Logical Fallacies and Flawed Reasoning

Introduction (last updated Aug 27, 2010)

We call any argument which fails to hold true if it fails to hold true due to some logical flaw. For instance, the simple argument

Some philosophers smoke cigarrettes.
Some smokers die from lung cancer.
Socrates died from lung cancer.

Fails logically, because two existentially quantified statements do not sufficiently imply any other statement. This particular argument also fails truth-functionally, since Socrates died from hemlock poisoning and not from lung cancer.

In order to understand what fallacies are and why certain argument forms are always fallacious, you need to know basic formal logic (forthcoming). In the meantime, if a particular argument resembles the logical form or structure of an argument on the following list, then that particular argument cannot hold true.

(Note: a fallacy may happen to contain or to yield a true statement, but if that’s the case, then the true statement stands true due to conditions in the world and not from the argument itself.)

List of Common Fallacies

These fallacies appear VERY frequently in everyday conversation, in the media, in workplaces and classrooms, and in many other environments where people exchange information and opinions.

Ad Hominem: the speaker either promotes or refutes a statement based on a judgment concerning the original speaker. Example: “Hitler once said the Alps divide northern and southern Europe; Hitler was a madman, therefore, the Alps cannot divide northern and southern Europe.” You do not need to know any geography to find the speaker’s argument fallacious. Another example: “Einstein once said a particle cannot effect another from a vast distance; Einstein was a genius, therefore, a particle cannot effect another from a vast distance.” Similarly, you don’t need to know anything about physics to reject this second speaker’s argument.

Bandwagon: the speaker attributes the truth of a statement to the readiness with which others accept its truthfulness. Example: “Nine out of ten dentists agree Colgate cleans teeth better than any other brand.” Nine out of ten dentists may be incorrect, or the polling may reflect some prior bias, or the study it relies upon may incorporate undisclosed flaws, or… the fact that a majority accepts a statement as truth does not attest to the actual truth of that statement. Thus, we must reject the argument in favor of a better demonstration of the facts.

Begging the Question: the speaker includes his conclusion among his initial premises. Example: “If it’s illegal then it’s not allowed.” Here the flaw can appear very subtle, but you’ll generally notice the way the information loops back to some initial assumption. We call this fallacy begging the question because the speaker solicits the listener to restate the basic assumption.

Cause and Effect Confusion: the speaker takes the correlation between two events to indicate a cause-effect relationship. Example: “The lights in the sky appeared at the same time the power cut out, therefore, the lights caused the outage.” Fallicies like this are very compelling, however, a simple counter-argument shows how weak the proposed cause-effect relation can be. For instance, the example may be refuted by showing the lights to result whichever event caused the power outage — thus making both the lights and the outage effects, neither causes.

(more to come!)

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