A short discussion of logic and logical fallacies

For practical purposes, the study of logic starts with Aristotle--if only because his writings have largely survived.

Logic has to do with how one makes valid inferences, i.e. if I know that some things are true, what can I know must also be true? A logical system consists of a set of postulates and a set of inference rules that determine what are valid inferences in that system.

Aristotle came up with what we call "syllogisms", schemata of valid inferences, such as "if all X are Y and all Y are Z, then all X are Z". The premises are the if part, and the conclusion is the then part. Under Aristotelian logic, if you know two premises of the given form (where you consistently replace the X, Y, and Z with classes of things), then you can infer the conclusion (again, replacing the variables consistently). For example, "If all men are mammals and all mammals are animals, then all men are animals."

Oh, yes... that "consistency" bit also forbids equivocation, i.e. using a word in two different ways. "Animal" not only means "one of those things that aren't vegetable or mineral," but is sometimes used to mean a beast or savage ("You animal!"); the consistency rule forbids switching meanings in midstream.

That's pretty much where things stood, aside from the work of the scholastics (who did things like invent mnemonics for the valid syllogisms, using vowels to represent the forms of the premises and conclusion), until the 19th century and the development of mathematical or symbolic logic.

The whole point about logical fallacies is that one cannot make valid inferences from them. Those who use them either don't know any better or do know better and are attempting to deceive; in either case, the supposed conclusion is suspect. (Not necessarily false, but suspect, since it doesn't follow from the premises.) You'll have to decide for yourself which of the two (ignorance or dishonesty) applies in any given case.