What Is A Philosophical Argument 2

Attacking The Argument Form

In the previous post What Is A Philosophical Argument we said, ” A PA will consist of reasons (or premises) leading to a conclusion.”  In other words it deals with evidences that point us to a final conclusion.  We also said that there are some things to look out for when evaluating an argument, namely the argument form and the truthfulness of its premises.

What does it mean to look at the argument form?  This means that we are going to look at the relationships between the argument’s premises and its conclusion.  The reason is if no relation exists then the conclusion does not follow from the premises.  It is important to also understand that evaluating the argument form is not evaluating the truthfulness of its premises.  At this stage in our evaluation all we want to know is whether or not the argument form is valid.

Modus Ponens: Affirming The Antecedent

For example a common argument form that is used:

If P then Q

P

therefore Q

This argument affirms the antecedent and is a valid argument form.  If you want to impress your friends you can drop the Latin modus ponens. However when we change things around we have something like this:


If P then Q

Q

therefore P

Notice in this argument form we are affirming the consequent.  This is an invalid argument form.  For example:

If the universe was created by God, then there would be no evil

there is no evil

Therefore the universe was created by God

This invalid form can be easily seen when we model the argument.  For example consider my dog Lady:

If Lady were a cat, then Lady would have four legs and a tail

Lady has four legs and a tail

Therefore Lady is a cat

Here we see a situation where both premises are true but the conclusion is false leaving us with an invalid form since we know that true premises cannot lead us to a false conclusion.

Modus Ponens: Negating the Consequent 

Some arguments consist of a negation. For example:

If the universe was created by God, then it would exhibit design.

But the universe does not exhibit design.

Therefor the universe was not created by God.

If P then Q

Not Q

Therefor not P

This is a valid form. However if we turn things around and affirm the antecedent the result is an invalid form:

If the universe was created by God, then it would exhibit design.

The universe was not created by God.

Therefor the universe does not exhibit design.

If P then Q

Not P

Therefor not Q

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