 1: Most American cats are domestic house cats.
Premise 2: Bill is an American cat.
Conclusion: Bill is domestic house cat.

Example of a Factual Error

Columbus is the capital of the United States.

Example of a Deductive Fallacy

Premise 1: If Portland is the capital of Maine, then it is in Maine.
Premise 2: Portland is in Maine.
Conclusion: Portland is the capital of Maine.
(Portland is in Maine, but Augusta is the capital. Portland is the largest city in Maine, though.)

  Example of an Inductive Fallacy

Premise 1: Having just arrived in Ohio, I saw a white squirrel.
Conclusion: All Ohio squirrels are white.
(While there are many, many squirrels in Ohio, the white ones are very rare).

Published in: on March 12, 2008 at 5:33 pm  Comments (1)  
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  1. […] people tend to be more influenced by poor reasoning than by good reasoning. As I tell my students, fallacies tend to be far more persuasive than logical arguments. After all, people tend to feel far more […]

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