Chapter 2: Deductive Arguments

Sections:

• ​Introduction to argument evaluation
• Validity
• Common argument patterns
• Basic structural fallacies
• Soundness

You’ll remember from Chapter 1 that an argument is an attempt to persuade someone of some conclusion by giving reasons in support of that conclusion.

Deductive arguments are ones which are intended to guarantee the truth of their conclusion.

For example, this is a deductive argument:

P1.    All politicians are dishonest.
P2.    Winston is a politician.
                                           
C.       Winston is dishonest.

You can see that if the premises were true, the conclusion would be guaranteed.

Not all arguments try to absolutely conclusively prove their conclusion, and an argument can still be a good argument if it makes its conclusion very likely to be true, rather than certain to be true.

For example, consider this argument.

P1. 92% of politicians are dishonest.
P2. Winston is a politician.
                                    
C.  Winston is dishonest.

​The premises, even if true, don’t guarantee the conclusion. But if the premises are true then the conclusion is very likely to be true.

The first argument above (with the “All” statement as its first premise)  is a deductive argument – it’s clear that the arguer intends to conclusively prove the conclusion. The second one, with the percentage claim as its first premise, is not: there’s no way you could guarantee, with premises like these, that the conclusion was true. (It’s always possible that Winston is in the honest 8%.) But the second one is still quite a good argument – it provides quite a good reason (supposing the premises are true) to think that Winston is dishonest.