Distinguish valid from invalid arguments and sound from unsound

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Presentation transcript:

Distinguish valid from invalid arguments and sound from unsound VALID VS INVALID Distinguish valid from invalid arguments and sound from unsound All dogs have 4 legs. All tables have 4 legs. Therefore, all dogs are tables.

All actors are robots. Tom Cruise is an actor. Therefore, Tom Cruise is a robot.

People from England speak English. Marc is from England. Therefore, Marc speaks English.

The president of the US must be born in the US. Donald Trump was born in the US. Therefore, Donald Trump is the president of the US.

All actors are robots. Tom Cruise is a robot. Therefore, Tom Cruise is an actor.

British people speak English. Marc speaks English. Therefore, Marc is British.

INVALID! The Pope speaks 13 languages. This man speaks 13 languages. Therefore, this man is the Pope. INVALID!

1. All female TV hosts are successful TV hosts. 2 1. All female TV hosts are successful TV hosts. 2. Oprah Winfrey is a successful TV host. 3. Therefore, Oprah Winfrey is a female TV host. INVALID!

Premise 1 says that all females who are TV hosts are successful. So, if you are a female, and you are a TV host, you are successful. However, premise 1 does not assert that ALL TV hosts are females. This implies that there are successful male TV hosts too. Now, what does premise 2 assert? It asserts that Oprah Winfrey is a successful TV host. But it does not say anything about her sex. I know what you’re thinking: “But I know Oprah! She is a female TV host.” Granted, but the question is NOT who Oprah in fact is. Once again, the question is whether the premises imply the conclusion. It is a matter of relationship between premises and conclusion, and not between reality and conclusion.

VALID! BUT UNSOUND Students who received an F pass the course. Philippa received an F. Therefore Philippa passed the course. VALID! BUT UNSOUND

INVALID If you are playing soccer, you have a soccer ball. Therefore you are playing soccer. INVALID

INVALID All trees are plants. This is a plant. Therefore, this is a tree. INVALID

INVALID All plants are green. Trees are green. Therefore, trees are plants. INVALID

INVALID All things with 3 sides are triangles. Some things with 3 angles are triangles. Therefore, some things with 3 angles are things with 3 sides. INVALID

Let’s change terms: Things with 3 sides become ants Triangles become insects Things with 3 angles become things that fly

INVALID All ants are insects. Some things that fly are insects. Therefore, some things that fly are ants. INVALID

VALID BUT UNSOUND All dogs are cats. Fluffy is a dog. Therefore, Fluffy is a cat. VALID BUT UNSOUND

? VALID But is it sound? Whatever begins to exist has a cause. The universe began to exist. Therefore, the universe has a cause. ? VALID But is it sound?

THIS IS THE END Questions?