Logical Fallacies When arguments go bad… Image: .

Logical Fallacies

When arguments go bad…

Image: www.colourbox.com

Non Sequitur Latin for “does not follow”; where the claim is

not logically linked to the supporting material

“Because the American people have elected such incompetent people to high office, we should raise the voting age back to twenty-one.” The claim (that we should raise the voting age)

doesn’t follow logically from the “evidence” (that we’ve elected incompetents to high office.)

“Non sequitur is the best logical fallacy because none of my meals so far today have involved asparagus.”

Circular Reasoning Where the claim merely restates, in

different terms, what is already given in the supporting material.

“Premarital sex is wrong because it is immoral.” To be “immoral” and to be “wrong” are two

different ways of saying the same thing, so what is really being said here is that “Premarital sex is wrong because it is wrong.” The “evidence” asserts what is claimed, rather than proving it.

“Circular reasoning is the best fallacy because it can prove anything. It can prove anything because it’s the best fallacy.”

Post hoc ergo propter hoc It happened after, therefore it happened

because Where correlation (things happening

together) is used to infer causation (something causing another)“It rained today because I washed my car.”Joey’s refrigerator

After I started assuming that post hoc is the best logical fallacy, I won the lottery three times.”

False Dilemma (Either-Or) Where two options are presented as if

there is no other option “Either you are with us, or you are

with the terrorists.” I’m the best there is “Either you think false dilemma is the

best logical fallacy, or you don’t think it’s a fallacy at all.”

Bandwagon Where something is

true/good/beneficial because it’s popular (or false/bad/harmful because it’s unpopular)

“Car XYZ is the best-selling vehicle in its class.”

Each line sounds pretty much the same-ea

“Most people think bandwagon is the best fallacy, so clearly it is.”

Ad Hominem Latin for “to the person”; when a person tries

to rebut another’s argument by using a personal attack rather than by addressing the argument itself.

“These bastards who run our country are a bunch of conniving, thieving, smug pricks who need to be brought down and removed and replaced with a whole new system that we control.”

Ad Hominem *and* Bandwagon (2:40) “Ad hominem is the best logical fallacy. If

you disagree with me, well, you suck.”

Slippery Slope Where one step leads to many others "We've got to stop them from banning

pornography. Once they start banning one form of literature, they will never stop. Next thing you know, they will be burning all the books!"

Fear is the path of the Dark Side (3:44) “If you don’t think slippery slope is

the best logical fallacy, then you’ll do poorly in class, drop out of school, commit crimes, and be thrown in jail for life.”

APPLICATIONS AND

A REALLY bad argument

Monty Python

Crowd: A witch! A witch! We've found a witch! A witch! A witch! Burn her! A witch! A witch!

Person 1: We have found a witch. May we burn her?

Bedemir: How do you know she is a witch?

Crowd: She looks like one! Yeah, yeah!

Bedemir: Bring her forward.

Accused Woman: I'm not a witch. I'm not a witch.

Bedemir: But you are dressed as one.

AW: They dressed me up like this. C: Ah, we did not. AW: And this isn't my nose it's a false

one. Bedemir: [examines false nose]

Well? P1: Well, we did do the nose. Bedemir: The nose?

P1: And the hat, but she's a witch. C: Yeah, burn her! Burn her! Bedemir: Did you dress her up like

this? C: NO! No! No. Yes. Yes. A bit. P1: She has got a wart. Bedemir: What makes you think

she's a witch? P2: Well, she turned me into a newt. Bedemir: A newt? Bedemir &C: [stare in disbelief]

P2: I got better. P3: Burn her anyway! C: Burn her! Burn her! <Arthur arrives> Bedemir: Quiet, quiet, quiet. There

are ways of telling whether she is a witch.

P1: Are there? C: Huh? What are they? Tell us. Tell

us. Do they hurt? Tell us. Bedemir: Tell me: what do you do

with witches?

C: Burn them! Bedemir: And what do you burn

apart from witches? P1: More witches! P3: Wood. Bedemir: So, why do witches burn? <much thinking> P2: Be-cause they're made of wood? Bedemir: Good. Crowd: Oh, good. Oh… Bedemir: So, how do we tell

whether she is made of wood?

P1: Build a bridge out of her. Bedemir: Ah, but can you not also

make bridges out of stone? P1: Oh, yeah. Bedemir: Does wood sink in

water? P1: No, no. P3: It floats. It floats! P1: Throw her into the pond! C: Yeah! Bedemir: What also floats in

water?

P1: Bread. P3: Apples. P2: Very small rocks. C: Cider, gravy, cherries, mud,

churches, churches, lead … King Arthur: A duck! Bedemir: Exactly. So, logically,

…? P1: If she weighs the same as a

duck, she's made of wood. Bedemir: And, therefore, …?

P3: A witch. C: A witch! A witch! [Bedemir and the crowd then

proceed to put AW on the scales opposite a duck, and the scales balance. She notes ironically, “this is a fair court…”]

A number of these arguments all rely on the same faulty argument structure. The structure of the argument is the standard

All A is C.All B is C.

○ Therefore, all A is B.

BA

C

BA

C

(D a ta )

R e jec te d :

(D a ta )

(W a rra n t)

R e jec te d :

(D a ta )

R e jec te d :

(D a ta )

(W a rra n t)

(C la im an d D a ta )

(W a rra n t)A rg um e nt b y S ign

P a rtia lly R e je c te d :

(D a ta )

(W a rra n t)A rg um e nt b y S ign

(M a jo r C la im )

S h e h a s a wa rt.(D a ta )

R e jec te d :T h e y g a ve h era fa lse n ose .

S h e h as a b ig no se(D a ta )

W itch e s h a ve w a rts an d b ig n ose s.(W a rra n t)

R e jec te d :S h e d id no t ch oo se

to we ar the ha t.

S h e w e ars a b ig h a t.(D a ta )

R e jec te d :S h e d id no t ch oo seto w ea r th e d re ss.

S h e w e a rs a b la ck d re ss.(D a ta )

W itch e s ch o ose to w ea r b ig h a ts an d b la ck d re sse s.(W a rra n t)

S h e loo ks like a w itch .(C la im an d D a ta)

T h in gs a re w h at th ey lo o k like .(W a rra n t)

A rg um e nt b y S ign

"I g o t b e tte r!"C la im to cou n te r

la ck o f s ig n o f n e w th oo d.

P a rtia lly R e je c te d :H e is cle a rly *n o t* a n e w t n o w .

"S h e tu rn e d m e in to a n ew t!"(D a ta )

W itch e s ca n tu rn p eo p le in to n ew ts.(W a rra n t)

A rg um e nt b y S ign

S h e 's a W itch .(M a jo r C la im )

(D ata) (D ata)

(W arran t)

(C laim an d W arran t)

(D ata)

(W arran t)

(D ata an d a C laim )

(W arran t)

(R ejec ted W arran t)< B u ild in g B rid ge fro m H er>

(W arran t)

(W arran t)

(U n related D ata)

(C laim an d D ata)

C laim : S h e's a W itch .

W itch es b u rn .(D ata)

W o o d b u rn s .(D ata)

T h in gs m ad e o f w o o d b u rn .(W arran t)

W itch es are m ad e o f w o o d .(C laim an d W arran t)

A .W . w eigh s th e sam e as a d u ck.(D ata)

T h in gs th at w eigh th e sam eare th e sam e.

(W arran t)

S h e is a d u ck.(D ata an d a C laim )

D u ck s fl o at.(W arran t)

T h in gs m ad e f ro m w o o d canb e p u t to th e sam e u ses.

(R ejec ted W arran t)< B u ild in g B rid ge fro m H er>

T h in gs m ad e o f w o o d fl o at.(W arran t)

O th er item s (b read , app les , v ery sm all ro ck s,c id er, grav y , ch erries , m u d , ch u rch es, lead)

fl o at.(U n related D ata)

T h in gs th at w eigh th e sam eare th e sam e.

(W arran t)

A .W . is m ad e o f w o o d .(C laim an d D ata)

M ajo r C laim : S h e is a w itch .