Fallacies of Presumption

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.

Accident

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.

 

  • Women earn less than men earn for doing the same work.
  • Oprah Winfrey is a woman.

 

  • Therefore, Oprah Winfrey earns less than male talk-show hosts.

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.

 

  • Dennis Rodman wears earrings and is an excellent rebounder.

 

  • Therefore, people who wear earrings are excellent rebounders.

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.

 

  • The moon was full on Thursday evening.
  • On Friday morning I overslept.

 

  • Therefore, the full moon caused me to oversleep.

Later we’ll consider what sort of evidence adequately supports the conclusion that a causal relationshipdoes 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:

 

  • All dogs are mammals.
  • All mammals have hair.
  • Since animals with hair bear live young, dogs bear live young.
  • But all animals that bear live young are mammals.

 

  • Therefore, all dogs are mammals.

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:

 

  • Have you tried to stop watching too much television?
  • If so, then you admit that you do watch too much television.
  • If not, then you must still be watching too much television.

 

  • Therefore, you watch too much television.

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.