You should do philosophy, and you will do philosophy whether you like it or not

In today's Bryn Mawr Classical Review is a review by Christopher Moore (available here) of an exciting  new monograph by Luca Castagnoli, Ancient Self-refutation: The Logic and History of the Self-refutation Argument from Democritus to Augustine (Cambridge University Press, 2010). Among the several other kinds of arguments Castagnoli investigates in the book, he also treats the argument about the inevitability of doing philosophy that originated in Aristotle's Protrepticus. Moore gives a good, information-dense review of this section of the work, which I quote below.

Castagnoli reconstructs Aristotle’s protreptic argument (“you should do philosophy; but anyway, you can’t avoid it”) at the book’s halfway point (187-196). A traditional reconstruction from the sources is this (p. 189):  

(1) p -->p If one must philosophize, one must philosophize
(2) ~p -->p If one must not philosophize, one must philosophize
. .. p In any case, one must philosophize

Castagnoli draws attention to several suspicious features of this reconstruction. First, it has no dialectical context; it is presented as establishing the truth about how one ought to live, not keyed to any particular claim about the necessity of philosophizing. Second, it would seem to expect reduction to (2), a Consequentia Mirabilis-style argument (if the negation of something entails its affirmation, then one can deduce the affirmation); but Castagnoli argues at length that no other ancient self-refutation arguments ever depended on that style. Third, the reconstruction leaves the conditional at (2) wholly unexplained. Fourth, such reconstructions do not follow from Cicero and Alexander of Aphrodisias’ testimony about Aristotle (the testimony Castagnoli argues is most trustworthy), only from later, likely Stoicizing, sources. Castagnoli goes on to reconstruct the protreptic argument anew:

I suggest that argument might have sounded like this: ‘If your position is that one must philosophise, you are definitely on my side of the barricade, and safe from the snares of Isocrates’ shallow rhetoric; if you contend, on the contrary, that one must not philosophise, you ought to vindicate this crucial choice of lifestyle, in front of me and yourself, by offering reasons for it; but don’t you realise that choosing what to do (and then defending your choices) on the basis of reflection and argument, and not, say, by ballot, is already doing philosophy, and thus you have already jumped over the fence to my side? In any case, therefore, whether you want this or not, you are bound to agree that one must philosophise.’

And he re-symbolizes it like this:
(a) q-->p If one must philosophise, then one must philosophise
(b) r<-->s ^ s>-->p If one must not philosophise, then one must philosophise
c) q v r Either one must philosophise or [your position is that] one must not philosophise
. .. p In any case, therefore, one must philosophise 

This argumentative strategy had fuller use in the anti-Skeptical arguments against proof, sign, and cause, which Castagnoli develops at length.

One thing we are especially interested to see is what Castagnoli has to say about the literary form of the Protrepticus, i.e. the fact that it was probably a dialogue. That interpretation would seem to fit well with his characterization of the argument, quoted above, in which Aristotle is speaking and explicitly mentions Isocrates.