Petitio principii

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Begging the question, which goes by the technical name petitio principii, is a logical fallacy that occurs when an argument implicitly assumes its conclusion in a premise.


Consider the following argument:

  • I know God exists because of all the good things He has done for me in my life.

Arranging this argument in the usual premise–conclusion sequence reveals the problem with it:

  1. God has done good things for me in my life.
  2. Therefore God exists.

The premise assumes that God exists in the first place, otherwise he couldn't "do" anything.

A more subtle example, perhaps, is the following:

  • I believe in the paranormal because I've actually seen a ghost.

The problem here is the assumption that what one saw was indeed "paranormal". Under most people's definition, ghosts are examples of paranormal phenomena. However, one must accept that paranormal phenomena are possible to accept the premise that a "ghost" was actually seen. Since everything we can see is made of ordinary matter — i.e., emits or reflects photons according to well understood laws of physics — the underlying assumption that the "ghost" was actually outside of the ordinary laws of physics — i.e., paranormal — cannot be accepted without question. Again, reformatting the argument (and rewording the parts slightly) clarifies the problem:

  1. I have seen a paranormal phenomenon (a ghost).
  2. Therefore I believe in paranormal phenomena.

Note that the structure of this argument is entirely valid. We can use it for any number of real phenomena (e.g., "I have seen the moon, so I believe in the moon"). Since examples of begging the question can occur in logically valid arguments, petitio principii is considered an informal fallacy. The above example is also based on an enthymeme (a missing premise), namely that the "ghost" was a paranormal phenomenon.

Different meanings

Sometimes the phrase "begging the question" refers to using a premise that requires at least as much justification as the intended conclusion — that is, that the use of the premise to justify the conclusion is at least as problematic as simply asserting the conclusion without any premise at all. For example:

  • The teachings of Jesus should be followed because he was the son of God.

In this case the premise, that Jesus was the son of God, requires at least as much justification as the claim that the teachings of Jesus should be followed. That is, the argument moves backwards, in a sense, to the more fundamental question, "How do you know Jesus was the son of God?"

People often refer to "begging the question" when they really simply mean "raising another question". For example:

"You say that theism makes more sense than atheism, but that just begs the question of which religion/philosophy I should choose to believe."

This is not an appropriate use of the term, since the question being raised is not implicit in the preceding statement.

v · d Logical fallacies
v · d Formal fallacies
Propositional logic   Affirming a disjunct · Affirming the consequent · Argument from fallacy · False dilemma · Denying the antecedent
Quantificational logic   Existential fallacy · Illicit conversion · Proof by example · Quantifier shift
Syllogistic   Affirmative conclusion from a negative premise · Exclusive premises · Necessity · Four-term Fallacy · Illicit major · Illicit minor · Undistributed middle
v · d Faulty generalisations
General   Begging the question · Gambler's fallacy · Slippery slope · Equivocation · argumentum verbosium
Distribution fallacies   Fallacy of composition · Fallacy of division
Data mining   Cherry picking · Accident fallacy · Spotlight fallacy · Hasty generalization · Special pleading
Causation fallacies   Post hoc ergo propter hoc · Retrospective determinism · Suppressed correlative · Wrong direction
Ontological fallacies   Fallacy of reification · Pathetic fallacy · Loki's Wager
v · d False relevance
Appeals   Appeal to authority · Appeal to consequences · Appeal to emotion · Appeal to motive · Appeal to novelty · Appeal to tradition · Appeal to pity · Appeal to popularity · Appeal to poverty · Appeal to spite · Appeal to wealth · Sentimental fallacy · Argumentum ad baculum
Ad hominem   Ad hominem abusive · Reductio ad Hitlerum · Judgmental language · Straw man · Tu quoque · Poisoning the well
Genetic Fallacies   Genetic fallacy · Association fallacy · Appeal to tradition · Texas sharpshooter fallacy
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