CPSP118G Fall Semester: Earth, Life & Time Colloquium
Logic and Logical Fallacies: Reasonable and Unreasonable Approaches to Thinking
Thomas R. Holtz, Jr.
From the Greek logos, word, logic can be defined as the
study of principles and rules to arrive at correct reasoning. It works primarily by the reasoning together of and statement of arguments.
NOTE: popular use of the term to the contrary, in logic and
rhetoric an "argument" is NOT simply contradiction, the "automatic gainsaying of any statement the other person makes." Instead,
an argument is a connected series of statements intended to establish a proposition.
In other words, to "argue" is to "state one's case".
There is a vast field of formal logic with its own sets of technical terminology, symbols, etc. What we will present is greatly reduced.
The each individual statement in the "series of statements intended to establish a proposition" is called a premise. An argument might
have a single premise, although many have a series of premises. In a properly constructed argument, the premise(s) should support a conclusion, the
proposition which you were trying to establish.
Logic is used (or at least is used in principle) in nearly every field of human endeavour: science and other academic fields (history, etc.), of course,
but also politics, business, advertising; indeed, anytime that someone is trying to convince other people about some proposition.
However, not all arguments are well-constructed. Logical fallacies can be unintentional (due to, among other things, Kida's 6 basic
mistakes we make in thinking), or they can be quite deliberate. In the latter case, they are used as rhetorical devices to convince others instead
of using correct, sound, valid reasoning.
The following is a sampling of some of the more commonly-encountered logical fallacies. There are many more. Also, some of the ones listed
below go by many different names (in fact, most of these have Latin names as well). Additionally, some of these shade into each other. We've
grouped these into general categories.
Premises are not actually related to the conclusion:
Non sequitur: This is Latin for "It does not follow." We apply the phrase to cases where someone presents two unconnected ideas as
if one were a logical consequence of the other.
A special case of the non sequitur is the red herring, a deliberately flamboyant non sequitur thrown out to confuse one's opponent.
Appeals to Non-Evidence:
Appeals to Ignorance: Not all events or phenomena are well-understood. In cases where we do not have evidence supporting or
rejecting a particular claim, the appropriate logical response is to withhold judgment. However, the Argument from Ignorance uses the
following: "We do not know what is going on in a particular event or phenomenon; therefore, MY claim is the correct one." Often formulated as
"because claim A has not been demonstrated to be false, claim A must be true."
Argument from Personal Incredulity: This argument boils down to "I personally can't imagine how claim A could be true, so
therefore claim A cannot be true" even when clear lines of evidence support claim A. Distinct from the Appeals to Ignorance in that in
this case the actual phenomenon or event is understood by other parties, but the person making the argument claims or chooses not to accept
Appeals to Emotion rather than Reason:
Ad hominem:Ad hominem means "at the person." Ad hominem arguments are those that say a claim should be
rejected (or supported) based on some attribute of the person making an argument rather than on the argument itself.
Argument from Authority: The idea that we should adopt (or reject) an idea because some respected person tells us to. The logical
problem here is that the truth of a claim doesn't change based on whether a respected person supports it or not; the authority may have no
expertise in the particular subject, or they might be mistaken. If the respected person has actual evidence for the claim, that evidence is
what is used to support it, not the person themselves. A short form of this argument: "Ideas by Merit, Not by Source."
A special case is the argument from false authority: this is extremely common in advertising, where a spokesperson is chosen to
advocate a product on the basis of popularity or fictional expertise.
Argument from the People (aka the Bandwagon): "If many believe it so, it must be so." In fact, while examining whether the
relative support for a claim might be useful in some contexts (e.g., finding out how many people support a claim…), it has no bearing on the
validity of that claim. For example, the more people in a community that believe in fire-breathing dragons does not make the fire-breathing
dragons more real (phrased differently, their belief is not evidence for the reality of the dragons.)
Arguments from Adverse Consequences: These are arguments in which one is asked not to accept a position because doing so would
require them to accept unpleasant consequences that stem from it. Of course, in reality the truth of a claim is independent of whether it
produces adverse affects.
Slippery slope: An unreasonable reluctance to accept a sound argument because of the fear of being drawn from it to accept a
similar but less sound argument, and so on until you have been persuaded, after many iterations, to accept a completely fallacious argument.
Straw Man: A weak parody of it (the "straw man") of an opponent's actual argument is proposed, and then dispatched. If the
deception is successful, listeners never notice that the substitution occurred and imagine that the original opposing argument was defeated.
Weasel words: When a forthright statement is more likely to offend than persuade, it is sugar-coated by the coining of new
terminology: e.g., "Ethnic cleansing" goes down easier than "Mass murder."
Observational selection: Noticing only the observations that tend to form the patterns that one wants to see and ignoring those
observations that either don't fit or form undesirable patterns. Often summarized as "Counting the Hits and Ignoring the Misses."
Statistics of small numbers: Attempting to infer a large pattern from too few observations.
Non-comprehension of statistics: Like it says.
Correlation equals Causation: It's easy to be convinced that because two things happen simultaneously, one must cause the other.
This is not something we can assume. The hypothesis of non-correlation has to be tested.
Post hoc ergo propter hoc: This is Latin for "it came after so it was caused by..." This is a special case of the
"correlation = causation" fallacy in which when one event follows another it is claimed to have been caused by it. While having one event
precede another is the first step in showing a causal relationship, it is ONLY the first step; additional evidence is needed in order to
establish an actual relationship between the two.
Special Pleading: Artificial resuscitation for an argument that has already been falsified by some reasonable standard. This
usually assumes the form of an argument that the special circumstances surrounding a particular situation invalidate the falsification. [NOTE:
don’t confuse with the "Straw Man" argument! In the Straw Man, you create a weak parody of your opponent’s argument to make them look bad; in
Special Pleading you change your argument to make you look good.]
The Disregarded Middle or Excluded Middle (aka False Dichotomy): In the real world, there is often a broad spectrum of possible
opinion on many topics. A serious fallacy is to assume there are only two possible positions (often, but not always, presented as the two