Informal Fallacy

In Logic, ‘fallacy’ is defined as an error in the structure or content of an argument which prevents a conclusion from being rationally drawn from the premises.

Fallacies divide into two distinct types:

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 questions:

  1. Sufficiency - is there enough evidence to make the conclusion more likely than not?
  2. Relevance - is the evidence relevant to the conclusion?
  3. 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.

Other fallacies of relevance occur when we introduce evidence that shifts focus away from the thesis being considered:

Fallacies of Sufficiency -

Another common mistake in induction is to fail to provide enough evidence to warrant the conclusion of the argument:

Fallacies of Clarity/Ambiguity -

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?