Apply Now
Home >

Enter Logic Tutorial

HOW TO IDENTIFY ASSUMPTIONS

"Tallmon's Definition of Practical Reasoning" is:

Cultivating the intellectual abilities to:

  1. Identify and evaluate assumptions.
  2. Follow an argument to its conclusion.
  3. Spot contradictions and faulty logic.
  4. Draw appropriate distinctions.
  5. Avoid extremes.
  6. Exercise foresight.

Three basic tools for developing practical reasoning in the classical model:

  • Syllogistic logic (to identify assumptions and follow an argument)
  • dialectic (to spot contradictions)
  • common material fallacies (to spot faulty logic)

Studying syllogistic logic equips one to do exactly the kind of practical reasoning you will be doing in the Analysis of Controversy and Persuasive speeches. So we will study it, briefly, and tie it to the Stock Issues Model, so you will have all the tools you need to do the A of C speech. Then, in preparation for the Persuasive speech, we'll study common material fallacies (as a means of equipping you to refute bogus arguments). Do you see how these tools will be practically applied?

(If, after you study the pages that follow, you do not understand how to apply this knowledge, email me or, better yet, post a discussion thread!)


SO—HOW DOES ONE IDENTIFY ASSUMPTIONS? (other than intuitively)


In traditional liberal arts pedagogy, kids, as pre-teens, were taught how to follow a train of thought by studying syllogisms. Categorical syllogisms are the easiest kind to understand.

Here's a common categorical syllogism:

  • All A are B.
  • C is an A.
  • C is a B.

The classical example is:

  • All men are mortal. (major premise)
  • Socrates is a man. (minor premise)
  • Socrates is mortal. (conclusion)

Aristotle's definition of syllogism: "discourse in which, certain things being stated, something other than what is stated follows of necessity from their being so." (Pr. Analytics 24b.20) What does this mean?

Consider this example:

  • All real computers are PCs.
  • John's machine is an Apple.
  • Therefore . . . . the unstated conclusion, that which "follows of necessity" (according to logic) is what?

Right!

"John's machine is not a real computer." How did you know that was the conclusion?


It's obvious!

But, what's not so obvious is, why it is obvious!

Because our minds operate according to laws of thought. It would be nonsense (in the most robust sense of the word) to conclude from the stated premises anything other than "John's machine is not a real computer." (Although, as we all know, Apple computers are superior to PCs, but that's another debate!!! :o)

What would you say to someone who, given the premises as stated, concluded that "John is disabled." What's that got to do with those premises? That conclusion does not follow from the premises! The syllogism is invalid. Got it?

So, Aristotle asserts that a syllogism is when things that are unstated follow, according to the laws of thought, necessarily from what was stated (if what was stated is "so").

We know an argument is valid when the conclusion follows from the premises logically. Whether or not it's TRUE is another question.

(We will study fallacies, in a couple weeks, as tests for truth.)


So, if one recognizes when the conclusion follows logically from premises, one can, with some precision, draw inferences about premises that are left unstated. In your upcoming speech assignment, you will need to identify some unstated premises. This knowledge helps one follow an argument to its conclusion and, by thinking "upstream" as it were, identifying assumptions.

Here's a syllogism that is valid but untrue:

  • All PHC students are skiers.
  • Hubert is an PHC student.
  • Therefore, Hubert skiis.

The argument is valid,but it is not necessarily true. It could be true, Hubert may ski, but only by coincidence, not by logical necessity.

What's wrong with it? Right! The major premise is flawed. There are obviously PHC students who do not know how to ski, so the major premise is untrue.


We will get into fallacies later. Right now we're trying to equip ourselves to identify assumptions, because, in order to fully analyze a controversy, one needs to be able to identify the questions at the heart of the controversy, and many times, those questions are left unstated.

If your argument has valid form and the premises are true, the argument is sound.

Here is another very critical point to understand: An enthymeme is a syllogism with one of the propositions missing.


Go to Richard M. Weaver's Chpt. 5 on Argumentation

Previous | Next