logic teaching question
I'm teaching Symbolic Logic this term, and we just introduced the notion that two syntactically distinct sentences can mean exactly the same thing (e.g. DeMorgan's Laws). In class yesterday, I asked the students to come up with a criterion for when two sentences are identical in meaning. We eventually reached the "official" answer: S_1 and S_2 are synonymous just in case they have the same truth-value in all models (ok, we're not using the notion of models; for us, it's "... in all possible arrangements of the board" in Tarski's World -- we're using Barwise and Etchemendy's Language, Proof, and Logic.)
Along the way to the official answer, though, a student gave the following characterization:
Sentences S_1 and S_2 are synonymous just in case the set of all sentences that follow from* S_1 is identical to the set of all sentences that follow from S_2.After class, I jotted down a proof-sketch showing that the student's characterization is equivalent to the "official" one. But I'm not 100% confident in it, so I'm just curious whether anyone can see a counter-example (i.e. are there any 2 sentences that meet one characterization but not the other?).
* EDIT: As Bryan made me realize in the comments, I should specify that 'follows from' here is semantic, not syntactic/ proof-theoretic; i.e. 'Conclusion C follows from premise P' means that every model (or world, or construction, or whatever it is that makes sentences true in your formal semantics) in which P is true is also a model in which C is true.