Two checks

About this video

An argument can go wrong in two different ways. The conclusion might not follow from the reasons, or one of the reasons might be false. So every argument gets two checks. The first is about its shape: if the reasons were true, would the conclusion have to be true? The second is about the facts: are the reasons really true? An argument that passes both checks is called sound.

This short animation shows why sound arguments are special. If the reasons are true, and the conclusion follows from them, there is only one possibility left: the conclusion is true. The example is "Every planet in our solar system goes around the Sun. The Earth is a planet in our solar system. So the Earth goes around the Sun." Both reasons are true and the conclusion follows, so it must be true.

Putting the two checks together gives four kinds of argument: it follows with true reasons (sound), it follows with a false reason (valid but not sound, like "all cats can fly"), it doesn't follow with true reasons, and neither. Only one of the four is sound. The first check anyone can do by thinking; the second often needs looking, measuring or asking someone who knows. This video goes with the Sound Arguments lesson of Logic Level 1 on Knaph.

Transcript

1. 0:00When does a conclusion have to be true?
2. 0:04Two checks. Does the conclusion follow? Are the reasons true? Pass both, and it's sound.
3. 0:11Every planet goes around the Sun. Earth is a planet. So the conclusion must be true.
4. 0:17Four kinds of argument. Follows or not, reasons true or not. Only one is sound.
5. 0:26Sound means it follows, and the reasons are true.