Logic: Misuse of Evidence

A. Evidence Missing

Vampires live in Transylvania.
There is life on other planets.

(Do we know for sure or do we only want to think so?)

B. Evidence Irrelevant

Tough guy Beer is the best beer made, because Mickey Spillane endorses it.

(Mickey may know his beer, but does his endorsement make this beer good?)

C. False Facts

Ninety-nine point nine percent of all beer drinkers we surveyed said Toughguy Beer is terrific.

(Not much help if you only surveyed five beer drinkers.)

D. Begging the Question

John: Why should I believe in the existence of God?
Henry: The Bible tells you so.
John: But why should I believe the Bible?
Henry: It's divinely inspired, so it couldn't be wrong.

As Professor de Vries explains this example, "Henry has begged the question, because his answer to John presupposed God's existence; John could not rationally accept the divine inspiration of the Bible while still questioning God's existence."

E. False Analogy

Treat kids like pets, with strict rules and feedings, and they'll be good.

(Children may need rules and feedings, but they aren't pets, so the analogy creates a false formula.)

F. Post Hoc, Ergo Prompter Hoc (If Y follows X, X caused Y)

Her husband hung a sock from the bedpost, and that's why she gave birth to a boy.

(Superstitious beliefs often induce this fallacious form of reasoning.)

Clyde joined a fraternity last summer; he'll probably flunk out.

(Unless the writer establishes that Clyde's joining a frat leads to bad academic performance and that a semester's bad performance automattically leads to flunking out, then this statement can't be true.)

G. "Mad Leapies": Non Sequiturs

He sleeps late on weekends; he's probably in love.
Nixon's five o'clock shadow cost him the 1960 election.
I put in three days writing this paper, so it's bound to get an A.

Non sequitur means it does not follow. Unless the writer explains the "causes" in these sentences, the "results" will seem illogical. More pertinent evidence will help: why is sleeping late necessarily a symptom of being in love? What effect did Nixon's beard have on his campaign; how important were his TV debates with Kennedy in portraying him as a candidate? And what did the writer do with those three days to ensure that her paper would get an A?

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