As said above, it’s circular reasoning. Another way of thinking about it is that an argument that begs the question has supporting steps that would only be accepted by someone who also accepts the conclusion, so the argument doesn’t provide any true support for the conclusion.
Here’s one: The Bible says that there is a God, and we know the Bible is true because it’s God’s word, therefore God exists.
Only someone who already believes in God would buy in to this reasoning. (But, as an aside, it’s worth noting, not everyone who believes in God will accept this.)
You can think of it as an argument in which the supporting reasons pre-suppose the conclusion. Let’s say for example we’re arguing about abortion. I am arguing that abortion is wrong. I give you as a reason for my position the premise that abortion kills babies, and it’s wrong to kill babies, so abortion is wrong. Notice how the premise (abortion kills babies) seems like a conclusion of my own position? A lot of people think abortion is justified precisely because it’s not a baby that’s being killed.
A good argument should use true and relatively uncontroversial premises to yield a conclusion. Begging the question slips in as a premise the very thing that should be demonstrated in the conclusion. A lot of times it’s less obvious than the example I gave, usually the wording will be different. But people do it all the time, probably without even realizing it
Here’s a real life example, from a 2004 George Bush press conference
>The reason I keep insisting that there was a relationship between Iraq and Saddam and al-Qaida is because there was a relationship between Iraq and al-Qaida.
He’s not actually giving the reason, he restates the original premise and acts as if it’s supporting evidence.
Here’s another example
>smoking cigarettes can kill you because they are deadly
It’s doesn’t answer the question WHY cigarettes are deadly. Just like George Bush didn’t answer the question of WHY he’s so sure there’s a relationship between Iraq and al qaeda.
The way I remember is that you need to beg to get your question answered.
A recent example on reddit was someone asked how people determined how many molecules were in a mole.
The answer was get a mole of charged molecules and divide it by the charge of one molecule.
“sure, but how do you know you have a mole of charged molecules?”
Begging the question is usually wrongly used when the answer given leads to a new question, not just having to reask the same.
The phrase “beg the question” to refer to the logical fallacy is actually [a weird mistranslation](https://languagelog.ldc.upenn.edu/nll/?p=2290) of the writings of Aristotle, who originally described the fallacy. It really should have been translated as, “**assuming the conclusion**”.
tl;dr, because of linguistic quirks of the time, the word the first translations into Latin used for “assume” was also a word that in some contexts meant “beg”, and similarly the word they used for “argument” was the same word for “question”. Simple confusion and a long game of telephone eventually gave us the English phrase we have today.
If we go with “assuming the conclusion”, it’s a lot simpler to piece together what this fallacy actually is. It involves making a statement, like say:
> /r/explainlikeimfive is the best subreddit.
and then “proving” it with another statement that you’ve simply assumed to be true, like:
> /r/explainlikeimfive is the best subreddit because the threads here are very good.
*Are* the threads in ELI5 very good? Is that a proven fact? No, it’s just something you’ve assumed. You’ve assumed the conclusion of your argument. Or you’ve “begged the question”, as the janky translation would have it.
Some people try to justify the janky translation by saying you’re “begging listeners to call your assumption into question”. That’s not really where the phrase comes from at all, as I mentioned earlier. But if that helps you remember somehow, more power to you, I guess. IMO it just makes me even more confused. I’d rather stick to “assuming the conclusion”.
All of this of course has absolutely nothing to do with the common use of “that begs the question”, which people use as a way to say, “that really makes me wonder”. Which royally pisses off some pedants out there. Personally I just think this confusion is even more evidence that the translation of the fallacy name really was a mistake and we need to phase it out ASAP.
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