for the purposes of this post, let:
<> be the possibility daimond
 be the necessity box
E be the existential quantifier
A be the universal quantifier
Let me run over Salmon's pen argument again. Suppose it's metaphysically possible for the pen to have originated from 5 blobs of material different than it actually did. We have the actual world @, world w1 at which the pen is from 4 blobs of material different, and w2 at which the pen is 4 blobs different than at w2. Salmon argues as follows:
1) w3 is an impossible world which legitimately exists
2) therefore Ex(x is an impossible world which legitimately exists)
Thus impossible worlds. Relative possibility always sort of bugged me. What's possility if not possibility simpliciter? What does possibly possible mean? Something like it could be possible? Well, the pen argument makes things a little clearer (at least it lit something up in my head).
Take Nathan Salmon. It's impossible for him to be a visa credit-card account. However, it's not impossible for anything to be a visa credit-card account(assuming we're realists about such things). This is possibility/impossibility relative to an object. Now imagine a world with no banks, no credit cards, nothing that could conceivably be a credit card account. It would be true that (N).
(N) ~Ex<>(x is a credit card account)
Unless you believe the barcan formula to be valid, this would not imply (N*)
(N*) ~<>Ex(x is a credit card account)
If (N*) were true, then credic card accounts would be truely impossible objects. As it stands they're only relative impossible objects. Everything's pretty simple so far.
Let @ be a proper name designating the actual world. Let 2x and 3x be complex predicates representing the maximal ways world 2 and world 3 are. Now (W) is true.
But (W) is silent with respect to (3).
In fact, (3) is obviously false, since w2 can possibly be the way w3 is. Now, a truely impossible world would fit into be predicated by (I).
(I) Ix iff ~<>ExIx
That is, something is impossible if it's not possible for anything that exists to be that way.
So, as far as Lewis goes, he can respond to Salmon using this machinery. He can claim that he only rejects impossible worlds that satisfy Ix (someday I'll try rephrasing this so I don't quantify over impossibelia). This isn't the logical metaphysical distinction, this is the relative possibility, absolute possibility distinction. He admits the existence of W3, though he may make such moves and denying W3 is a counter-part of @ (this would be denying that @ is possibly the way w3 is).
So what about Salmon's pen? It still has the modal property of not being able to have come from 5 blobs different material. It also has the counter-factual modal property that if it had come from 4 blobs different material, it could have come from 8 blobs different material(as the material it actually came from).
That's my rant. Anyone well-versed in Salmon want to respond? Personally, I'm worried that (I) is trivially un-satisfiable (Lewis might be ok with that), although I'm unsure of how else to represent impossibility simpliciter.