Tuesday, September 25, 2012

Goldfinger Takes Fort Knox! (Propositional Fallacies, Part 2)

Bond villain Auric Goldfinger
In propositional calculus, we can describe certain arguments in mathematical terms. Some arguments are true if the component statements are true. The statement “It is raining here now, and it is raining where you are now as well” can be written as P⋀Q. It is true if both its component statements are true. On the other hand, “It is raining here now OR it is raining where you are now” (written as P⋁Q) is true as long as at least one of the statements is true.

Propositional fallacies involve fallacies of mathematical reasoning. They are fallacious regardless of the truth value of the component statements. Last time, we discussed affirming a disjunct, the fallacy of turning an inclusive OR into an exclusive one. The two remaining propositional fallacies are known as affirming the consequent and denying the antecedent.

Affirming the Consequent

If Auric Goldfinger owned Fort Knox, then he would be rich. Auric Goldfinger is rich. Therefore, Auric Goldfinger owns Fort Knox. Even if the first two statements are true, the conclusion is invalid because there are other ways to be rich besides owning Fort Knox.

Here's how to cast the argument in propositional calculus:

∴ P 

(If P, then Q. Q is true. Therefore, P.)

This is different from the argument "if and only if." If Auric Goldfinger is rich if and only if he owns Fort Knox, then the statement "Auric Goldfinger is rich" makes "Auric Goldfinger owns Fort Knox" necessarily true. But that's the case only if the first statement is true — which it isn't. In propositional calculus, we'd write that:

∴ P

Affirming the consequent is sometimes called converse error.

Denying the Antecedent

The opposite fallacy, denying the antecedent, is also known as inverse error.

If Auric Goldfinger owned Fort Knox, then he would be rich. Auric Goldfinger does not own Fort Knox. Therefore, Auric Goldfinger is not rich. This is wrong for the same reason as the previous argument was wrong: there are other ways to be rich.

In propositional calculus, this takes the form:

∴ ¬Q

If P, then Q. P is false (not-P). Therefore, Q is false (not-Q). As in the previous case, the rules for if and only if are different from if alone.

Monday, September 17, 2012

The Seven Deadly Sins — and Where To Find Them

Researchers at Kansas State University decided to create a series of county-by-county maps of the United States showing the relative distribution of the Seven Deadly Sins (Envy, Greed, Wrath, Sloth, Gluttony, Lust, and Pride). For each sin, they identified a measurable criterion that could serve as a stand-in, and mapped the results showing the deviation from the norm expressed in terms of the standard deviation (σ). Measures from -1.65σ to + 1.65σ are normal; lower levels shade toward the blue and higher levels toward the red.

It's very easy to critique the criteria used for each sin, or to suggest alternative metrics, but I thought it was quite interesting nonetheless. You can learn more about the project and the researchers here, starting on page 8 of the PDF.


Metric: Total thefts (robbery, burglary, larceny, grand theft auto) per capita.

Maps of the Seven Deadly Sins


Metric: Number of fast food restaurants per capita.


Metric: Average income compared with the number of people living below the poverty line.


Metric: Number of STD cases reported per capita.


Metric: Expenditures on art, entertainment, and recreation compared with employment.


Metric: Number of violent crimes (murder, assault, rape) per capita.


Metric: Aggregate of the other six offenses — because pride, as they say, is the root of all sin.

Tuesday, September 11, 2012

Propositional Fallacies, Part 1

There’s a branch of math known as propositional calculus that treats arguments like mathematical propositions. Using propositional calculus, you can demonstrate the truth or falsity of certain arguments.

Take the statement “It is raining here now.” Depending on when you make the statement, it can be either true or false. In propositional calculus, you’d represent the statement as “P,” and the opposite, “It is not raining here now” as “¬P.” If P is true, then ¬P has to be false; if ¬P is true, then P has to be false.

You can link together statements with connectors. Common connectors are AND, OR NOT, ONLY IF, and IF AND ONLY IF. If we say “It is raining here now, and it is raining where you are now as well,” we can label the second statement as Q. Represent AND with the symbol ⋀, and we can write “It is raining here now, and it is raining where you are now as well” as P⋀Q.

Of course, maybe it is raining here or it isn’t; maybe it’s raining at your house and maybe it isn’t. Because the individual statements can be true or false, we can prepare a truth table.

P                    Q                    P⋀Q
True         True            True
True         False              False
False             True            False
False             False           False

With and as a connector, the proposition P⋀Q is only true if both statements are true.

The connector OR (represented as “⋁”), on the other hand, makes the proposition true as long as at least one of the statements are true. “It is raining here now OR it is raining where you are now” results in the following truth table.

P                    Q                    P⋁Q
True         True           True
True         False          True
False          True              True
False           False             False

Notice that OR is used here inclusively rather than exclusively. That is, P doesn’t exclude Q from being true. If it’s raining at my house, that doesn’t mean it’s not raining at yours.

Given the idea of propositional logic, it's easy to conclude that there are fallacies to go with it. The first of these is known as affirming a disjunct.

Affirming a Disjunct

Also known as the fallacy of the alternative disjunct, or the false exclusionary disjunct, this particular fallacy occurs when you change an inclusive OR into an exclusive one. “It is raining here now or it is raining where you are now” gets interpreted as “If it is raining here now, then it isn’t raining where you are now.”

In our symbolic structure, that gets represented as the following argument (with “therefore” represented by ∴).


That’s a fallacy because it could be raining both places. One doesn’t preclude the other.

While OR in logic always means an inclusive “or,” that doesn’t mean you don’t sometimes want to be more concrete. The logical operator XOR is an exclusive or. When you use it, you’re saying “one or the other, but not both.” The symbol for that is ⊻.

More next week.

Tuesday, September 4, 2012

Who Was That Masked Man? (Formal Fallacies Part 3)

Formal fallacies are arguments that are always wrong, regardless whether the argument's premises (statements claimed as fact) are true or false. For example, in the appeal to probability, someone makes a claim that because something could happen, therefore it will happen. That’s false even if it's true that the something in question could indeed happen.

Masked Man Fallacy

I know who Bruce Wayne is.

I do not know who Batman is.

Therefore, Bruce Wayne is not Batman.

In the masked man fallacy, a substitution of identical designators in a true statement can lead to a false one. The statement "I do not know who Batman is" gets treated as if it excludes Bruce Wayne simply because I do know who he is. Of course, as long as I don’t know that Bruce is actually Batman, both statements can be absolutely true, and yet the conclusion does not follow logically.

The general form of the argument is:
X is known.
Y is unknown.
Therefore, X is not Y.
A similar argument, however, is valid.

Clark Kent is Superman (X is Z).

Batman is not Superman (Y is not Z).

Therefore, Clark Kent is not Batman (therefore, X is not Y).

That’s because being something is different from knowing something. Lack of proof of one proposition doesn’t serve as proof of the counter proposition.