ARGUMENTS GOOD AND BAD
A good argument shows that its conclusion is wor- thy of belief or acceptance; a bad argument fails to show this. A good argument gives you good rea- sons to accept a claim; a bad argument proves nothing. So the crucial question is, How can you tell which is which? To start, you can learn more about different kinds of arguments and how they get to be good or bad.
There are two basic types of arguments: deduc- tive and inductive. Deductive arguments are sup- posed to give logically conclusive support to their
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conclusions. Inductive arguments, on the other hand, are supposed to offer only probable support for their conclusions.
Consider this classic deductive argument:
All men are mortal.
Socrates is a man.
Therefore, Socrates is mortal.
It is deductive because the support offered for the conclusion is meant to be absolutely unshak- able. When a deductive argument actually achieves this kind of conclusive support, it is said to be valid. In a valid argument, if the premises are true, then the conclusion absolutely has to be true. In the Socrates argument, if the premises are true, the conclusion must be true. The conclusion follows inexorably from the premises. The argument is therefore valid. When a deductive argument does not offer conclusive support for the conclusion, it is said to be invalid. In an invalid argument, it is not the case that if the premises are true, the con- clusion must be true. Suppose the first premise of the Socrates argument was changed to “All ducks
are mortal.” Then the argument would be invalid because even if the premises were true, the conclu- sion would not necessarily be true. The conclusion would not follow inexorably from the premises.
Notice that the validity or invalidity of an argument is a matter of its form, not its content. The structure of a deductive argument renders it either valid or invalid, and validity is a separate matter from the truth of the argument’s state- ments. Its statements (premises and conclusion) may be either true or false, but that has nothing to do with validity. Saying that an argument is valid means that it has a particular form that ensures that if the premises are true, the conclusion can be nothing but true. There is no way that the prem- ises can be true and the conclusion false.
Recall that there are indicator words that point to the presence of premises and conclusions. There are also indicator words that suggest (but do not prove) that an argument is deductive. Some of the more common terms are it necessarily follows that, it must be the case that, it logically follows that, con- clusively, and necessarily.