In defending the teaching of Formal Logic, Dorothy Sayers notes, “Another cause for the disfavor into which Logic has fallen is the belief that it is entirely based upon universal assumptions that are either unprovable or tautological. This is not true. Not all universal propositions are of this kind.”

I want to make two comments. First, it appears that Sayers is committing the fallacy of apriorism here. The discreditors of Logic attack it by arguing that the universal assumptions upon which Logic is based are unprovable or tautological (and are thus worthless). She implies, in what appears to be a hasty generalization, that they are attacking all universal propositions (when she argues that not all universal propositions are of this kind), when in fact they are only attacking some of them. Maybe I am missing something here, but it seems that the universal propositions she defends may not be the same ones the discreditors are attacking.

Second, if I were to respond to the discreditors given her assumption, I would argue this way: “Do you really believe that all universal propositions are unprovable or tautological? Then how about that claim? Is it unprovable, or tautological?” All universal propositions are unprovable or tautological is itself a universal proposition, and is thus open to the same refutation. Thus this attack on universals lays itself wide open to a classic reductio.

It would be an instructive exercise to take Logic or Rhetoric students through a defense of Sayers claim that not all universal propositions are unprovable or tautological. Ask your students, “How do we know the truth of universal propositions?” Discuss the value of an inductive defense of universals. Discuss also the proving of universals by authority, by definition, and by deduction from other universals.

In her essay “The Lost Tools of Learning,” Dorothy Sayers has identified for many classical Christian schools of our day an outline for a modern education following the medieval Trivium: Grammar, Logic, and Rhetoric. I am interested in what she says about Logic and the Dialectic Stage, and plan to occasionally post some thoughts about these and related topics. I will start my posts with a comment she makes that I have found helpful in my own teaching of Logic.

In describing a student of the medieval Trivium, Sayers writes, “Secondly, he learned how to use language; how to define his terms and make accurate statements; how to construct an argument and how to detect fallacies in argument.” This short statement gives us what I have come to believe are the four primary lessons to be learned in a Logic class, and in the proper order.

First, the logic student learns about terms, which are the building blocks of statements. They learn what a term is, how terms differ from words, the methods and rules of defining terms, and how to use the tools that relate terms to one another, such as genus and species charts.

Second, the logic student learns about statements. They learn what a statement is, how to identify the different types of statements, how to relate statements to one another, and how to determine the truth of a given statement.

Third, the student learns “how to construct an argument.” Logical arguments are built out of statements, which are connected as premises to make conclusions. Students learn how to distinguish between valid and invalid arguments, what validity means, and why it differs from truth. Once they are able to identify valid arguments given to them, they learn how to construct valid arguments of their own.

Fourth, the logic student learns “how to detect fallacies in argument.” A fallacy is an invalid form of argument. They learn to identify not only the formal fallacies discovered by the rules of validity, but also informal fallacies such as ad hominem and post hoc.

Thus, Sayers has given us the outline of a complete introductory logic curriculum. I would only add that we should not limit our learning of the above to categorical logic, but include the tools of propositional (or symbolic) logic as well. Students should be given the powerful tools of relating symbolic propositions, determining the validity of propositional arguments, and learning how to construct propositional proofs.