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Fallacies of Presumption

Unwarranted Assumptions

The fallacies of presumption also fail to provide adequate reason for believing the truth of their conclusions. In these instances, however, the erroneous reasoning results from an implicit supposition of some further proposition whose truth is uncertain or implausible. Again, we'll consider each of them in turn, seeking always to identify the unwarranted assumption upon which it is based.


The fallacy of accident begins with the statement of some principle that is true as a general rule, but then errs by applying this principle to a specific case that is unusual or atypical in some way.

As we'll soon see, a true universal premise would entail the truth of this conclusion; but then, a universal statement that "Every woman earns less than any man." would obviously be false. The truth of a general rule, on the other hand, leaves plenty of room for exceptional cases, and applying it to any of them is fallacious.

Converse Accident

The fallacy of converse accident begins with a specific case that is unusual or atypical in some way, and then errs by deriving from this case the truth of a general rule.

It should be obvious that a single instance is not enough to establish the truth of such a general principle. Since it's easy for this conclusion to be false even though the premise is true, the argument is unreliable.

False Cause

The fallacy of false cause infers the presence of a causal connectionsimply because events appear to occur in correlation or (in the post hoc, ergo propter hoc variety) temporal succession.

Later we'll consider what sort of evidence adequately supports the conclusion that a causal relationship does exist, but these fallacies clearly are not enough.

Begging the Question (petitio principii)

Begging the question is the fallacy of using the conclusion of an argument as one of the premises offered in its own support. Although this often happens in an implicit or disguised fashion, an explicit version would look like this:

Unlike the other fallacies we've considered, begging the question involves an argument (or chain of arguments) that is formally valid: if its premises (including the first) are true, then the conclusion must be true. The problem is that this valid argument doesn't really provide support for the truth its conclusion; we can't use it unless we have already granted that.

Complex Question

The fallacy of complex question presupposes the truth of its own conclusion by including it implicitly in the statement of the issue to be considered:

In a somewhat more subtle fashion, this involves the same difficulty as the previous fallacy. We would not willingly agree to the first premise unless we already accepted the truth of the conclusion that the argument is supposed to prove.

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Last modified 12 November 2011.
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