Logical Fallacies

All of the fallacies and descriptions here are also found on the Skeptics Guide to the Universe website.

How does an argument work?

All arguments have the same basic structure: A therefore B. They begin with one or more premises (A), which is a fact or assumption upon which the argument is based. They then apply a logical principle (therefore) to arrive at a conclusion (B). An example of a logical principle is that of equivalence. For example, if you begin with the premises that A=B and B=C, you can apply the logical principle of equivalence to conclude that A=C.

What is a logical fallacy?

A logical fallacy is a false or incorrect logical principle. An argument that is based upon a logical fallacy is therefore not valid. Valid logic applied to one or more false premises, leads to an invalid argument. However, if an argument is not valid the conclusion may, by chance, still be true.

It is important to note that if the logic of an argument is valid and all the premises are true, then the conclusion will always be a true statement.

Examples: We will begin with a few basic fallacies. I will post more fallacies in the future.

Non-Sequitur

In Latin this term translates to "doesn't follow". This refers to an argument in which the conclusion does not necessarily follow from the premises. In other words, a logical connection is implied where none exists. For example, we should consult a psychic because the police haven't been able to solve the case. The fact that the case hasn't been solved using traditional police methods is irrelevant to whether consulting a psychic is a method that should be used.

Post-Hoc Fallacy

The "post-hoc ergo propter hoc" fallacy follows the basic format of: A preceded B, therefore A caused B, and therefore assumes cause and effect for two events just because they are temporally related (the Latin translates to "after this, therefore because of this"). There are many examples of post-hoc reasoning: You have a cold, so you drink fluids and two weeks later your cold goes away, therefore drinking fluids cured your cold. Another example is: You perform some task exceptionally well after forgetting to bathe, so the next time you have to perform the same task you shouldn't before hand bathe.

Straw Man

Arguing against a position which you create specifically to be easy to argue against, rather than the position actually held by those who oppose your point of view. A common straw man argument made by evolution denialists is: We did not evolve from monkeys because... This is a straw man because no one believes we evolved from modern prime-apes, the real argument is that we share a common ancestor.

Tu Quoque Fallacy

Literally, you too. This is an attempt to justify wrong action because someone else also does it. "My evidence may be invalid, but so is yours." For example, I may be wrong about the date of creation, but you are wrong about the age of the universe.