Scott Postma

A blog about the Great Books, the Craft of Writing, and Human Flourishing.

Saturday School: Lesson #6 – Supporting Your Thesis, Part 2 (Logic)


Lesson #6 – Supporting Your Thesis Statement, Part 2 (Logic)

  • Complete the reading assignment
  • Complete the writing exercise
  • Post your assignment in the comments
  • Share the lesson with a friend



Last week we discussed the fact that a writer must have real and reasoned arguments to support his thesis. The way this is accomplished is by defining and developing terms. Last week we discussed definition–clarifying terms, ideas, and positions for your reader. This week we’ll turn to the first of two important ways a writer can develop his ideas.

Today’s Lesson

How a writer develops the ideas that support his paper’s thesis can be classified into two main categories. A writer can use either logic or topics of invention. Of course, he can and will use both, and they are often employed together, but we will deal with these individually for clarity’s sake.

This week I’ll introduce the first, logic. Next week I’ll expound on it a bit. Following that, I will introduce the second, topics of invention.

Logic, or what is sometimes referred to as a syllogism, is fundamental to thinking well about a subject. As we have previously established, thinking well and writing well are synonymous, so it’s important to understand the basics of 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 the writer is that he cannot support his thesis by arguing for something and against something at the same time. Arguing this way will only confuse the reader, and hardly convince him. As Scott Crider states in his book, The Office of Assertion, “Arguments that contradict themselves are not very persuasive.”

Believe it or not this is one of the most common mistakes new writers make. They argue in favor of an idea on one page, discover something down the road that changes their mind or debunks their argument, then they turn around and argue against their original idea a few pages later.

As obvious as it should be, it must be stated that good writing adheres to logic’s foundational law of non-contradiction.

One reason so many fall into the subtle trap of contradicting themselves, and subsequently fail to support their thesis with adequate proofs is because fallacies in one of the categories of logic, called deductive argumentation, or sometimes assumptive argumentation, are sometimes difficult to recognize.

This is because deductive arguments mostly take place in the mind and depend on us making appropriate assumptions about unstated syllogisms. Let me explain.

In logic, there are two kinds of arguments, deductive arguments and inductive arguments. An inductive argument is the observation of particular examples. Think empirical or scientific observation. Deductive arguments don’t focus on particular examples; instead, they move from an established principle to an established principle in order to establish a new principle.

For example, without ever really thinking about it, we might argue deductively to ourselves that someone has run out of gas when we observe that person walking along a desolate road carrying a gas can. We make reasonable assumptions (their car must have run out of gas) based on established presuppositions (i.e. cars run on gas–in this case multiple established presuppositions are actually in play; we will discuss these in the next lesson).

To be sure we are making good arguments that support our thesis, we must learn how to syllogisms work. This is where we will pick up next week.

Writing Exercise

Using the thesis statement you’ve been working with, write three pairs of supporting statements that literally contradict each other. The more subtle you can be the better.

Example: Students who hope to succeed in life and work must master the craft of writing before graduating high school.

  • Students who know how to write well are better at problem-solving.
  • Students who don’t know how to write well are still pretty good at problem-solving.

Now it’s your turn. Post your contradicting statements in the comments.

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!


Aristotle, Aristotle in 23 Volumes, Vols.17, 18, Translated by Hugh Tredennick. (Medford, MA: Cambridge, MA, Harvard University Press; London, William Heinemann Ltd., 1933, 1989).

Crider, Scott F. The Office of Assertion: An Art of Rhetoric for the Academic Essay. Wilmington, DE: ISI Books, 2005.

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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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