Common Logical Fallacies: Understanding Invalid Arguments

Common Logical Fallacies

Ad Hominem

Description: Attacking the person making the argument, rather than the argument itself. This fallacy occurs when the attack on the person is completely irrelevant to the argument they are making.

Logical Form:

• Person 1 is claiming Y.
• Person 1 is a moron.
• Therefore, Y is not true.

Ad Populum

Description: Claiming that most or many people accept a belief as true, and presenting this as evidence for the claim. Accepting this without demanding evidence as to why they accept the belief is lazy thinking and a dangerous way to accept information.

Logical Form:

• A lot of people believe X.
• Therefore, X must be true.

Argumentum ad Misericordiam

Description: An attempt to distract from the truth of the conclusion by the use of pity.

Logical Form:

• X is true because person 1 worked really hard at making X true.

Reductio ad Absurdum

Description: A proposition is disproven by following its implications logically to an absurd conclusion.

Logical Form:

• Assume P is true.
• From this assumption, deduce that Q is true.
• Also deduce that Q is false.
• Thus, P implies both Q and not Q.
• Therefore, P itself must be false.

Non Sequitur

Description: When the conclusion does not follow from the premises. In more informal reasoning, it can be when what is presented as evidence or reason is irrelevant or adds very little support to the conclusion. Any good argument must have a conclusion that follows from the premises.

Logical Form:

• Claim A is made.
• Evidence is presented for Claim A.
• Therefore, claim C is true.

Post Hoc Ergo Propter Hoc

Description: The Latin phrase means “after this, therefore because of this.” This fallacy is committed when it is assumed that because one thing occurred after another, it must have occurred as a result of it. Mere temporal succession does not entail causal succession.

Logical Form:

• X happens after Y, so Y is the result of X.

False Dilemma

Description: When only two choices are presented yet more exist, or a spectrum of possible choices exists between two extremes. False dilemmas are usually characterized by “either this or that” language, but can also be characterized by omissions of choices.

Logical Form:

• Either X or Y is true.

Slippery Slope

Description: These arguments falsely assume that one thing must lead to another. They begin by suggesting that if we do one thing, then that will lead to another, and conclude that we therefore shouldn’t do the first thing. The problem with these arguments is that it is possible to do the first thing that they mention without going on to do the other things.

Logical Form:

• If you do X, you will end up doing Y, which is bad.