Introduction to argument evaluation

There are two things you always need to think about in deciding whether or not an argument is a good argument:

1) Are the premises and the conclusion connected in such a way that the premises, if they were true, would provide good reasons to accept the conclusion?

Here is an example of an argument in which the premises are not connected to the conclusion in this way:

P1. All mothers are female.
P2. Justine is female
                               
C. Justine is a mother

If you think about the premises, you can probably see that even if they are true, they still don’t give you any reason to believe that Justine is a mother.  It would be different if P1 said “All females are mothers” – from that and P2, you certainly could conclude that Justine is a mother (though then there would be a new problem with the argument: “All females are mothers” is clearly not true).  But if you’re given the argument as stated, you should think “Hang on a minute. Just because Justine is female and all mothers are female, that doesn’t mean that Justine is a mother. She might be, or she might not be. These premises aren’t enough reason to think she is.”

2) Are the premises actually true? 

Here is an example of an argument in which the premises are properly connected to the conclusion but the premises aren’t true:

P1. All rugby players sing opera.
P2. Kiri Te Kanawa is a rugby player.
                                     
C. Kiri Te Kanawa sings opera.

​If it were true that all rugby players sing opera and that Kiri Te Kanawa is a rugby player, then it would just have to be true as well that Kiri Te Kanawa sings opera. The connection between the premises and the conclusion is the strongest possible connection – the premises, if they were true, would guarantee the truth of the conclusion. So this argument passes the first test for being a good argument.

But it doesn’t pass the second test. It’s an argument in which, if the premises were true, the conclusion would be guaranteed. However, the premises aren’t true. All rugby players sing opera? Surely not! Kiri is a rugby player? Don’t think so. If one or more of the premises is false, we shouldn’t accept the conclusion on the basis of those premises.

In evaluating an argument, we always have to consider these two things.  We have to think about what the connection is between the premises and the conclusion.  To do this, we have to forget about whether the premises are actually true and think about whether, if they were  true, they would give us good reason to believe the conclusion.

And we have to decide whether we think the premises are true.  Obviously, if we don’t believe in the premises then, even if the conclusion follows logically from the premises, we don’t have to accept the conclusion.  On the other hand, if we think that the premises are true, and if the conclusion follows logically from the premises, then in the interests of consistency we should believe in the conclusion as well.