petitio principii

Written by  on February 19, 2014

My SO is currently taking a course that uses this book as a text book. The argument to be analyzed for fallacies is:

The death penalty is wrong because it’s murder.

She answered by saying the argument includes a persuasive definition, but commits none of the fallacies asked for in this homework assignment. The listed fallacies included “begging the question/circular argument/petitio principii”. Her reasoning for her answer was that the argument is basically this:

  • Murder is wrong.
  • The death penalty is murder.
  • Hence, the death penalty is wrong.

Now, the professor wrote off her answer almost flippantly, without even considering it. I can understand why, since this argument bears a striking similarity to an argument that is presented almost ubiquitously as canonical begging the question. That canonical example is: “The death penalty is immoral because killing is immoral.” This fits the typical tautological structure of begging the question because the death penalty is most definitely a form of killing. So, the conclusion is simply a more specific statement of the premise.

But the argument we’re considering, here, is most definitetly not that simple. To someone not familiar with the technical definitions, Murder may be synonymous with killing. After all, murder is simply illegal killing. Hence, murder and the death penalty are specific types of killing. What makes the canonical argument petitio principii is that the generality of killing contains the specific death penalty. But that’s not what this argument is doing.

Rather, the flaw in this argument is simply an undefended premise: the death penalty is murder. We can’t infer whether the arguer intends the death penalty to be a sub-type of murder or synonymous with it. But it’s a substantive premise, not simply an obvious rephrasing of the conclusion.

Now, I have seen some definitions of begging the question that include implicit premises as a way to commit the fallacy. But I think that’s a serious miscategorization. Concealed premises that cause fallacy are premises that would not be acceptable to lots of people, i.e. obviously unsound or “tricky” premises (arguably as Aristotle points out).

I can only assume that if my SO had included additional argument for that second premise, the professor might have paid closer attention.

Why do I care? Because petitio principii holds a special place in modeling and simulation, identified as “inscription error” by Brian Cantewell Smith, or better termed “pre-emptive registration”. The idea is that when building a simulation from which you intend to predict or discover interesting behaviors, the behavior you observe is not very interesting if you purposefully programmed the simulation to behave that way. Now, there’s an interesting twist to this in that magic word “purposefully”. One can make the argument that accidental inscription is what simulation is all about! And this twist extends all the way out into the foundations of logic and mathematics, perhaps even physics and chemistry. The entire enterprise of science may well be about computing forward from premises to conclusions so as to see what “concealed premises” are hidden therein. Indeed, this is, I think, the fundamental criticism of computationalism. If the universe is simply deductive computation and all deduction is tantamount to tautology, then we’re all just wasting our time, here… really. ;-)