What is a logical fallacy?
What is a example of a logical fallacy?
All cats are animals.
All dogs are animals.
Therefore, all cats are dogs.
That’s a false syllogism. So’s this:
All cats have tails.
Manx cats don’t have tails.
Therefore, Manx cats aren’t cats.
here’s an excellent list:
http://www.datanation.com/fallacies/index.htm
How is this one a logical fallacy? The conclusion may be false, granted, but only because one of the premises is; the logic itself is as sound as a button, as far as I can tell.
The classic logical fallacy goes:
All cats have tails.
Fido has a tail.
Therefore, Fido is a cat.
My favorite non-syllogism is:
Socrates was a man.
Socrates was homosexual.
Therefore, all men are Socrates.
Logical fallacies apply to either an invalid conclusion, as in RealityChuck’s first example, or an invalid premise, as in the second. You are correct that if both premises were valid, then the second argument would be correct.
Are you sure about that? I’ll agree that RealityChuck second argument is a fallacy, but I don’t think it should be called a logical fallacy, as the logic is sound. But maybe that’s me, and I Am Not A Logistician.
Had RealityChuck just said “cats have tails,” it could have been an example of the fallacy of division. With “all” in there, though, it is simply a “factual error,” which the Nizkor Project lists as an example of a fallacy. But that’s just an argument from authority. 
Another fallacies resource simply points out that it’s possible to make valid logical arguments with incorrect premises, which leads to a valid but unsound argument. The only way to know an argument is correct is to show that it is both valid and sound. Any conclusions reached through an argument which lacks either validity or soundness are questionable, at best.
They are usually divided up into informal fallacies (common errors that occur in argumentation) and formal ones (where the conclusion cannot be derived from the premises according to the rules of logic).
RealityChuck’s first example is a formal fallacy which could be restated as
All A = B
All C = B
All A = C <-- does not follow
It can also be diagrammed as a Venn diagram. Draw a circle and label it “A”. Draw an intersecting circle and label it “B”. Draw a third circle that intersects the other two (including part of the intersection of the other two, so that there’s an area where all three intersect), and label it “C”. Since all A = B, shade out the part of “A” that doesn’t overlap “B” (there’s nothing in that part of “A”, it exists as a category but it’s an empty category). Since all C = A as well, shade out of the part of “C” that doesn’t overlap “B” (it’s also an empty category). The diagram now indicates all that can be stated from the two premises, and it does not say that all A’s are C’s or that any A’s are C’s, or for that matter that all A’s are non-C’s. Can’t say from the premises.
Informal fallacies are things like this:
appeal to authority: TuvaDiva says jazz is good music. TubaDiva is a board goddess and a jazzy tuba player. Therefore jazz is good music.
argument ad hominem (against the person): JDT argued that circumcision is bad. JTD was a troll and an asshole. Therefore circumcision is not bad.
begging the question: The Bible is the Word of God. We know because it says so in the Bible. We know the Bible is accurate in saying so because it is the Word of God and the Word of God is obviously infallible.
argument ad baculem (appeal to force): It is immoral and wrong to smoke marijuana. After all, it would be immoral and wrong to do something that brings you to a bad end and hurts people who depend on you, and if I catch you smoking marijuana I’m going to arrest you and put you in jail, and you wouldn’t want that, would you?
equivocation: We need nervy brave daring soldiers, i.e., soldiers with balls. Chicks ain’t got balls, obviously, so they would not be nervy brave and daring, so all soldiers should be males.
etc.
And yes, RealityChuck’s second example is logically consistent and contains no fallacies. Logic will not tell you whether or not your premises are correct. It will only tell you what you can or can’t conclude based on what you are “given” as pre-established truth.
Hijack - help. Is there any non-posting way I can be notified when this thread is updated?
Um, yes. scroll way down there and click on the `email notification’ checkbox. Then again, sometimes that doesn’t work, and sometimes the hamsters seem to feel like keeping you informed even if you don’t check it, but what can ya do?
Click “subscribe to this thread” (right below the options box). It’s worked for me in the past.
If you think about this a moment you might restate it, because it looks suspiciously like transitivity.
Of course, given
A->B and
C->B,
A->C doesn’t follow, that’s true.
Because of the qualifier “all”, A and B are necessarily congruent.
j.c., there is a “Subscribe to this thread” link at the bottom… no posting required.
Various semantic tricks are common enough:
- Nothing is better than eternal happiness.
- A ham sandwhich is better than nothing.
- Therefore, a ham sandwhich is better than eternal happiness.
Lots of logical fallacies involve attempts at persuasion. The logic is not just in error, instead it’s intentionally deceptive.
Great sites about semantic trickery, etc.
Conversational cheap shots
http://www.vandruff.com/art_converse.html
Table of slick manuvers
http://attitudeadjustment.tripod.com/Books/Logic.htm
Propaganda analysis
How to be persuasive
http://www.rinkworks.com/persuasive/
Sham reasoning, pseudo-inquiry
http://www.csicop.org/si/9711/preposterism.html
Also:
The fallacy files (large)
http://gncurtis.home.texas.net/index.html