Logic (Earth)

[rojse]

Originally Posted by SoyLeche

Actually, the point was to show that an argument can be sound and the conclusion false. In order for this to happen at least one of the premises must be false.

I don't think anyone said that since P1 was false then the conclusion was false. At this point it is pretty self-evident to most of us that this particular conclusion is false - that's why it was an effective example.

The argument is not sound, because the premise is incorrect. Even though the conclusion is correct, the premise is not.

[Comprehend]

Originally Posted by rojse

It is fallacious logic - one of the in-built premises behind the conclusion is incorrect.

no it isn't. It is completely sound logic. I think you might be unfamiliar with the rules of logic...

[Comprehend]

Originally Posted by SoyLeche

No, the logic is fine, that's the point. Any logical argument is only as good as it's premises. That's where the argument always should lie.

If we could all get a good basic understanding of logic we could get to the real discussions - which premises we should start with.

I am beginning to think that some of us are talking about actual logic and other are talking about "folk logic".

[Comprehend]

Originally Posted by rojse

You are phrasing the logic behind the original question wrong.

If A, then B.
A is false.
B is false.

The third conclusion is incorrect, as I have stated before in a previous post; disproving A does not disprove B.

Argument from fallacy - Wikipedia, the free encyclopedia

uh.. I didn't make that argument... go back and look at it.

[Aqualung]

Originally Posted by rojse

The argument is not sound, because the premise is incorrect. Even though the conclusion is correct, the premise is not.

You are making the Denying the antecedent fallacy by stating that if the premise is false, the argument is not sound. Once again, the only necessary thing is that, for the time being, all those wishing to participate simply assume that the premises are false. If we can't even agree on that, the argument can't go forward at all. If we can, then we can make logically sound arguments, even if we know that the premises are false, simply by assuming or accepting them as true for a few moments.

The Modus Ponens page also deals with this by saying

The fact that the argument is valid cannot assure us that any of the statements in the argument are true; the validity of modus ponens tells us that the conclusion must be true if all the premises are true. It is wise to recall that a valid argument within which one or more of the premises are not true is called an unsound argument

[Comprehend]

Originally Posted by rojse

The start and conclusion was the first argument - if the earth appears flat, then it is:

"If A, then B."

That was Comprehend's argument.

We say that things are not as they appear, so A is false.

"A is false."

Because A is false, we conclude that B must be false.

"B is false."

However, A being false does not automatically mean that B is false.

You have a severe lack of understanding of logic. My argument is 100% sound and also 100% false which was the entire point.

I was proving that logic cannot prove whether something is true or not, it can only prove soundness.

[Aqualung]

Originally Posted by rojse

The start and conclusion was the first argument - if the earth appears flat, then it is:

"If A, then B."

That was Comprehend's argument.

We say that things are not as they appear, so A is false.

"A is false."

Because A is false, we conclude that B must be false.

"B is false."

However, A being false does not automatically mean that B is false.

Truth table of an If/then statement

P   Q    P -> Q
-----------------
T   T       T
T   F       F
F   T       T
F   F       T

If A is false, then actually, the entire argument is true, no matter the truthiness of B.

[Comprehend]

Originally Posted by SoyLeche

Which fallacy is this (Comprehend's logical argument with variables):

If A then B
A
Therefore B

(hint - it isn't one, it's sound logic)

actually, my argument wasn't an if/then but:

a=b
b=c
(therefore)
a=c


Not that it will make any difference to the folk logicians.