Recap on last week: we looked at what makes a deductive argument SOUND and at what makes such an argument VALID

If it is snowing the mail will be late

It is snowing

----------------------

Therefore the mail will be late

This is an example of the valid argument form of modus ponens

If it is snowing the mail will be late

The mail will be late

------------------------Therefore it is snowing

This is an example of the

fallacy of affirming the consequent

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The fallacy we have just looked at is a formal fallacy…

….a fallacy of form…

…but we are going to look at fallacies informally

In particular we are going to look at fallacies of:

Ørelevance

Øvacuity

Øclarity

Fallacies of relevance:

Øciting in support of a conclusion something that is true but irrelevant (non-sequitur)

Øattacking the person making the argument rather than the

argument that is made (ad

hominem)

Non-Sequitur:

Bill lives in a large building, therefore his apartment is large.

Every year many people are supported through life by their religious beliefs, so their religious beliefs must be true.

These arguments work because people don’t notice the irrelevance, and because they are overly:

a)  generous (they are reluctant to point out the irrelevance);

b)  proud (they don’t want to admit they can’t see a connection)

Ad Hominem:

Nick Griffin is leader of the BNP therefore his claim that some people worry about immigration is rubbish.

Von Daniken's books about ancient astronauts are worthless because he is a convicted forger and embezzler.

Be careful to distinguish:

Øad hominem attacks: attacks on someone’s right to say something

Øad hominem fallacies: attacks on the truth of what someone says

An ad hominem attack:

Nick Griffin is a self-professed racist, so you should take care when listening to his claims about immigration.

An ad hominem fallacy:

Nick Griffin is leader of the BNP therefore his claim that some people worry about immigration is rubbish.

Fallacies of vacuity:

Øciting in support of a conclusion  that very conclusion (circular arguments)

Øciting in support of a conclusion a premise that assumes the conclusion (question-begging)

Øoffering an argument that cannot be questioned (self-sealing)

In a circular argument the conclusion IS one of the premises

In a question-begging argument the conclusion is ASSUMED by one of the premises.

Circular argument:

All whales are mammals, therefore all whales are mammals Question: is this valid?

All circular arguments are valid…

…. because there can’t be any possible

situation in which the premises are all true…

…. and the conclusion false…

… if the conclusion is one of the premises.

Circular arguments often convince ….

…because there will be many premises other than the premise that is the conclusion…

…so the fact that the conclusion is amongst the premises can go unnoticed

Begging the question:

It is always wrong to murder human beings

Capital punishment involves murdering human beings

-----------------------------

Capital punishment is wrong

Explain the circles or the question-begging premises in each