In my last post I described the syllogism "Photogenic people look good in photograps; Michelle Pfeiffer is photogenic; therefore, Michelle Pfeiffer looks good in photographs" as "begging the question". A few people commented on that, so I thought I'd address this point of English usage.
In modern usage, "begging the question" has come to mean nothing more than "the situation suggests that an obvious question to raise at this time is blah blah blah." For example, "The global financial meltdown begs the question: was there insufficient federal oversight of the American mortgage industry?" Though this usage is certainly common in civic discourse and the media, it is entirely a modern departure from the historic usage of the phrase. I try to eschew this modern usage when I say "begs the question".
"Begs the question" is also sometimes used to mean "this argument raises additional questions which require additional investigation before we can accept the argument". Though this is considerably closer to the traditional definition of the phrase, this is also not exactly what I mean.
When I say "begs the question", I mean it in the traditional sense of "this argument is fallacious because it takes as a premise an assumption which is at least as strong as the thing being proven, and is therefore an unwarranted assumption."
Let me give you another example of question begging, in the traditional sense, which might be more clear.
Suppose I asked "why are diamonds very hard but butter is very soft?" and you answered "diamond and butter are both made out of atoms; the atoms of diamonds are hard and the atoms of butter are soft." You would have begged the question; your answer to my question "why are some things hard and some things soft" is "because some things are made out of stuff that is hard and some things are made out of stuff that is soft" -- that is, you've avoided answering the question by providing an "explanation" that itself cannot be understood without answering the original question -- namely, why is some stuff hard and some stuff soft? This pseudo-explanation has no predictive power; it doesn't tell us anything new, it just circles back on itself. The explanatory assumption -- that some atoms are hard and some atoms are soft -- is stronger than the thing we are trying to investigate -- the hardness and softness of two substances.
A non-question-begging answer would be "diamond and butter are both made of atoms; the atoms of a diamond are all identical and arranged in a stable, rigid lattice where every point in the lattice is reinforced by a strong bond to four other points. The atoms of butter are a disorganized collection of many different atoms grouped into different kinds of relatively complex molecules; though the molecules themselves are quite strong, each molecule of butter holds weakly to each other molecule. It takes only a small force to disrupt the loose arrangement of butter molecules but a very large force to disrupt the strong arrangement of diamond atoms. We perceive this difference in required force as 'hardness' on the human scale, but in fact it is a property that arises from the sub-microscopic-scale properties of each substance."
Now, this explanation does *raise* more questions. It raises questions like "why are some lattices strong and some weak?" and "why are some objects composed of many different kinds of atoms organized into molecules, and some composed of just one atom?" Question-begging is not the act of raising more questions. Every good explanation raises more questions. What makes this explanation a good one is that it is testable and has predictive power; we can investigate the hardness or softness of other substances, and make predictions about what sorts of atomic structures they will have -- or, vice versa, we can look at an atomic structure and try to figure out from it how hard the substance will be. We can invent other techniques for determining atomic structure, like x-ray diffraction crystallography or spectroscopic analysis, and use those to cross-check our "atomic theory of hardness".
But the "because she's photogenic" pseudo-explanation is clearly question-begging. Why does she look so good? Because she's photogenic. Why is she photogenic? Because she looks so good. We have learned nothing about photogenicity (or the lovely Ms. Pfeiffer).
Similarly, if you ask "why is this code thread-safe?" and the answer is "because it can be correctly called on multiple threads", we've begged the question. Why is it thread-safe? Because it's correct. Why is it correct? Because it's thread-safe. Again, we have learned nothing about the nature of thread safety.