Logical fallacies

False Dilemma

A limited number of options (usually two) is given, while in reality there are more options. A false dilemma is an illegitimate use of the "or" operator. Putting issues or opinions into "black or white" terms is a common instance of this fallacy.

Appeal to Popularity (argumentum ad populum)

A proposition is held to be true because it is widely held to be true or is held to be true by some (usually upper crust) sector of the population. This fallacy is sometimes also called the "Appeal to Emotion" because emotional appeals often sway the population as a whole.

Affirming the Consequent non sequiturs

Any argument of the following form is invalid: If A then B B Therefore, A

Denying the Antecedent non sequiturs

Any argument of the following form is invalid: If A then B Not A Therefore, Not B

False Dilemma

Either you're for me or against me.

False Analogy

Employees are like nails. Just as nails must be hit in the head in order to make them work, so must employees.

Hasty Generalization

Fred, the Australian, stole my wallet. Thus, all Australians are thieves.

Affirming the Consequent non sequiturs

If the mill were polluting the river then we would see an increase in fish deaths. And fish deaths have increased. Thus, the mill is polluting the river.

Coincidental Correlation (post hoc ergo propter hoc)

Immigration to Alberta from Ontario increased. Soon after, the welfare rolls increased. Therefore, the increased immigration caused the increased welfare rolls.

False Analogy

In an analogy, two objects (or events), A and B are shown to be similar. Then it is argued that since A has property P, so also B must have property P. An analogy fails when the two objects, A and B, are different in a way which affects whether they both have property P.

Inconsistency non sequiturs

John is taller than Jake, and Jake is taller than Fred, while Fred is taller than John.

Appeal to Authority (argumentum ad verecundiam)

My friend heard on the news the other day that Canada will declare war on Serbia. (This is a case of hearsay; in fact, the reporter said that Canada would not declare war.)

Appeal to Popularity (argumentum ad populum)

Polls suggest that the Liberals will form a majority government, so you may as well vote for them

Begging the Question (petitio principii)

Since I'm not lying, it follows that I'm telling the truth.

Inconsistency non sequiturs

The author asserts more than one proposition such that the propositions cannot all be true. In such a case, the propositions may be contradictions or they may be contraries.

Coincidental Correlation (post hoc ergo propter hoc)

The name in Latin means "after this therefore because of this". This describes the fallacy. An author commits the fallacy when it is assumed that because one thing follows another that the one thing was caused by the other.

Attacking the Person (argumentum ad hominem)

The person presenting an argument is attacked instead of the argument itself. This takes many forms. For example, the person's character, nationality or religion may be attacked. Alternatively, it may be pointed out that a person stands to gain from a favorable outcome. Or, finally, a person may be attacked by association, or by the company he keeps.

Appeal to Pity (argumentum ad misercordiam)

The reader is told to agree to the proposition because of the pitiful state of the author

Hasty Generalization

The size of the sample is too small to support the conclusion.

Non-Sequitur

The term non sequitur literally means "it does not follow". In this section we describe fallacies which occur as a consequence of invalid arguments.

Begging the Question (petitio principii)

The truth of the conclusion is assumed by the premises. Often, the conclusion is simply restated in the premises in a slightly different form. In more difficult cases, the premise is a consequence of the conclusion.

Appeal to Pity (argumentum ad misercordiam)

We hope you'll accept our recommendations. We spent the last three months working extra time on it.

ad hominem (circumstantial)

We should discount what Premier Klein says about taxation because he won't be hurt by the increase.

Appeal to Authority (argumentum ad verecundiam)

While sometimes it may be appropriate to cite an authority to support a point, often it is not. In particular, an appeal to authority is inappropriate if: the person is not qualified to have an expert opinion on the subject, experts in the field disagree on this issue. the authority was making a joke, drunk, or otherwise not being serious A variation of the fallacious appeal to authority is hearsay. An argument from hearsay is an argument which depends on second or third hand sources.

ad hominem (abusive)

You may argue that God doesn't exist, but you are just following a fad

ad hominem (tu quoque)

You say I shouldn't drink, but you haven't been sober for more than a year.

Denying the Antecedent non sequiturs

if you get hit by a car when you are six then you will die young. But you were not hit by a car when you were six. Thus you will not die young. (Of course, you could be hit by a train at age seven, in which case you still die young.

ad hominem (circumstantial)

instead of attacking an assertion the author points to the relationship between the person making the assertion and the person's circumstances.

ad hominem (abusive)

instead of attacking an assertion, the argument attacks the person who made the assertion

ad hominem (tu quoque)

this form of attack on the person notes that a person does not practice what he preaches.