Logical Fallacies PDF Print E-mail
rubiks_cubeOver the past several months, during our Wednesday night gatherings, we devoted some of our time to studying logic, using Come Let Us Reason – An Introduction to Logical Thinking by authors Dr. Norman L. Geisler and Ronald M. Brooks.

I trust it was educational, edifying, and fun too!

What is logic? According to Dr. Geisler “Logic is the study of right reason or valid inferences and the attending fallacies, formal and informal.”

Here are a few of my favorite informal fallacies:

Simple ambiguity (or equivocation)

This happens when a word or phrase is used with more than one meaning.

Ad Verecundiam – Appeal to authority

This happens when you appeal to someone who is an authority in one area, but is not an authority in the area that you are trying to make a claim about.

Argument ad Hominem (against the man)

This happens when you attack the person instead of the argument.

Argument ad Ignorantiam (argument from ignorance)

This happens when you assume something is true until it is proven false.

Argument ad Misericordiam (appeal to pity)

This happens when you appeal to one’s emotions rather than truth.

Argument ad Populum (appeal to popularity)

This happens when you appeal to the ignorant masses rather than the facts.

Petitio Principii (begging the question)

This happens when you sneak the conclusion into the premise. We also call it circular reasoning.

Straw man

This happens when a person distorts the other person’s view then attacks that view rather than what the other person is really saying.

Red Herring (diverting the issue)

This one is a bold attempt to change the subject. The name of this fallacy comes from the sport of fox hunting in which a dried, smoked herring, which is red in color, is dragged across the trail of the fox to throw the hounds off the scent.

Dicto Simpliciter (fallacy of the general rule)

This happens when you apply a general rule to a particular case that has significant differences from the general cases that you get the general rule from.

Hasty generalization

This is the opposite of the previous fallacy. Rather than arguing the validity of a particular case based on a general rule, you are trying to come up with a general rule based on an unusual or atypical case.

Genetic fallacy

This is a special type of reductive fallacy that focuses on the source of the idea. It argues that if the source is bad, the idea must be wrong.

Faulty dilemma

This type of fallacy only suggests two “either or” alternatives and forces you to choose between one or the other.

Slippery slope

The slippery slope fallacy says that I should reject this, because it will inevitably slip into that.

Post Hoc Fallacy

This type of fallacy happens when we assume an antecedent event is the cause.

Fallacy of Emphasizing Irrelevant Factors

This is similar to the post hoc fallacy in that it confuses a concomitant factor with the cause.

Fallacy of Neglecting Negative Evidence

This happens when you ignore data that doesn’t fit your hypothesis, writing it off as an anomaly or an irrelevant exception.


For further study, I recommend you pick up Dr. Geisler’s book and check out some of these web sites:

Definitions of logic

Overview of logic

Logical fallacies, and here, and here

Logic puzzles, and here, and here