In logic, there are generally reckoned (for disagreement see Etchemendy 1990) to be two types of signs: logical signs, and extra- or non-logical signs. So, for example, we might have a sentence like the following, which can be approximated in English as “If p, then q.”

p → q

The material conditional ? is a logical sign; it indicates a syntactical relationship between p and q (roughly speaking, p implies q). p and q are non- or extra-logical signs; they’re names, or variables. p might stand for “It is raining” and q for “You should take your umbrella”, giving us the sentence “If it is raining, then you should take your umbrella.”

Anyway… now you know. The new feed is here, the old one’s redirecting to it and will die in a couple of weeks. Let me know if I’ve broken any links anywhere.