We haven’t had a Logical Fallacies Friday in a long time, due mostly to distractions. Here’s a fun one with a twist at the end: Affirming the Consequent. This one runs a little long, but stay with it. You won’t be sorry.
Affirming the Consequent is the logical equivalent of throwing your car’s stick shift into reverse while going forward at 55 mph. (Don’t try this!) A sudden reversal of logic that strips the gears of the mind. Here’s a simple example:
The streets are wet today,
Therefore it must be raining.
Can you hear the gears clash? Do you feel that nagging little pebble of illogic grinding in the cylinders of your brain? Of course, there are a variety of reasons why the street might be wet. One of them could be rain. Others are melting snow, the neighbor’s sprinkler system, a leaky fire hydrant, a burst water main, etc. While the sentence works in one direction, it doesn’t necessarily work in reverse.
Harry lied under oath,
Therefore Harry must be guilty of a crime.
Murders increased last year,
Therefore there must have been more guns.
The Bible is inerrant;
Therefore the Bible is God’s Word.
Pastor X unconditionally subscribed the Book of Concord,
Therefore, Pastor X is a confessional Lutheran.
Affirming the Consequent is the way of circumstantial evidence in court. Circumstantial evidence speaks indirectly to the case. It isn’t necessarily wrong, but it isn’t necessarily right, either.
The defendant had concrete in his garage;
Therefore the defendant killed the victim.
Tax and spend liberals love to increase taxes.
Taxes increased under my opponent’s watch;
Therefore, my opponent is a tax and spend liberal.
Here’s one you hear all the time around the creationist water cooler:
Sam believes in evolution;
Therefore, Sam is an atheist.
From the worship wars. Perhaps you’ve said it yourself, or had someone say it about you.
Pastor Y uses contemporary music;
Therefore Pastor Y is a church growthist.
The twin sister of “Affirming the Consequent” is called “Denying the Antecedent.” It works like: If A then B; not A therefore not B. If the positive is true, the negative must also be true. Right? Well…not exactly.
I don’t eat a lot,
Therefore I won’t get sick.
(Unless someone with the flu coughs in your face.)
The glove doesn’t fit,
therefore you must acquit.
(The trouble is the victims were killed with a knife, not a glove. Small, but significant detail.)
Denying the antecedent is a great way to defend the status quo. It’s a common conservative ploy to say, “We’ll stick with the old, and things will always be good.” Or maybe things will just be old.
We’ll stay with the old hymnal,
Therefore, we’ll never have a problem with heresies.
(Yeah, ask the Episcopal Church about that one.)
Double predestination rests on a fallacy of denying the antecedent. Think about it:
Some are not saved,
Therefore some are not elect to salvation.
(That’ll teach you to play logical games with God!)
So far, so good. One big problem, however. Affirming the Consequent is a formal fallacy only in “deductive” reasoning, which is the safest, but least interesting, way of thinking, going from rule to conclusion by way of definition.
Socrates is a man.
Therefore, Socrates is mortal.
All very true, and quite boring. Mathematicians work this way. That’s why they’re generally as interesting as accountants and statisticians. One thing follows another by definition. Scientists, historians, and detectives think in a different direction, called “retroductive reasoning.” Connecting the dots. Here’s an example:
Observation: Socrates is mortal.
Conclusion: Therefore, Socrates is a man.
Ah hah! you say. That’s “Affirming the Consequent.” Right you are, my deductive friend. Yet without this way of thinking, we’d never be able to come up with reasonable explanations for anything. Here’s a simple example from science:
Substance X is composed of 2 parts hydrogen and 1 part oxygen.
Therefore, X is water.
The fallacy occurs when you make the conclusion an absolute statement. A little healthy skepticism is always in order when reasoning retroductively. Good science knows to be skeptical Socrates might be a man, or he might be a cat, which is also mortal. Substance X might be water or it might be something else with the same ratio of hydrogen to oxygen. The fallacy can be avoided by use of the phrase “might be,” or if you are fairly certain, “probably is.” Socrates might be a man; Substance X probably is water.
Retroductively speaking, you can never be absolutely certain of anything, just “reasonably certain.” It’s the way it works in court: Beyond a reasonable doubt. Nothing that we observe is absolutely certain. A circumstantial evidence case is built on a series of retroductive conclusions. Any one of them may be questionable, but the sum total paints a picture that is difficult to deny beyond a reasonable doubt.
So parents, while it is generally true that kids who stay out late are up to no good, and it is most certainly true that Sally is out late tonight, it is not necessarily true that Sally is up to no good. You’ll have to talk to her when she gets home. You know the drill.