If you weigh the same as a duck, then, logically, you’re made of wood and must be a witch. Or so goes the reasoning of Monty Python’s Sir Bedevere. Obviously something has gone wrong with the knight’s reasoning – and by the end of this lesson, you’ll know exactly what that is. This lesson will focus on 10 fallacies that represent the most common types of mistakes in reasoning.
Background
Arguments can be bad for one of several reasons. They might fail because one of the premises is false. For example:
If Burger King sells Big Macs, then McDonald's will go out of business.
Burger King does sell Big Macs.
So McDonald's will go out of business.
In this argument, the first two lines are premises and the third line is the conclusion. The argument is formally valid (that is, if the premises were true, then the conclusion would have to follow). It’s also unsound, since Burger King is not in the business of selling Big Macs, and thus McDonald’s franchise is safe. (Or at least it won’t fail for this reason!)
Arguments also fail when the conclusion does not properly follow from the premises.
Baptists are often politically conservative. (premise)
Republicans are often politically conservative. (premise)
Therefore, Baptists are often Republicans. (conclusion)
This sort of argument can be extremely seductive, but logically it does not work. Consider another argument that has exactly the same form:
John Elway is a great quarterback. (premise)
Michael Vick is a great quarterback. (premise)
Therefore, John Elway is Michael Vick. (conclusion)
Both arguments are invalid, for even if their premises are true, their conclusions can still be false. Many logical fallacies are of just this sort: They offer reasons that fail to support their conclusions.
Finally, some arguments are bad not because they make false claims or because they commit some logical error, but rather because they are booby traps for unsuspecting readers.
Dr. Roy Spencer, who is a prominent climate scientist at the University of Alabama at Huntsville and winner of NASA’s Medal for Exceptional Scientific Achievement, doesn’t think that humans are causing global warming (premise). So humans are probably not causing global warming (conclusion).
Formally speaking, there is nothing fallacious about this argument. It appeals to authority, but Dr. Spencer is fairly clearly an appropriate authority on the matter of global warming. So as far as it goes, this is a good argument. The problem, however, is that the argument leaves out an important bit of information, namely that the overwhelming consensus in the scientific community is that global warming is being caused by humans. But by suppressing important evidence, the argument is potentially a booby trap for unwary readers.