by Scott F. Aikin and Robert B. Talisse
We’re currently finishing work on the manuscript for our forthcoming book, Why We Argue (And How We Should), so we’ve been thinking a lot recently about argumentation. We’ve been especially concerned with how arguments can go wrong. When evaluating an argument, one of the central questions to ask is whether the stated premises support the proposed conclusion. When the premises fail to provide the right kind of support for the conclusion, we often call the argument (and its form) fallacious. Fallacies are so pervasive precisely because they are cases in which it looks as if the stated premises provide propose support for a proposed conclusion, but in fact they don’t. Take, for example, a simple textbook fallacy, that of asserting the consequent:
If Bill’s a bachelor, Bill is male.
Bill is male, therefore Bill is a bachelor.
The trouble with an argument of this form is that it presents an invalid inference — the premises, if true, don’t guarantee the truth of the conclusion. So even were the premises and the conclusion true, the proposed argument fails. Note that the failure is a matter of the proposed argument’s form rather than its content. The objective of fallacy detection in the formal mode is to reveal cases in which the truth of the stated premises fail to provide the proper kind of support for the conclusion.
In the formal mode, we also can identify different degrees in which premises provide support for a conclusion. The highest degree of support that premises can provide for a conclusion is the guarantee of its truth, given the truth of the premises. Arguments that manifest that feature are called deductively valid. But note that deductive validity does not depend on the stated premises actually being true. That is, with a valid argument, the conclusion is guaranteed to be true, if the premises are true. Accordingly, an argument can be deductively valid even if every one of its stated premises is false.
Thus we require an additional metric of formal success. It would seem that an argument that is both deductively valid and has premises that in fact are all true would be bombproof. Such arguments are called deductively sound. Notice that deductive soundness encompasses deductive validity in that every sound argument is valid. A deductively sound argument is a deductively valid argument that has true premises. Since a deductively valid argument is one that guarantees the truth of its conclusion provided that its premises are in fact true, it should be no surprise that deductive soundness is often considered the gold standard for argumentative success. Every deductively sound argument actually establishes the truth of its conclusion. Who could ask for more than that?
