Scott Postma

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Saturday School: Lesson #7 – Supporting your Thesis, Part III (Syllogisms)


Lesson #7 – Supporting your Thesis, Part III (Syllogisms)

  • Complete the reading assignment
  • Complete the writing exercise
  • Post your assignment in the comments
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Last week we discussed how the ideas a writer develops to support a paper’s thesis can be classified into two main categories: logic and topics of invention.

We addressed the first, logic. The first thing every writer, speaker, or thinker needs to know about logic is that all logic is based on a principle of non-contradiction. This means, for example, something cannot exist and not exist at the same time. Aristotle says it this way in Metaphysics: “it is impossible at once to be and not to be…”

What this teaches a writer is that a thesis cannot be supported by arguing for something and against something at the same time. Arguing this way will only confuse the reader, and convince no one. As Scott Crider states in his book, The Office of Assertion, “Arguments that contradict themselves are not very persuasive.”

Additionally, there are two kinds of logical arguments writers can use, deductive arguments and inductive arguments. An inductive argument is made from the observation of particular examples. Deductive arguments are made moving from one established principle to another established principle in order to establish a new principle.

Today’s Lesson

Today we’re going to learn how syllogisms–the framework of a deductive argument–work.

In the first place, there are three kinds of syllogisms writers need to be familiar with. These are categorical, hypothetical, and disjunctive syllogisms. (Because this is not a class in formal logic, we will not be developing these fully, just enough to give you some tools to help you formulate an idea of what should be going on in a persuasive essay.)

Each of these three kinds of syllogisms have three parts to them: a major premise, a minor premise, and a conclusion. And it is by the nature of and the relationship between these three parts that we distinguish between the three kinds of syllogism.

First, we’ll tackle the categorical syllogism. This is the one most of us are familiar with by virtue of its famous example:

  • All people are mortal (Major premise: All A are B.)
  • Socrates is a person (Minor premise: C is A.)
  • Socrates is mortal (Conclusion: therefore, C is B.)

The way it works is rather simple, but requires us to put on our thinking caps and focus a little.

Typically, the major premise is universal in nature. It tells us something that is generally known or can be easily observed. In our example, it is saying all people (A) have the attribute of mortality (B). The minor premise is usually a premise that is more particular in nature: Socrates (C) is a person (in category A). Therefore, by logical necessity, Socrates (C) must be mortal (have attribute B).

An important aspect of categorical syllogisms that is helpful for writers to understand is the ability to qualify an argument by using qualifying language in the construction of the syllogism. Perhaps in another lesson we will tackle this more completely, but for now recognize that one powerful tool for writers to use is to substitute All in the major premise with expressions like Some or No to qualify which category a thing belongs in.

The last two syllogisms, hypothetical and disjunctive, are not unlike the categorical syllogism in structure, but are different in operation. Whereas the categorical syllogism imagines groups or categories for things to belong in, the hypothetical imagines conditions, and the disjunctive imagines alternatives. Thus, the hypothetical looks like this:

  • If P, then Q (Major premise: if condition P, then condition Q)
  • P. (Minor premise: the condition for P is met)
  • Therefore, Q (Conclusion: it follows by necessity that condition Q is true also)

Example: If it rains, the car will get dirty. It rained. Therefore, the car is dirty.

The disjunctive looks like this:

  • Either X, or Y (Major premise: one alternative or the other)
  • Not X (Minor premise: not the first alternative)
  • Therefore, Y (Conclusion: therefore, it is the second alternative)

Example: He either went to Dallas or to New York. He is not in Dallas. Therefore, he must be in New York.

All syllogisms are in danger of failing due to specific fallacies associated with their nature and structure. We’ll tackle these at a later time, but do beware of the fallacy! In the mean time, let’s practice.

Writing Exercise

Try your hand at creating deductive arguments using each of the three syllogistic structures we’ve just studied.

Do you want to touch readers with your words, learn the craft of writing, or simply improve your writing skills from a classical perspective? Join me each week for Saturday (Writing) School. Every Saturday I’ll send a lesson to your inbox you can complete in an hour, or you can work on it at your leisure. It’s free!


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About Scott Postma

Scott is a writer and teacher living in North Idaho. He loves teaching the Great Books, writing and blogging, and collecting more books than he’ll ever read in a lifetime. You can subscribe to the tribe and follow him on Twitter, Facebook, and Google Plus.

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