If we rely on experiences as evidence for our inferences and explanations, we must sift through those that offer themselves so as to focus on ones that are relevant to the conclusions that we seek to draw. Inferences that rely on irrelevant “evidence” fail, being guilty of the fallacy of non sequitur.
There’s another kind of logic, usually called informal logic, commonly covered in books or courses about critical thinking. Although it’s less technical and less demanding, it is no less important than the formal matters of logical inference. Informal logic concerns the standards that need to be satisfied in order for us to get formal reasoning underway.
If we rely on experiences (or anything else, for that matter) as evidence for our inferences and explanations, we must sift through those that offer themselves and focus on the ones that are relevant to our enterprise. Evidential relevance is a prerequisite for useful inference drawing. Unless our purported evidence is relevant to the inferences we are trying to draw, we are not even in the ballpark, much less in the game.
Inferences that rely on irrelevant “evidence” commit non sequitur in one form or another. Here are descriptions and examples of seven forms that such bad reasoning can take:
A. Ad vericundium. This fallacy amounts to an appeal to an improper authority (often due to some equivocation over the notion of authority itself). Example: “Don’t question the President. He is the highest authority in the land.”
B. Post hoc ergo propter hoc. This fallacy amounts to the inference that because one thing follows another in time, the later of the two must have been caused by its predecessor. Example: Keeping Mark Twain’s story in mind, any wino with good teeth will serve.
C. Ad populum. This fallacy amounts to inferring that a point of view or opinion must be true on the grounds that it is widely held. Example: “Fifty million Frenchmen can’t be wrong!” “Join the swing to Dodge!”
D. Ad baculum. This fallacy amounts to inferring that a point of view or opinion is true (or false) on the grounds that the one who holds it has (or lacks) the power to impose it on others. Positive Example: “You do exactly what I said, young man, or else!” Negative Example: “And exactly how many tanks does the pope have?”
E. Ad misericordiam. This fallacy amounts to inferring that a point of view or opinion must be true on the grounds that those who hold it deserve (or are, at least, natural targets for) our sympathy. Example (a defense lawyer at the sentencing hearing after a conviction for matricide): “Please be lenient with my client. He is, after all, a motherless child.”
F. Ad hominem. This fallacy amounts to inferring that a point of view or opinion must be true (or false) because of the character and/or the position of those who hold it. Positive Example: Teresa must have been right about her visions coming directly from God. She was a good and virtuous person. Negative Example: Bill Clinton’s improper liaisons prove the illegitimacy of his political policies.
G. Accident and converse accident (hasty generalization). These fallacies amount to inferring that a member of a group has certain characteristics on the grounds that they are common to the members of the group or that all the members of a group must have certain characteristics on the grounds that some of its members do.
Example: Any case of stereotyping will do for the accident fallacy: “White men can’t jump.” Any case of jumping to conclusions will do for the converse accident fallacy. Where, after all, do the stereotypes come from?
We should not be misled by the fact that such fallacies are common, by the fact that some of them “sound OK” to careless ears, or by the fact that contrived examples of them can be amusing. They are always dangerous. They never settle an issue.