how do "signs" fit into this discussion? It seems that words are signs; what about "first mental images," concepts... what else can be considered a sign? Do the Stoics have a different idea of what constitutes a sign than the Epicureans do? Just defining the language with which to discuss logic and methods of inference is confusing!
Godfrey if you are getting started in Philodemus I hope you will post all sorts of things like that in the Philodemus thread. Yes it appears that signs are a big issue, as well as the terms "contraposition" and also "reasoning by analogy"
Articulating this question of what signs means and how the Epicureans took a position on them is going to be key to unwinding all these issues. They clearly took the position that "contraposition" is not the key to truth.
One way of stating what i remember the ultimate point to have been is that Plato and the boys were taking the position that nothing can be said to be "true" unless the proposition could be stated in some form of symbolic logic (if A=B and B=C then A=C, might be an example, but I think their examples were much more complex).
The Epicureans took the position that truth has to be "defined" in terms of evidence from the canonical faculties, not from symbolic logic.
But that is a very primitive way of stating the issue and our goal ought to be to make things a lot more clear than that.