Formal Fallacies:

illicit major:  The formal fallacy committed in a categorical syllogism that is invalid because its major term is undistributed in the major premise but distributed in the conclusion.

Examples:

 

"All dogs are mammals. No cats are dogs. Therefore, no cats

are mammals."

“All students have to eat at Downer.  No faculty members are also students.  Therefore, no faculty members have to eat at Downer.”

If the first “All” was changed to “Only”, then it would be valid, but as it stands, faculty members could possibly have a meal plan as part of their contract.  This line of reasoning does not prove that they do not or could not, even though it is true that they do not have to eat at Downer.

illicit minor:  The formal fallacy committed in a categorical syllogism that is invalid because its minor term is undistributed in the minor premise but distributed in the conclusion.

Example:

"All poodles are mammals. All poodles are pets. Therefore, All

pets are mammals."

The reasoning goes from a more specific to a more general category, and then equates the two general categories, when it is possible that the general categories mearly overlap, instead of contain all of the same elements.

Informal Fallacies:

ignorance, appeal to (argumentum ad ignoratiam):  The informal fallacy of supposing that a proposition must be true because there is no proof that it is false.

Example

"The F.B.I. investigation was never able to establish that Smith

was not at the scene of the crime on the night of June 25th, so we may safely

conclude that he was there."

More Examples:

“If no one has proven that a god does not exist, then a god must exist.”

“No one has ever proven that Santa Claus does not exist, therefore he does exist.”

If a claim is not proven false, it does not mean it is true—it could also be false, even though we have not proven it, even if no one ever proves it.  Truth and falsehood does not seem to be related to whether we know a claim to be true or false.  According to at least one theory of truth, the truth or falsehood of a claim depends on if the claim corresponds with the facts.  The claim “Santa Claus exists” is true only if Santa Claus really does exist, not whether we can or do prove that claim.

authority, appeal to (argumentum ad verecundiam):  The informal fallacy of claiming that we ought to accept the truth of a proposition because of some personal feature of the individual who affirms it.

Example:

"The former Governor believes that aliens have landed in the

Arizona desert, so aliens must have landed in the Arizona desert."

“If Ryckman doesn’t believe that propositions exist, then they must not, because Ryckman is a smart guy.”

We depend on experts to know (true) things that we don’t have time or the desire to investigate for ourselves, but experts are sometimes wrong.  So while we might trust someone’s claim to be true, that is not the same thing as knowing that the claim is true.  “Be skeptical.  Question authority.  Make up your own mind.  No one holds a monopoly on truth.”  All these sayings are attempts to give advice contrary to the appeal to authority.

ad hominem argument (argument against the person):  The informal fallacy of supposing that a proposition should be denied because of some disqualifying feature of the person who affirms it. This fallacy is the mirror image of the appeal to authority. In its abusive form, ad hominem is a direct (and often inflammatory) attack on the appearance, character, or personality of the individual.

Example:

"Jeremy claims that Susan was at the party, but since Jeremy

is the kind of person who has to ride to work on the city bus, it must be false

that she was there."

A circumstantial ad hominem accuses the person of having an alternative motive for defending the proposition or points out its inconsistency with the person's other views. Tu quoque (the "so do you" fallacy) uses a similar method in response to criticism of a position already held.

“Peter says that there is a wolf coming, but since he has lied about this same thing before, it isn’t true that he has spotted a wolf.”

We should instead say “It might not be true that he has spotted a wolf” or “I think it is unlikely because he has lied about this before”, but as the story goes, there actually was a wolf there that final time.

Circumstantial: 

1)  “John says that my argument for the existence of God is flawed, but then again John is an Atheist, so he biased and therefore his claim is false.”

If John’s criticism is valid, regardless of whether he is an atheist (or even if he has never before had a valid criticism), then John’s criticism is valid.  His personal characteristics or his past history has no bearing on the truth of the claim.  John only happened to claim something—the truth of the claim has no relation to John himself.

2)  “Ryckman claims that there are no such things as propositions, but then he uses talks as if there are, therefore his claim that propositions don’t exist is false.”

Ryckman might be inconsistent, or have some other reason for continuing to act as if propositions exist, even if he thinks they don’t exist, but this has no bearing on whether they do actually exist or not.

This fallacy occurs quite often in political debates, informal discussions, and Junior High School, so be careful.  More extreme examples of this fallacy are exhibited in name-calling, prejudice, and racism.

Andrew McDuffee, September 27, 1999