J.D. vs. F.d.S. vis à vis p

Though he won me over with examples of Saussure's curmudgeonly attitude towards French pronunciation, dinging Saussure for prescriptivism is clearly not Derrida's main point. Whatever that point is, it suffuses the long paragraph on page 43 that begins "What do these limits and presuppositions signify?" J. D. is winding up to a big finish here, and I'm actually on board, I want to see what he's getting at, but I just can't make heads or tails of the thing. And it's not just because I don't have the background in Heiddeger or whatever; J.D. is just flat out unclear, which I said I wasn't going to complain about but c'mon. Here is a sketch of the argument he presents.

1. Saussure distinguishes between speech and writing by claiming that the latter is outside linguistics' domain of study.
2. J.D. claims that F.d.S. claims that writing is out of the game because it possesses some property which I'm not clear on so I'll call it p.
3. In part, p = "being the sign of a sign, i.e. lacking semiotic autonomy".
4. Ironically, says J.D., F.d.S. was right about the existence of p, but he was wrong about the entity it applies to. In fact, p is true of the whole of language, written and spoken.
5. This whole issue is not some a priori philosophical debate that would evetually surface whenever one talked about language, but has a specific historical origin in the evolution of western thought, which in turn was shaped by the use of a phonetic writing system in Europe. (So presumably things played out differently in China, but that is beyond the scope of this book.)
   

This is a very charitable reading. The paragraph is nowhere near this clear. I've used this quasi-algebraic p as a placeholder for all the areas where Derrida drifts off into the ether. It may be that there is no there there, in which case p is a placeholder for nothing.

I'm finding that I can do something like the above with a lot of J.D.'s arguments. That is, I can sketch out the general scheme of things and make a list of the rhetorical tricks he employs, but his actual point remains elusive. Of course it may be that J.D. thinks this is all philosophy can ever do, and intends Of Grammatology to be a shaggy dog illustration of that fact. I have to remember to skip ahead and see if the last sentence of the book is "Gotcha!"