Philosophy - Philosophy Course Notes

Evaluating Arguments

Lecture 6

April 24, 2007

 

In this lecture we will take a general look at what is involved in evaluating arguments.  What happens when someone is presenting an argument?

a. Their goal is to convince someone to believe it is true.

b. They are saying all their premises are true, or at least it is reasonable to believe it.

c. They are also saying there is a particular relationship between the premises and the conclusion.  The conclusions follows from the premises.

d.  Finally they are saying that the premises adequately support the conclusion.

Two kinds of things can go wrong when someone is presenting an argument:

a. There is a problem with one or more of the premises.

b. The conclusion might not be supported by the premises.

 

I. Notes About Arguments

 

1. Premises supporting the conclusion.

The amount of support premises can provide to a conclusion differs radically:

a. From rock solid (if premises true, the conclusion must be true.)  ie. deductive validity.

b. All the way down to weak support... maybe or low probability.

Validity in logic is a technical term that applies to arguments.

 

2. Premises and conclusions are either true or false.

 

3. Arguments are either valid or not-valid.

Validity has nothing to do with the truth values of the premises or the conclusion.

 

4. Not all good arguments are valid arguments.

a. How can a non-valid argument be a good argument?

b. There are different levels of support that a premise can provide or the conclusion.

c. You cannot use the word valid as a synonym for the word good. (See point #3)

d. We need another term to describe good arguments.

Formal logic uses the concept of a sound argument.

 

An argument is sound if it meets two conditions:

i. All the premises are true.

ii. The argument is valid.

 

Sound arguments are the best possible kind of argument, not everyone believes sound arguments.

 

5. You cannot always know a premise is true for certain.

Sometimes we have to ask is it reasonable to believe this premise.  You do not have to know a premise is true for it to be something that is reasonable for you to believe.

 

II. What to Do If You Think The Conclusion Is Not Acceptable

 

You basically have four options:

a. You can deny that all the premises are acceptable.

b. You can deny that the conclusion is supported by the premises.

c. Admit to yourself that you are being irrational about this issue.

d. Postpone judgment.  (If this option is available.  You cannot postpone judgment forever.)

 

III. Cogency

 

A cogent argument is one whose acceptable premises provide sufficient support for the conclusion.  Cogent arguments should be persuasive.

How do we tell when an argument is cogent?

 

We use the ARG conditions to evaluate the argument.

A - Acceptability (of the premises)

R - Relevance (of each premise to the conclusion)

G - Grounds (premises taken together supporting the conclusion)

 

Ask of each premise, is it acceptable to find this premise true? It makes a lot more sense to consider the G condition before considering the R condition.

Both the R and G conditions both have to do with how much support the premise provide for the conclusion.

 

IV. Acceptability

 

A premise is acceptable if it is reasonable for us to think that the premise is true or if we know it is also true.

 

V. Grounds

 

If an argument is valid the the G condition has been met.

Have to consider the premises taken as a group.  Collectively do the premises support the conclusion.

Once you have determined that the argument is not valid, the question of grounds becomes a matter of degrees.  How much support do the premises supply for the conclusion.

Sometimes the answer is clearly yes or clearly no.  Sometimes it is less obvious.

Do the G test before the R test.

 

VI. Relevance

 

If you know an argument is valid, you do not have to consider the R condition at all.  In non-valid arguments, you should check for the relevance of each premise.

 

Take each premise individually and ask:

a.  Is this premise relevant to the conclusion?

b. Does this premise (if true) provide any reason at all to believe the conclusion?

 

If the premises provide linked support, you will have to take linked premises together when considering the R conclusion.

 

 

VII. Using the ARG Conditions

 

The ARG conditions are supposed to help us identify if the argument presented is cogent or not.  But an argument can still fail the ARG conditions and be cogent... ie. have one irrelevant premise and two very relevant true premises.

 

Use the follow steps to determine if an argument is cogent:

 

1. Check all premises against the acceptability condition.

2. Consider whether the argument is valid or not.

a. If it is valid, then both the R and G conditions will be met.

b. If we eliminate from the argument any premises that do not meet the A condition:

i. Is the argument still valid?

ii. If it is then you have a cogent argument.

3. Dealing with non-valid arguments.

a. Decide which premises are relevant and which ones are not. (R condition)

b. If there are remaining relevant premises, do they provide adequate grounds (G condition) for believing the conclusion?

 

This method allows two otherwise imperfect types of arguments that other methods might leave out:

1. You might have irrelevant premises added to an otherwise good argument.

2. You might have a good argument that has some unacceptable premises.

 

Note: Having a bad argument does not mean the conclusion is false, there could still be good reasons to believe the conclusion.

 

When you want to try and show that something is wrong with someone's argument, you need to either show:

1. One or more premises are unacceptable.

2. The conclusion does not follow from the premises.

 

 

 

 

 

< Prev		Next >