This being philosophy, you can be sure we're going to spend plenty of time criticizing arguments. (Although don't make the mistake of thinking that's all philosophy is about. If we're attacking arguments, it's because we at least want to take a shot at finding out the truth.) Now that we've thought a little about the structure of arguments, we're in a position to consider the various ways in which a person can criticize an argument. There are essentially three ways.
1. Challenging the Acceptability of the Premises
This is, of course, no surprise. Clearly, one of the ways of challenging an argument is to call its premises into question. If one of its premises is false, then it isn't sound. We should keep in mind, however, that an argument isn't always shown to be unconvincing if one of its premises is made to look doubtful. Consider two different ways in which a set of premises might support a conclusion. They can do so cooperatively or independently. Consider:
P1: All cats like to eat yogurt.Here, neither premise alone gives us any reason to believe the conclusion. They only support the conclusion when taken together. As such, showing either of these premises to be false puts the argument into serious jeopardy. However, this isn't always so. Consider the following argument:
P2: Bob is a cat.
C: Bob likes to eat yogurt.
P1: The Blue Jays have a very effective bullpen.Here, each individual premise independently provides us with some reason to believe the conclusion. With this sort of argument, it might be possible to shoot down one of the premises without shooting down the conclusion (e.g., even if P4 is false, if we believe P1-3, we still have good reason to believe the conclusion). In other words, although soundness is what we ideally want out of an argument, it is possible for an argument to be convincing even if it's not sound.
P2: The Jays have the finest starting pitching in the American League.
P3: The Blue Jays have one of the top five batting line-ups in all of baseball.
P4: All the other American League East teams have agreed not to win more than eight games next season.
C: The Jays will win the American League East next season.
2. Showing the Argument isn't Logically Strong
Again, I'm sure it's no surprise to see this listed as one of the ways of criticizing an argument. If you can show the argument isn't logically strong, then it's not sound. To be more specific, if an argument is not logically strong, then it's unconvincing.
Consider the following:
P1: All the great Presidents cheated on their wives.Is this a good argument?
P2: Clinton cheated on his wife.
C: Clinton is a great President.
It's important to keep in mind that showing an argument isn't logically strong isn't the same thing as showing that its conclusion is false. Even if we can show that the above argument isn't logically strong, Clinton may still be a great President. All we do when we show an argument isn't logically strong is to take away a reason for believing its conclusion. There may be others.
3. Offering a counter-argument
We don't always have to use the direct approach as in 1 and 2 above. Sometimes, rather than directly criticizing an argument, we might introduce a counter-argument, i.e., one that shows the argument's conclusion is false, but otherwise doesn't address the argument.
P1: Campbell was Prime Minister of Canada.Even if, as is the case here, the premises are acceptable and the argument is logically strong (inductively strong, in this case), we can use a counter-argument to render the argument unconvincing.
P2: All but one of the Prime Ministers of Canada have been men.
C: Campbell was a man.
P1: Campbell was always referred to as Madam Prime Minister when addressed by the Canadian press.[Philosophy 1200]
P2: Only women are addressed as 'Madam' by the press.
C: Campbell was a woman.