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Petitio principii

(Lat.: “presuppose what is yet to be proven”) refers to a fallacy of relevancy in which a proof is given, which in its premises assumes to be true, what is yet to proven. Such an argument may sound plausible at first sight, but it actually does not prove anything.

For example:

The Bible is God’s Word, because it is written [in the Bible] that “all Scripture is inspired by God”.

Obviously, this proves nothing, because the basic assumption is already exactly the question to be proven.

Other names

  • Begging the question
  • Assuming the conclusion
  • Tó en archeí aiteísthai [Τὸ ἐν ἀρχῇ αἰτεῖσθαι] (Grk.: “asking the original point”)

The Latin expression “petitio principii” is a direct translation of the above Ancient Greek expression that was already used by Aristotle. This can also be translated as “asking for the starting point” or somewhat more freely as “presupposing the premise”.


Petitio principii describes the fundamental fallacy, which manifests itself a form of circular reasoning, i.e. for a proof which already presupposes (in its premises) the thing to be proven (the conclusion).

It is not always as easy as in the example above to recognise what actually needs to be proven in the first place, and to what extent this may already have been assumed. This is especially true when it comes to rather abstract or very complex topics.


#TODO FIXME This article is still under construction. More examples will follow.

See also

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