Entropy at Work:
I don’t think the example posted by the OP would constitute petitio principii. 'Tis always possible that I’m the one misconstruing the concept, but I think petitio principii is more like this:
Example A: I go for a ride in the country and after a while I smell the smell that indicates that some unfortunate driver has run over a skunk. I roll up the window while wrinkling my nose and I exclaim, “No wonder they call them skunks!”
OK, that’s not a pure form because it isn’t making an actual logical claim, but it’s implicit in there.
Example B: I get into a Great Debate on the subject of involuntary psychiatric treatment, and someone argues that people with a psychiatric diagnosis of schizophrenia need to take psychiatric medication, and if they won’t take it voluntarily they need to be forced to do so. I post a response asking on what basis this need has been determined, pointing out that I’ve received that diagnosis myself and I don’t take psych meds and seem to be doing OK. The other person posts back that I should be forced to take psych meds because I’m a schizophrenic, which is in and of itself the reason I need to take psych meds, which is that schizophrenics are people who need to take psych meds, and if I don’t need to take psych meds that means I’ve been misdiagnosed because schizophrenics need to take psych meds. So I post the rejoinder that this is begging the question.
That one is close to pure form. ‘Schizophrenics need to take psych meds because they are schizophrenics, and schizophrenics are people who need to take psych meds’. Incorporated along with it in there is a special case called No True Scotsman, namely the part that says ‘Schizophrenics need to take psych meds, so if you say you’re a schizophrenic but you don’t need to take psych meds, well then you aren’t a true schizophrenic because real schizophrenics need to take their psych meds’.
Example C: A silly one from the realm of religious studies — a student writes that Jesus of Nazareth was taken to be killed at the place known as Golgotha, the place of the skull, which was given its name by the fact that Jesus of Nazareth was put to his death at that place.
Example D: Here’s one you’ll hear people putting forth, in all seriousness, another religious one — 'We know the Bible to be the infallible word of God, it says so in the Bible, and since the Bible is infallible that means it has to be so"
Example F: In social sciences research design, petitio principii often makes an apearance when playing with the regression analysis to see how controlling for one variable in the equation washes out the impact of another variable that previously looked like it was explaining a lot of the observed behavior. Problem is with social science variables is that they are often heavily interknitted with others, to the point that when you control for that variable x you’ve washed out the impact of variable y because you’ve controlled for variable y in the process of controlling for x, doing your study at this point of how your data looks when you only compare subjects with identical variable y values (because they are so closely bolted onto variable x values), and after having erased the effect of y by controlling for x you find that y makes no difference. Umm, well, yes, it certainly doesn’t if we’ve made sure we never compare subjects that differ according to variable y. This kind of study can often be a begging of questions. Two intervowen concepts, let’s say education level and income levels, are available as data and have been coded for. Any time you’re studying the effects of either of them on an outcome, if you toss the other in as a control variable it’s going to wash out the “explanatory power” of the other when you do regression. Geeky arguments in the hallways: “Don’t you be controlling for that, you’re controlling my main player here right off the screen, controlling him to death” Because the variables are, theoretically, supposed to be discrete and separate phenomenae, but often they are nothing of the sort, and are actually different manifestations of the same thing. 'Gee, surprisingly enough it looks like the educational courses that students from lower economic classes were offered and had recommended to them was about equal to those suggested for and made available to students from well-established economically successful families. At first the data made it look like they weren’t getting equal access to planning resources and were not steered towards towards the advanced courses we’re studying, but then when we thought to control for income levels and include that in our regression formulas, we found it doesn’t have much to do with it after all. The students from the lower economic classes who personally have a low income level were given access to those classes about the same as the students from successful families who personally have a low income level. And the students from the lower economic classes who had an impessive personal income sufficient to take whatever they wanted to were found to be taking those classes in about as high a percent as the students from well-off families who had their own substantial personal income.
I think a lot of these kinds of studies get to beg the question as a consequence of how they operationalize their variables. Poor people spend as effectively as rich people if you control for how much money they’ve got in their pockets, but to most of us the money you got in your pockets is part of the definition of “are you poor or not”, so if you control for that in that fashion you’re just begging the question.
