The web page I pinked to earlier in Miscellany has a good entry for
begs the question:
An argument that improperly assumes as true the very point the speaker is trying to argue for is said in formal logic to “beg the question.” Here is an example of a question-begging argument: “This painting is trash because it is obviously worthless.” The speaker is simply asserting the worthlessness of the work, not presenting any evidence to demonstrate that this is in fact the case. Since we never use “begs” with this odd meaning (“to improperly take for granted”) in any other phrase, many people mistakenly suppose the phrase implies something quite different: that the argument demands that a question about it be asked—raises the question. If you’re not comfortable with formal terms of logic, it’s best to stay away from this phrase, or risk embarrassing yourself.
