Fallacy?

Begging the question

The conclusion is already built into one of the premises.

Also called: Circular reasoning · Petitio principii

The test

Could someone who doubts the conclusion still accept every premise?

Circular arguments are rarely stated in one breath. More often a definition, a method or a dataset carries an assumption, and the conclusion is drawn later without noticing that it was there from the start.

The test is simple to state and hard to apply: imagine a careful person who does not yet accept the conclusion. Would they accept every premise? If not, the argument cannot persuade them, however tidy it looks to someone who already agrees.

Seen on both sides

In mainstream arguments
It would beg the question to define homology as similarity due to common ancestry, and then offer homology as evidence for common ancestry.
In critical arguments
It would beg the question to set aside every measurement that conflicts with a young age as unreliable, and then cite the set-aside measurements as proof that the method is unreliable.
Not a fallacy when

The circle is broken by an independent line of evidence, for example a definition or a dating method that does not presuppose the conclusion under test.

Diagnosed on this site

  • Pillar 3 · Common DescentClear case
    In the pillar as a whole

    Where similarity is defined by ancestry, it cannot then be counted as independent evidence of ancestry. Every card in this pillar has to keep the pattern and its explanation apart.

  • Claim № 3.01 · HomologyClear case
    In the claim as taught

    Homology is defined as similarity due to common ancestry and then offered as evidence for common ancestry. The circle is broken only if homology is identified by criteria that don't assume ancestry, such as position and connection, as Owen did. Then the step to descent is a separate inference that has to be argued.

Further reading

  1. Walton DN. “Begging the question as a pragmatic fallacy.” Synthese 100:95–131 (1994).