Lesson 13-3 of the U of Chicago text is called "Ruling Out Possibilities." In the modern Third Edition of the text, ruling out possibilities appears in Lesson 11-1.
This is what I wrote about Lesson 13-3 last year:
Here are a few things that I want to point out. First of all, some texts refer to the Law of Ruling Out Possibilities in Section 13-3 by another Latin name, modus tollens. Here is a link to the Metamath reference to modus tollens.
As we can observe in the proof at the above link, modus tollens is essentially modus ponens (The Law of Detachment) applied to the contrapositive (Law of the Contrapositive, or contraposition.)
Section 13-3 is another section that lends itself to an activity, since many of its questions are actually logic problems, like the ones that often appear in puzzle books.
I don't have much else to say today. After so many lengthy posts about circle clocks and circle constants, the Queen of the MTBoS and the King of Traditionalism, and so much subbing, today is a much-needed short post.
(OK, I can at least mention today's Google Doodle since it honors a scientist -- Guillermo Haro, the first Mexican member of the Royal Astronomical Society.)