Math Is Fun Forum

rune2402

Banned

Registered: 2007-10-23

Posts: 32

last one and Ill reply valid or invalid

Identify the argument as valid or invalid.
The argument has a true conclusion. All dogs have fur. All cats have fur. ______________ A cat is not a dog.

I think it is valid.

JaneFairfax

Member

Registered: 2007-02-23

Posts: 6,868

Re: last one and Ill reply valid or invalid

No, it is not valid! Just because a statement is true in the real world does not mean that the argument leading to it is valid.

The validity of an argument DOES NOT DEPEND on the truth/falsity of its conclusion in the real world. This is a very common psychological mistake of students in logic. It is possible for an argument with a conclusion that is true in the real world to be invalid, just as it is possible for an argument with a conclusion that is false in the real world to be valid.

Consider the following argument instead.

All birds have feathers.
All parrots have feathers.
∴ A parrot is not a bird.

Now you can see why the argument is invalid. The premises are true while the conclusion is false; since a false conclusion cannot be logically derived from true premises, the argument is therefore unsound.

Last edited by JaneFairfax (2007-10-31 01:23:10)

rune2402

Banned

Registered: 2007-10-23

Posts: 32

Re: last one and Ill reply valid or invalid

This one actually got me. Heheh. Thanks.