Chapter 1: Reconstructing and analyzing arguments

A good argument is not only valid, but also sound. Soundness is defined in  terms of validity, so since we have already defined validity, we can now rely on it  to define soundness. A sound argument is a valid argument that has all true  premises. That means that the conclusion of a sound argument will always be  true. Why? Because if an argument is valid, the premises transmit truth to the conclusion on the assumption of the truth of the premises. But if the premises  are actually true, as they are in a sound argument, then since all sound  arguments are valid, we know that the conclusion of a sound argument is true.  Compare the last two Obama examples from the previous section. While the  first argument was sound, the second argument was not sound, although it was  valid. The relationship between soundness and validity is easy to specify: all  sound arguments are valid arguments, but not all valid arguments are sound  arguments.

Although soundness is what any argument should aim for, we will not be talking  much about soundness in this book. The reason for this is that the only  difference between a valid argument and a sound argument is that a sound  argument has all true premises. But how do we determine whether the premises  of an argument are actually true? Well, there are lots of ways to do that,  including using Google to look up an answer, studying the relevant subjects in  school, consulting experts on the relevant topics, and so on. But none of these  activities have anything to do with logic, per se. The relevant disciplines to  consult if you want to know whether a particular statement is true is almost never  logic! For example, logic has nothing to say regarding whether or not protozoa  are animals or whether there are predators that aren’t in the animal kingdom. In  order to learn whether those statements are true, we’d have to consult biology,  not logic. Since this is a logic textbook, however, it is best to leave the question  of what is empirically true or false to the relevant disciplines that study those  topics. And that is why the issue of soundness, while crucial for any good  argument, is outside the purview of logic.