Introduction to Informal Fallacies

Fallacy: a defect in an argument that consists in something more than merely false premises. Both deductive and inductive arguments may contain fallacies, and, if they do, they are either unsound or uncogent, depending on the kind of argument. Also, if an argument is either unsound or uncogent, then it has one or more false premises or contains a fallacy or both.

Two kinds of fallacies: Formal fallacies and informal fallacies.

Formal fallacies:

Consider this argument:

All bullfights are grotesque rituals.

All executions are grotesque rituals.

Therefore, all bullfights are executions.

which has the form:

All A are B

All C are B

All A are C

This argument form is, of course, a "categorical syllogism." This particular one is invalid. Notice that reversing the terms in the second line would make it valid, though it would make the original argument unsound.

 Consider this next form, a hypothetical syllogism:

If apes are intelligent, then apes can solve puzzles.

Apes can solve puzzles.

Therefore, apes are intelligent.

which has the form:

If A then B.

B._______

A.

These are examples of formal fallacies. They are valid or invalid depending on the form of the argument. Note that inductive arguments CANNOT contain formal fallacies, only informal. Deductive arguments may contain both.

Informal fallacies, on the other hand cannot be detected by form, but require an analysis of content, regardless of whether the argument is inductive or deductive. Consider:

All factories are plants.

All plants have chlorophyll.

Therefore, all factories have chlorophyll.

On first glance, the form would appear to be:

All A are B.

All B are C.

All A are C.

which is valid. But in actuality, given the fact that "plant" is used as two different words, the form is:

All A are B.

All C are D.

All A are D.

which is clearly invalid. Notice, then, the argument contains both a formal and an informal fallacy.