Intro to Logical Fallacies

[Masterhawk]

The point of logic is to reach the truth. Fallacies are tactics and otherwise arguments which do not help reach the truth.

Fallacies can be split into two categories:

Formal fallacies: Arguments which are not logically sound (i.e. non sequiturs)

Informal fallacies: Arguments which are sound at face value but are wrong.

To really understand the difference between the two, we need to understand what makes an argument sound. A sound argument is basically one which logically follows for example:

Asia is the largest continent and also the most densely populated one. Therefore, it's also the most populous one.

Nunavut is the least populous subregion in Canada and also the largest. Therefore, it's also the least densely populated one.

John murdered Mary. People who commit murder get the death penalty. Therefore, John will get the death penalty.


Now let's break one of the arguments down:

Premise 1: Asia is the largest continent

Premise 2: Asia is the most densely populated continent

Conclusion: Asia is the most populous continent

This is logically sound because the conclusion is strictly contained within the premises. If the entity with the most resources is putting n the most effort that entity will come in the first place (assuming that resources + effort = victory).


Now let's look at some deviations:

Premise 1: California is the largest state

Premise 2: California is the most densely populated state west of the Mississippi River.

Conclusion: California has the largest population

For those of you who passed american demographics 101, you know that this conclusion is true. However, this argument is not logically sound. In fact, premise 1 isn't even true and premise 2 doesn't even rule out any of the states east of the Mississippi River.


Premise: Bush did 9/11

Conclusion: Bush is one of the biggest criminals in American history and must be brought to justice.

This is the opposite example. Even though the argument is false, it's still logically sound. If Bush did indeed do 9.11, that would make him a pretty despicable president who sacrificed Americans just to go to war. The conclusion can be contained within the premise. It's just problematic because it's based on misinformation.


Premise: A man is an entity with two legs and no feathers

Conclusion: A featherless chicken is a man

This was actually Plato's definition of a man. Then Diogenesis's followers found a loophole and pulled the feathers off a chicken. As is the case with the example above, the argument is false but sound nonetheless.


These above arguments demonstrate the difference between a correct argument and a sound one. A sound argument is one that assuming that each of the premises are true, the conclusion must be true as well.


Now let's look at arguments which are neither sound nor correct:

Premise: Bananas fit around our hands and are easy to eat

Conclusion: This is proof that God made the world

This argument is unsound for a couple of reasons. Even ignoring domestic cultivation, man's ancestors were primates who ate, you guessed it, bananas. Furthermore, there are plenty of fruits which don't bear the traits of bananas. You have coconuts which are difficult to open, cranberries which have many seeds in them, and even fruits which are poisonous. If we're not restricting ourselves to fruit, plenty of mushrooms are poisonous as well. If the banana is proof that the world was intelligently designed, are death cap mushrooms proof that those designers were sadists?


Premise: Jet fuel cannot melt steel beams

Conclusion: The twin towers were not brought down by planes. Perhaps by internal explosives installed secretly.

This argument appears to be sound but upon close inspection, the premise is not all-encompassing. Jet fuel may not be able to melt steel beams but it can make them more flexible. What this does is that it makes the ability of the beams in question to be less able to hold everything above it. Eventually, the beams weakened to the point where the towers collapsed.

Now here are some BSC examples:

Premise1: Wookies live on Kashyyk

Premise 2: Chewbacca lives on Endor which does not make sense

Conclusion: You must acquit


Premise: Charlie hates God and wonders if he even exists

Conclusion: You can't hate something that doesn't exist making this proof that God exists


Premise: There are an infinite number of worlds, meaning that God exists in one of them

Conclusion: If God exists in one of them, he exists in all of them


Premise: Nature is so beautiful

Conclusion: Goddidit


The above arguments are unsound and completely wrong. Formal fallacies are based on unsound reasoning while informal fallacies have sound reasoning with faulty premises.

 

[HIP56948]

I believe bananas are a fairly recent thing. (700-6000 years ??) Not that you're entirely wrong with the other things.

 

[Loulit01]

Where's validity? An argument can be valid but not sound. Does the term "correct argument" exist in the field of logic? Arguments can be valid or not valid, sound or not sound. Sure, they can be "correct," but that term isn't used in logic. A sound argument must be valid but valid arguments are not necessarily sound. What does "correct" have to do with anything?

And syllogisms consist of two premises and a conclusion.

Where are you getting your information?


You lost me.

 

[Loulit01]

Examples:

Some stars are white.
The sun is a star.
____________________
The sun is white.

This is an invalid syllogism and therefore cannot be sound.

All humans are mortal.
A chimpanzee is a human.
____________________________
A chimpanzee is mortal.

This is a valid syllogism that is not sound because it has a false premise.

All humans are mortal.
Michael Jordan is a human.
____________________________
Michael Jordan is mortal.

This syllogism is sound and therefore must be valid.

 

[Gaius46]

Premise: A man is an entity with two legs and no feathers

Conclusion: A featherless chicken is a man

This was actually Plato's definition of a man. Then Diogenesis's followers found a loophole and pulled the feathers off a chicken. As is the case with the example above, the argument is false but sound nonetheless.

Question. How can the above even be a sound argument when the premise isn’t exclusive to men?

 

[Loulit01]

Question. How can the above even be a sound argument when the premise isn’t exclusive to men?

It is in no way a sound argument.

A sound argument must have true premises. The first premise, as you point out, isn't true. The conclusion isn't a conclusion because conclusions aren't drawn from a single premise. He's completely lost me.

 

[soylentgreen]

An argument is valid if the premises and conclusion are related to each other in the right way so that if the premises were true, then the conclusion would have to be true as well.

All toasters are items made of gold.
All items made of gold are time-travel devices.
Therefore, all toasters are time-travel devices.

is a valid argument.

 

[Loulit01]

An argument is valid if the premises and conclusion are related to each other in the right way so that if the premises were true, then the conclusion would have to be true as well.

All toasters are items made of gold.
All items made of gold are time-travel devices.
Therefore, all toasters are time-travel devices.

is a valid argument.

But not a sound argument.

 

[soylentgreen]

But not a sound argument.

I would agree.

However I disagree that the point of logic is to reach the truth. At best it is a useful tool for pointing out what is not true.

 

[BabyAugustine]

Question. How can the above even be a sound argument when the premise isn’t exclusive to men?

Yes, but you are arguing about essence. Unless a man has the same essence as all men then you are a Nominalist and even if I say 'man' and you say 'man' we can differ on what is a man.