In Logic, ‘fallacy’ is defined as an error in the structure or
content of an argument which prevents a
being rationally drawn from the premises.
Fallacies divide into two distinct types:
- Formal - a structural error in a deductive argument
- Informal - a substantive error in an inductive argument
The success of induction is dependent on the evidence supporting the conclusion, not the from or
structure of the argument. When thinking about the evidence supporting the conclusion we must think about three distinct
- Sufficiency - is there enough evidence to make the conclusion more likely than not?
- Relevance - is the evidence relevant to the conclusion?
- Clarity - is the evidence unambiguous?
Unlike formal fallacies, Informal fallacies are difficult to spot because there are so many ways to have insufficient,
irrelevant, or ambiguous evidence which fails to warrant the conclusion; and judging this this can take on a degree
of subjectivity that never enters into the evaluation of deductive arguments. Also, because there are an almost infinite variety
of inductive arguments, and because the conclusions of these arguments only have to be probably (never certain), there is
an almost infinite variety of errors that could occur.
However, we can broadly categorize informal fallacies around the type of error committed: there are common
mistakes surrounding the relevance of the evidence given, there are common errors surrounding
the sufficiency of the evidence, and, there are common errors surrounding the clarity of the
evidence. Therefore, we organize Informal Fallacies into these three categories.
Fallacies of Relevance -
The point of giving an argument is to persuade using reason alone. One of the most common errors that infect inductive
arguments is to use emotional appeals as evidence. But emotional evidence is dangerous for two reasons: it is unstable
and it is subjective. The truth (i.e., what the world is really like) is not determined by how we feel about it. Injustice
may make me feel angry, but that is not sufficient evidence for me to dismiss its existence in order to feel better.
There are several common fallacies of relevance making various forms of emotional appeal:
In each of these cases we are using emotions which are irrelevant to the thesis being considered in the argument.
- Argumentum ad Baculum (Appeal to the Stick) - appealing to fear
- Argumentum ad Misericordiam (Appeal to Pity) - appealing to pity
- Argumentum ad Populum (Appeal to the People) - appealing to popularity
Other fallacies of relevance occur when we introduce evidence that shifts focus away from the thesis being considered:
- Argumentum ad Hominem (Argument against the Person) - shifting focus from the thesis to a person's character
- Red Herring - introducing irrelevant information to draw attention away from the thesis
- Ignoratio Elenchi (Missing the Point) - drawing the wrong conclusion from the evidence
- Straw Man - oversimplifying an opponent's argument to defeat it
- Accident - misapplying a general rule to a specific case
Fallacies of Sufficiency -
Another common mistake in induction is to fail to provide enough evidence to warrant the conclusion of the argument:
- Hasty Generalization - drawing a conclusion form an insufficient sample
- Post Hoc Ergo Propter Hoc (False Cause) - confusing correlation with causation
- Slippery Slope - predicting negative consequences with insufficient evidence
- Weak Analogy - drawing conclusions from cases that are insufficiently parallel
- Argumentum ad Verecundiam (Appeal to Authority, unqualified) - using the wrong kind of authority as a witness
- Argumentum ad Ignorantiam (Appeal to Ignorance) - asserting something based on the lack of evidence against it
- Petitio Principii (Begging the Question) - assuming as a premise something that itself needs to be established first
- Complex Question - posing two questions in one, or hiding a question within a question
- False Dichotomy - to assert an exclusive disjunction
when more alternatives are possible
- Suppressed Evidence - deliberately leaving out evidence that would weaken one's conclusion
Fallacies of Clarity/Ambiguity -
- Equivocation - applying different definitions to a single term in an argument
- Amphiboly - drawing a conclusion from a grammatical ambiguity
- Composition - asserting what's true of the parts must be true of the whole
- Division - asserting what's true of the whole must be true of the parts
- Natural Fallacy - confusing what is natural with that which is good
It is important to emphasize again that this is by no means an exhaustive list, but rather some common errors
associated with inductive reasoning. As a general rule you should ask yourself three question when determining the strength of
an inductive argument: 1) Is there enough evidence to make the conclusion more likely than not? 2) Is the evidence relevant to
the conclusion of the argument? 3) Is there ambiguity that would weaken the conclusion?