# Validity of a simple logical argument

Discussion in 'General Philosophy' started by Speakpigeon, Jan 23, 2019.

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## Is the argument valid?

Poll closed Feb 22, 2019.

54.5%

45.5%

0 vote(s)
0.0%

0 vote(s)
0.0%
1. ### DaveC426913Valued Senior Member

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16,960
Pretty much nailed, it. QQ had the guts to say it out loud.

At least three relatively new members off the top of my head use this tactic prodigiously.

3. ### James RJust this guy, you know?Staff Member

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37,194
One wonders what happens when these kids are confronted by a cake divided into three parts. Since 1/3 + 1/3 + 1/3 = 3/9 = 1/3, they must find it inexplicable that the three pieces apparently make a whole cake.

5. ### Write4UValued Senior Member

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18,617
Utter nonsense.
P5. Joe is an elephant, not a squid, nor a giraffe, he is an elephant as declared in the conditions of the deductive exercise!
P4. is a logically flawed premise. It posits an impossible contradiction both ways.

The conclusion is already provided before the false deduction. The OP question may be logical in format but it contains false premises. Therefore the entire logical exercise is "unsound" and "not valid".

Last edited: Feb 3, 2019

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Derail.
EB

8. ### SpeakpigeonValued Senior Member

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Here I'm using the 2,400-year-established logic of Aristotle. Would that be good enough, do you think?
Between Aristotle and Kant, no known major intellectual figure ever articulated any substantial disagreement with Aristotle's definition of logical validity implicit in his notion of syllogism.
So, yeah, that has to be good enough.
And, by the way, that's indeed deciding what is validity by vote.
So, I guess this just goes on to show you don't know much about what you're talking about.
Derail, but, still, that's a good story. It sure helps me understand you guys.
-------------------------------------------
So, you think there's a correct definition of validity and that's the one you use?
OK, go on, provide the definition of validity you think is correct and then what you think would be the justification given by experts as to why this definition would be the correct one.
Let's see if you know what you're talking about.
EB

9. ### SpeakpigeonValued Senior Member

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Do you at all understand that I take the "proof" I provided to be utter tosh?!
Or are you so beside your shoes that you can't get yourself to read what you pretend to be responding to?!
So, yeah, Hallelujah?
Still, this shows again that we agree as to validity.
Little comfort, though.
EB

Write4U likes this.
10. ### iceauraValued Senior Member

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30,994
Nope.
I have used a criterion of invalidity.
Show, don't tell.
It's not a derail.

11. ### SpeakpigeonValued Senior Member

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So, the provisional results are that 5 think the argument is not valid and 5 think the argument is valid.
There are a few possible biases, here.
First, some people here just can read very well and some will cast the wrong vote. Still, same chance either way, so it shouldn't matter too much.
Second, people with even slight cognitive disorders or mental issues may be more likely to vote "not valid" and this whatever the argument. However, there will be even more likely to vote "The argument doesn't make sense", so, again, this bias shouldn't be too much of an issue.
Third, people with a training in formal logic are much more likely to be dogmatic in their assessment of this particular argument and therefore to vote valid. Here, we have at least three people like that, James R., Sarkus and Baldeee.
So, overall, I think it's fair to surmise that a substantial majority of people assessing the argument on an intuitive basis will say it is not valid.
Thank you all to have contributed to this experiment and to have contributed your own subjective evidence. Now we have the better empirical evidence of this objective result.
Thanks.
EB

12. ### SarkusHippomonstrosesquippedalo phobeValued Senior Member

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9,561
Great, so you get to conclude that formal logic can seem unintuitive. What's new! This was highlighted on the first page of this thread, that the definition of validity in classical logic throws up seemingly counterintuitive examples of valid arguments. But I'm glad you are satisfied with the result of your experiment, even it is one that has been known for rather a long time.
Does that mean you'll now stop littering this forum with your threads?

13. ### James RJust this guy, you know?Staff Member

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37,194
People assessing it on an intuitive basis are unlikely to be overly concerned about the distinction between validity and soundness. Everybody agrees it isn't sound.

14. ### James RJust this guy, you know?Staff Member

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Any conclusion, C, validly follows from contradictory premises. For example:

Formal argument:
P1: P
P2: not P
Conclusion: Therefore C.

An example of this in practice:
P1: John is an elephant.
P2: John is a squid.
P3: A squid is not an elephant.
Conclusion: Donald Trump is America's greatest ever President.
----

Proof that the formal argument is valid:
1. P. (from P1)
2. P or C. (necessarily true because P is true - from step 1)
3. not P. (From P2)
4. Therefore C. (true because either P or C must be true (step 2) and we know that P is false (step 3)).

Application of the formal proof to the example:
1. John is an elephant. (P1)
2. Either John is an elephant or Trump is the greatest President, or both. (True because John is an elephant (step 1).)
3. John is not an elephant. (because a squid is not an elephant (P2) and John is a squid (P3).)
4. Therefore Trump is the greatest President. (Deduced because we have shown that John is not an elephant (step 3), but, as we have shown previously, one or both of "John is an elephant" or "Trump is the greatest President" must be true (step 2).)

That is, the example is a valid proof that Donald Trump is America's greatest ever President.

Quantum Quack likes this.
15. ### SpeakpigeonValued Senior Member

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Easy to say. Now prove it. Go, on, provide quotes of people here who seemed to have voted "not valid" because they found the argument unsound.
People who assess an argument as not valid on the basis of their intuition will find the argument absurd, and this will usually shows in the way they report their position, and this even though that understand ship about formal logic.
EB

16. ### SpeakpigeonValued Senior Member

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The way you do it, this argument is only valid in reference to the definition of validity you accept. If the definition you use is not correct, your "validity" here is shit.
So what is the justification that the definition of validity you use here is correct?
Is it arbitrary? If it is arbitrary, why are people here insisting I'm wrong. The question was,"Is the argument valid", and not "Is the argument valid according to mathematicians".
And if you think it's not arbitrary, then surely you should be able to explain why you think so, unless it's just a case of deferring to what you perceive as "the authority"? Is your argument here, i.e. you, Sarkus, Baldeee, an argument from authority?!
So, please explain what is the justification given by mathematicians, logicians, philosophers etc. that the definition of validity you use here is correct.
I'll be waiting anxiously not.
EB

17. ### SpeakpigeonValued Senior Member

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Please explain what is the justification given by mathematicians, logicians, philosophers etc. that the definition of validity you use here is correct.
EB

18. ### SpeakpigeonValued Senior Member

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Please explain what is the justification given by mathematicians, logicians, philosophers etc. that the definition of validity you use here is correct.
EB

19. ### SpeakpigeonValued Senior Member

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Please explain what is the justification given by mathematicians, logicians, philosophers etc. that the definition of validity you use here is correct.
EB

20. ### iceauraValued Senior Member

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30,994
Once again:
I don't use a definition of validity here.

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LOL.
EB

22. ### iceauraValued Senior Member

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30,994
Perhaps, as I tried to forestall above, you have both
1) confused my posting here with that of some other thread, and
2) confused a criterion for invalidity with a definition of validity.
That would explain your insistent rhetoric.
As would mental incapability on your part - losing track of your own argument. You do have several different posters to deal with, not a simple task.

23. ### James RJust this guy, you know?Staff Member

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37,194
How do you know?

Can you give me a workable definition of validity in which the argument I gave is not valid?

Recall that what you're comparing to is this:
"An argument is valid if the truth of the premises guarantees the truth of the conclusion."

or, equivalently:

"An argument is valid if it is impossible for the premises to all be true and for the conclusion to be false, at the same time."

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