Must it be true?

About this video

Your aunt says every cookie in the jar is chocolate chip, and you take one without looking. Before you look, you already know your cookie is chocolate chip. If the two reasons are true, every cookie in the jar is chocolate chip and yours came from the jar, then the conclusion has to be true. When an argument works like this, the conclusion follows from the reasons, and the argument is called valid.

Change one word: most of the cookies are chocolate chip. Now the reasons can be true while the conclusion is false, because you might have picked the one plain cookie. The conclusion is likely, but it doesn't follow. This short animation shows the two jars side by side: must, and might.

Then it shows the test for validity in three steps. Pretend the reasons are true. Try to imagine the conclusion false at the same time. If you can't, the argument is valid. Validity is about the shape of the argument, not its facts: "All cats can fly. Tom is a cat. So Tom can fly." is valid, even though its first reason is false. Whether the reasons are true is a separate question. This video goes with the Must It Be True? and Good Shape, Wrong Facts lessons of Logic Level 1 on Knaph.

Transcript

1. 0:00You take a cookie without looking. What do you already know?
2. 0:04Every cookie in the jar is chocolate chip. So yours must be.
3. 0:10If only most are, yours might be the plain one.
4. 0:16The test. Pretend the reasons are true. Try to imagine the conclusion false. If you can't, it's valid.
5. 0:23All cats can fly, Tom is a cat, so Tom can fly. Still valid!
6. 0:29Valid means, if the reasons are true, the conclusion must be.