Beg the question

Example:

  Premise: Death should be prevented.
  Premise: Life always leads to death.
  Conclusion: Life should be prevented.

To prove that something is begging the question, prove that the premises may not be true. Or, prove that the premises depend on the conclusion.

The Latin term for the fallacy is petitio principii, a translation of the Greek to en archei aiteisthai="at the beginning to assume"; but aiteisthai does literally mean "to beg". The phrase can be traced back to Aristotle (4th century B.C.): "Begging or assuming the point at issue consists (to take the expression in its widest sense) in failing to demonstrate the required proposition. But there are several other ways in which this may happen; for example, if the argument has not taken syllogistic form at all .... If, however, the relation of B to C is such that they are identical, or that they are clearly convertible, or that one applies to the other, then he is begging the point at issue." (Prior Analytics II xvi)

I have only heard Begging the Question used, in philosophical context, to mean that the argument assumes what it is trying to prove.

This is different from assuming something that needs to be proven. You can always say that any or all premises in any argument need to be proven. Philosophers have a lot of fun questioning Time, Space, Material Objects, Personal Identity, Good, Bad, and anything else that stumbles into their path. If you find something that doesn't need to be proved, you will go down in history (along with Descartes -- but we're still arguing about whether or not 'I think, therefore I am' really works.)

As far as the form of an argument goes, it can be valid even while containing misinformation, and it can never be sound if you require a perfectly rigorous proof for everything. But an argument is not useful if it assumes what it is trying to prove, therefor Begging the Question is an informal fallacy*.

1) God, by definition, is a necessary being
2) By definition, a necessary being exists
-----------------------------------------------
3) Therefore, God exists.


Should be
1) If God exists, he is, by definition, a necessary being.
2) By definition, a necessary being exists
-----------------------------------------
3) Therefore, if God exists, he exists.

* I had originally claimed that begging the question invalidated an argument; skeller has pointed out that an argument can be valid even if it begs the question:

X
------------
Therefore X

This is an entirely correct argument form, even if you have no reason to believe that X is true. The problem with begging the question is that it doesn't give you sufficient evidence to believe the conclusion, not that the argument form is incorrect.

Bells' definition of begging the question is correct. But whenever you see a journalist or hear a sports- or news-caster use this term, e will be using it in a different sense. 99% of the time, the mass media (in the USA) uses 'begs the question' to mean 'raises the question.'

These people probably aren't all that bright (although every last one of them has had a college education), and they probably have no idea that begging the question is a logical fallacy. But on the other hand, many of the people who come to this node for information may be wanting to know what these TV personalities are talking about. Like it or not, the popular usage should be defined. And then eradicated.