Ad hominem

The phrase "ad hominem" means, "to the man." In logic it refers to an informal fallacy where the person giving the argument is attacked instead of the argument itself. It is designed to deflect attention away from the weakness of the attacker's position or the strength of the defender's. This is common in politics, and is not always without merit.

If an environmentalist candidate flies exclusively in private jets, it is right to point out this hypocrisy, but hypocrisy doesn't affect the truth or falsity of his positions on global warming or other issues. Further, too often in politics and religion ad hominem attacks are used in lieu of valid reasoning skills.

For example....how can you trust a draft-dodger's views on foreign policy? .... How can you believe in that religion with so many bad people leading it? ... That politician sleeps around and he wants to tell me what to do with my body? ... that man opposes gay marriage just because he's a homophobe...that person supports that bill because she's racist...

And so on. All of the above statements attack the person and not the idea. They may help us feel better but not think better.

posted from Bloggeroid

Denying the Antecedent

A logical condition has two parts: the antecedent and the consequent.  The antecedent (with the prefix ante-, meaning coming before, like anteroom or antebellum), indicates the first part of the condition.  If I say, "If P then Q" or "If it is raining, then the ground is wet" the antecedent is "P" or "it is raining."  The consequent is the other part of the condition, the Q, "the ground is wet."

There are two common fallacies: affirming the consequent and denying the antecedent.  If I affirm the consequent, I am saying that the ground is wet, so therefore it's raining, ignoring the possibility of the ground being wet for other reasons, such as a busted fire hydrant.  If I deny the antecedent, I say that since it is not raining, the ground cannot be wet, also ignoring the possibility of the ground being wet for other reasons.  Just because it is not raining does not mean the ground is dry.

Modus ponens & Affirming the Consequent

One of the most basic rules of validity is known as modus ponens, "the affirming way."  In short, it says that if I have a conditional statement and the first part of the statement is true, the second part must also be true.  This is often written as "If P, then Q.  P is true, so Q must be true."

Here are a few examples:

If it is raining, the ground is wet.  It is raining, so the ground is wet.

If you are human, you are also a mammal.  You are a human, therefore you're a mammal.

If you get 3 strikes, you're out.  You have three strikes, so you're out.

Each example follows the same logical structure.  If (something happens), then (something else will happen).  Since (the first thing happened), I know that (the second thing happens).  I know that the ground is wet because it is raining.  I know that I am a mammal because I am human.  I know that I'm out because I got 3 strikes.

Keep in mind that this does not work in reverse. I do not know that it is is raining just because the ground is wet (someone could be washing their car).  I do not know that I am human because I'm a mammal (I could be a horse).  I do not know that I got 3 strikes because I'm out (I could have hit a pop fly).  If I were to make an argument this way, I would commit the formal fallacy known as affirming the consequent.

The consequent is the Q part of "If P, then Q."  This fallacy effectively says, "If P, then Q.  I know that Q is true, so P must be true."  As shown in the examples above, this isn't true.  There could be other reasons Q is true without P being true.

Equivocation

When we construct an argument, our terms must be unambiguous.  Their meanings must be clear and known to all involved.  Politicians and sophists of various stripes love to exploit ambiguity.  This is a fallacy known as equivocation.

Consider the following bit of dialogue:

Are you aware that Jim is gay?
Really?  He doesn't seem that happy to me.

The ambiguous term gay here is exploited by the meanings homosexual and happy, carefree.

Another example in the form of a syllogism:

God is love.
Love is blind.
Therefore God is blind.

This argument is valid, but is not sound because the terms love and blind are ambiguous.  Their meaning changes throughout the syllogism, and the multiple meanings of the words are exploited.  The first love refers to love of a self-sacrificing the nature, the second to erotic love or sacrificial love, depending on whom you're talking to.  The first blind is metaphorical, whereas the second is literal.  Arguments like this are sound and fury signifying nothing.

Non-Contradiction

Given that everything that is, is, something cannot both be true and false at the same time under the same circumstances.  Either A or not A.  Either I am married or I am unmarried.  Either I am alive, or dead.

This idea forms the second and third laws of thought: noncontradiction and the excluded middle.

Identity says that everything is itself; whatever is, is.  Noncontradiction says that it is itself and not something else; whatever is, cannot not be.  The law of the excluded middle says that no third way is possible; everything either is or is not.

The rules include things that are mutually exclusive of one another, but not do not include either/or statements that admit other possibilities.

For example, I have a glass of something to drink.  It is either water or something else.  Water or not water.  I can say that about any liquid: this liquid, or not this liquid.  But under the LNC, I cannot say "it is water or milk."  I may be able to make that assertion if someone poured a drink for me and the only drinkable liquids in my house are water and milk.  In that case, I have eliminated all other possibilities.   However, it does not follow from the LNC that my drink is either water or milk.  Water and milk do not logically contradict.

To attempt to say logically that my drink is either milk or water is to commit the fallacy of the excluded middle, i.e. to say that there is no third way when there actually is.