Two Standards for Evaluating Arguments

The most basic distinction, then, is not between two kinds of argument but is instead between two standards for evaluating arguments. The deduc- tive standard is validity. The inductive standard is strength. Arguments themselves are classified as either deductive or inductive in accordance with the standard that they are intended or claimed to meet.

There are several important differences between deductive and inductive standards. One fundamental feature of the deductive standard of validity is that adding premises to a valid argument cannot make it invalid. The defi- nition of validity guarantees this: In a valid argument, it is not possible for the premises to be true without the conclusion being true as well. If any fur- ther premises could change this, then it would be possible for this relation- ship not to hold, so the argument would not be valid after all. Additional information might, of course, lead us to question the truth of one of the premises, but that is another matter.

The situation is strikingly different when we deal with inductive argu- ments. To cite a famous example, before the time of Captain Cook’s voyage to Australia, Europeans had observed a great many swans, and every one of them was white. Thus, up to that time Europeans had very strong inductive evidence to support the claim that all swans are white. Then Captain Cook discovered black swans in Australia. What happens if we add this new piece of information to the premises of the original inductive argument? Provided that we accept Cook’s report, we now produce a sound deductive argument in behalf of the opposite claim that not all swans are white; for, if some swans are black, then not all of them are white. This, then, is a feature of the inductive standard of strength: No matter how strong an inductive argu- ment is, the possibility remains open that further information can undercut, Induction versus Deduction perhaps completely, the strength of the argument and the support that the premises give to the conclusion. Because inductive strength and inductive argu- ments can always be defeated in this way, they are described as defeasible. Valid deductive arguments do not face a similar peril, so they are called indefeasible.