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Lecture on Relevance

Fallacies of Relevance:  Ch. 15

Premises that have no bearing on the truth of the conclusion are irrelevant.
Sometimes called Red Herrings.
A Red Herring is a fish that fox hunters would drag across a fox’s path in order to train their hounds.  The smell of the fish distracts the hounds from the smell of the fox.

Irrelevant reasons are offered to mislead or divert attention from the real issue.

A reason is relevant when it has some bearing on the truth value of the conclusion.


1. Ad Hominem

Literally, an attack “against the person” making a claim, rather than against the claim itself.

Three subtypes of Ad Hominem:  1) Denier; 2) Silencer; 3) Dismisser

A. Denier

Denies the truth of what is claimed based on something about the person making the claim.

Some deniers are justified: “Louie is a jailhouse snitch who gets paid to testify and always perjures, so his testimony is probably false now.”

Others are unjustified: “The OWS folks look like a bunch of bums, so they are probably wrong.”

Ask: Does the information about the person give me reason to think the claim might be false?

If yes, then it is a justified denier – not ad hominem fallacy.
If no, then it is an unjustified denier – ad hominem fallacy.

B. Silencer

Silencers question a person’s right to speak without denying the truth of the claim.
Random Guy: “No nukes! No nukes!”
Senator: “Would the gentleman please GTFO of the Senate Chamber?  We are in session.”

Senator: “We should raise taxes!”
Other Senator: “Would the ‘esteemed colleague’ STFU?! He is Junior Senator from Wyoming!”

C. Dismisser

Dismiss the speaker as a reliable source of good information.  These do not deny the claim, but seek to undermine its support.

Justified: When the speaker lacks integrity and stands to gain from his claim.

Unjustified: When the speaker has integrity.

2. Appeal to Authority

Remember Ethos?
Usually it is okay to appeal to authority to support a minor claim.
When it is abused, it is a fallacy.

So when is it abused?  Ask yourself:
1. Is the cited authority really an expert in the appropriate field?
2. Is this the kind of question that an expert can settle?
3. Has the authority been cited correctly?
4. Can the cited authority be trusted to tell the truth?
5. Why is an appeal to authority even being made?

If the answers to (1)-(4) are “yes”, then the appeal may be relevant and justified.  But, it is still weak most of the time.

If one answers “no” to any of the above (1)-(4), then the appeal is irrelevant and unjustified.

3. Appeal to Popularity

Lots of people believe X, so X is (probably) true.

Ask: Is the opinion actually widely held?

Ask: Is popular opinion likely to be right about this sort of thing?

Ask: Why appeal to popular opinion at all?

4. Appeal to Emotion

P makes me Angry/Sad/Afraid…. So, not P.

Emotions generally cloud judgment, and often have no bearing on the truth of a proposition.  Often, they are irrelevant.


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