Got it in one. I remember SoullessAutomaton saying that rhetoric was called one of the "Dark Arts" in the originating post, so it had to have been this one.
I should chat with Yvain sometime: it sounds like he? she? knows the old myth of Rhetorica as the dark sister of the goddess Philosophy. Yes, Rhetoric is very much one of the "Dark Arts"--for, unlike Philosophy--Rhetorica looks to derive her knowledge by paying attention to what actually works in the observed world. Her light sister Philosophy derives her knowledge from some great "Truth"--be it a deity, Plato's Forms, or such. Or so one can read the myth.
Not to say that Rhetoric hasn't fallen prey to the great truths repeatedly over the centuries--St Augustine, for instance, was a trained rhetor before he converted, and one of his major treatises is his attempt to Christianize rhetoric so that it would reify the "great truth" of his religion.
Yet as I read the history of Rhetoric, I find that it fits very nicely with Science, thank you very much. Yes, Rhetoric is the study of how to persuade, but that study assumes that all comers can learn the techniques, and so they can learn to disregard those techniques when the substance of a communicative act necessitates it. And Rhetoric has mostly been using observed results to establish its knowledge, not imposing philosophical ideals onto the perceived world.
(Well, mostly. We do go through periods when Rhetoric is subsumed by some of those groups that believe deeply in "great truths"--right now the political "truths" of a particular stripe has Rhetorica in chains. And don't get me started on that soapbox of mine.)
See Dark Side Epistemology for the origin of the term in this community.
This site is very likely impenetrable to the newcomer. You one-box and defect on the True Prisoner's Dilemma, but is that just because of a cached thought, or is it your Tsuyoku Naratai? So I've created the LessWrong Wiki on Wikia. I'd like this to become a respository of useful definitions and links: it can support our discussions here, and create something lasting from the ephemerality of a blog.
badger already created a Wiki, but as you can see in the updates to that article badger and others pretty quickly concluded that TiddlyWiki wouldn't be up to the job. MediaWiki, the software Wikipedia and Wikia use, is the monster of them all, and will give us good support for practically anything we want to do, including mathematical notation. I've ported across a couple of articles from the old wiki onto the new, but many more are needed. The "download" link in TiddlyWiki and a text editor may help.
EDIT: Usernames are global across all of Wikia, so you may not be able to use the same name there as here. Sorry.