2. So what exactly is logic? Who cares?
Informally, logic is about saying things that make
sense. You can think of it in that way if you like.
“It's pretty sunny today, so you should wear sunscreen.”
Formally, logic is the art of arguing – not like a fight
or debate, but by using the information we already
know to draw new and useful conclusions.
“If it's sunny today, you should wear sunscreen.
Indeed it is sunny today.
Therefore, you should wear sunscreen.”
3. “But wait. That just looked like the exact same
thing you said before, you hack.”
Well, yes. But that's how an argument looks in standard
form! You can break down any argument into this form;
that makes it easier to think about.
If it's sunny today, you should wear sunscreen.
It is sunny today.
___________________________________________
Therefore, you should wear sunscreen.
Premises/
givens
Conclusion
(Premises always come first, and the conclusion always comes last.)
4. How about a more complex argument?
1. This piece of fresh fruit is fuzzy.
2. It also has seeds.
3. If a fruit is fuzzy, it's either a kiwi or a peach.
4. Peaches have a pit; they don't have seeds.
5. So the fruit can't be a peach.
6. So the fruit must be a kiwi.
Which of the above sentences is a conclusion?
5. How about a more complex argument?
1. This piece of fresh fruit is fuzzy.
2. It also has seeds.
3. If a fruit is fuzzy, it's either a kiwi or a peach.
4. Peaches have a pit; they don't have seeds.
5. So the fruit can't be a peach.
6. So the fruit must be a kiwi.
Good logic lets us cobble together lots of
different pieces of information, and tell from
them what's probably or definitely true.
6. But what counts as “good logic”?
That argument was good (made sense), because the
conclusion followed from the premises. We'll see what
this means in a moment.
Why don't we look at a bad argument?
7. But what counts as “good logic”?
Some people have fallen off cliffs and lived.
Therefore, if I jump off this cliff, I will definitely be fine.
“Come on.
What could
possibly go
wrong?”
8. But what counts as “good logic”?
Some people have fallen off cliffs and lived.
Therefore, if I jump off this cliff, I will definitely be fine.
This argument is weak. Although the premise is true,
it's easy to think of ways (very painful ways) that the
conclusion could be false. The easiest way to spot bad
logic is to do just that: try to think of another way out.
(Philosophers call these counterexamples).
9. But what counts as “good logic”?
Let's look at two kinds of arguments.
1. Deductive reasoning:
All interns can breathe fire. So Philip can
breathe fire.
Is there a piece of the puzzle missing?
10. But what counts as “good logic”?
Let's look at two kinds of arguments.
1. Deductive reasoning:
All interns can breathe fire. Philip is an intern.
So Philip can breathe fire.
Sometimes you may encounter “hidden” statements
and ideas, which the writer sneaks in but doesn't
say outright.
11. But what counts as “good logic”?
Let's look at two kinds of arguments.
1. Deductive reasoning:
All interns can breathe fire. Philip is an intern.
So Philip can breathe fire.
Are the premises true?
If so, then the conclusion's
100% guaranteed true.
No getting around it!
12. But what counts as “good logic”?
Let's look at two kinds of arguments.
2. Inductive reasoning:
I touched a stove and it burned me. I did this
fifty times, and the same thing happened. So the
next time I touch the stove, it will burn me.
Are the premises true?
If so, then the conclusion's
probably true. There might
still be exceptions.
LIKE WHAT?
13. How is this useful to me?
Like Mr. Morgan said, the ability to make strong logical
arguments will become more and more important later
on in high school and college.
Pathos and ethos are still valuable! But your audience
will be a lot better at questioning them. Logos is handy
because, if you use it well, it can't really be disproved.
14. How is this useful to me?
But even better is the superpower to spot weak logic.
Next time you watch TV or go online (with your parents'
permission, of course), try to keep track of how many
different arguments are being pitched to you by ads.
How much info is given to you? How much is left out?
15. What time is it?
Adven- wait, no. Activity time!
Pair off into groups of four. Each group will receive an example
of a poor argument (these may be either inductive or deductive).
With your group, you will have 5 minutes to try to come up with
one counterexample - one way in which the argument
could be wrong, even if the premises are definitely true.
Poke it full of holes!
Also, choose a group representative to tell us your reasoning.
(It's OK to imagine unlikely or weird explanations;
don't be afraid to think outside the box.
Oddly enough, logic has very little to do with facts.)
16. Some examples:
ARGUMENT: “I pulled an all-nighter studying for last week's big
test, and I ended up with an A. Tiredness must make me smart!”
COUNTER: What if you got an A because you actually studied?
Or maybe the test was going to be easy for you all along?
ARGUMENT: “If I play with Dad's power tools, he'll yell at me.
But Dad is yelling at me for something. So I guess I must have
played with the power tools.”
COUNTER: What if he's yelling at you for a different reason:
scratching the car, or hammer-throwing the cat onto the roof?
