Critical Thinking, Logic, and Skepticism for Children Course: Lesson 2

Lesson 2

Opening Song (First two verses):

If there’s a question bothering your brain
That you think you know how to explain
You need a test
Yeah, think up a test

If it’s possible to prove it wrong
You’re going to want to know before too long
You’ll need a test

Fundamental Rules of Logic:

  • One of the primary principles of logic is the law of (non-) contradiction. Basically it states that no statement (proposition, assertion, etc.) can be both true and not true.

My cousin rode an elephant.  My Cousin has never ridden elephants before.

  • The second primary law of logic is the principle of excluded middle (principium tertii exclusi). The law of (non-)contradiction simply states that A cannot equal, or be, non-A.

Either your cousin rode an elephant, or he/she did not.

  • The third primary law of logic is called the law of identity. It states that A = A, or that “if any statement is true, then it is true.”

My cousin rode an elephant on a merry go round.  It was wooden.

  • The fourth primary law of logic is the law of logical or rational inference, meaning that facts can prove other facts.

Your cousin rode on a model of an elephant, not an actual elephant, therefore, your cousin did not ride on an elephant.  He/she rode on an elephant-like structure.

  • Activity 1:  Have the children come up with a proposition.  Then have them write the negation of that proposition.  Have them write the excluded middle format. Have them identify facts that would establish the proposition.  Encourage them to try, even if this is not clear. Older children may need to create facts for younger ones.

Imagination can be very helpful here.  Comic books can be a great use here, as children can comprehend that Spiderman does not exist, but if one states that he exists, he should be able to be established by various facts.  This can help the kids move into a layer of abstraction.

Logical Deduction 1: Descarte’s Thinking Thing

Tell the children to think about what they are.  Explain that in the early 1600s there was a philosopher named Rene Descartes who had the same question.  His fundamental question was “Am I a body with a mind, or am I a mind with a Body?”

To illustrate use the following example:
Imagine that we are scientists or doctors and that, to treat someone to make them better, we need to cut away everything that is “not them.”  If we cut off a finger, would the person still be “them”?  An Arm?

At what point would they no longer be them?

Explain that Rene Descartes was trying to answer a similar question.  He came up with the idea that we are “thinking things.”  That is to say that the body is added on to a mind that exists independently.  He talks about “What if we are someone else’s dream like the Tweedledee  posits to Alice in ‘Through the Looking Glass’?”  He thinks about “What if he is a brain in a jar somewhere, just imagining it has a body?”

How could we test these hypotheses?

Phineas Gage Video.

Explain that scientists can actually show that different parts of our brain are connected to different behaviors because of accidents like Phineas Gage’s, where bits of brain were cut out.

Another philosopher who knew Descartes was named Thomas Hobbes.  Hobbes is probably most famous for having a tiger named after him. Hobbes posited that we are a body and that the mind is a creation of it.

How could we test this hypothesis?

Logic isn’t about guessing

Both Rene Descartes and Thomas Hobbes were, in the primary case, guessing.  They had some reasons for their guesses, but neither one actually put it to the test.

Anyone can guess. Anyone can make statements that sound reasonable but are not (such as in our elephant example).  What separates logic from guesses is that it is built on “deductions,” that is, that it considers facts as building blocks to come to a conclusion.

  • Activity 2:  Build a block tower out of Jenga-like blocks.  Start to remove a block from the bottom.  Ask the children what will happen if you remove the bottom blocks.

Logic relies on basic building blocks that work together with rules, just like a block tower.  Follow the rules, and you have a “firm foundation”–your argument is strong.  Skip pieces and ignore rules, and your tower will be weak.

  • Activity 3:  Have the children throw the blocks (Guessing).

Explain that guessing is like throwing blocks.  Although you might get a few blocks to stack up, it will never be as firm as the tower.


We can know more about ourselves and the world around us by using the principles of logic to establish firm “towers” of belief that do not rely on guesswork.

There are four primary laws of logic (have the children repeat them, possibly hand out candy/cookies to children who can remember them):

  1. Law of (non-) contradiction – A statement cannot be both true and false.
  2. Principle of excluded middle- There is no third proposition. Either it happened or it did not (no quibbling).
  3. Law of identity – A true statement is true.
  4. Law of logical or rational inference – Statements can follow each other to a conclusion.

Homework:  Observe a person for a day and then write a story from their point of view.  Take it as accurately to how that person would think and feel about their decisions.  Feel free to ask the person why the make the choices they do, or how they would react.  The point is to get a feeling of how that person thinks and feels; as well as to understand how they are different form you.

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Last edited by EmmaHS on February 2, 2013 at 11:13 pm

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