Here are two questions I’d appreciate any thoughts on. Firstly, I’ve recently detected an apparent tension in my beliefs. In my paper for Phil Compass on the grounds of necessity, I argue that the Lewisian realist needn’t be worried about the epistemological objection. The objection goes: how could we know what’s (merely) possible if what’s possible is what’s true at a spatially-temporally isolated concrete world – such worlds do not interact with us causally, so how can we come to know what they’re like? Lewis responds by saying that causal interaction is necessary only when the subject matter is a contingent truth – when the claim to be known is a non-contingent matter, causal acquaintance with what the claim is about is not necessary even when the claim is about the realm of concreta. It’s not obvious to me that this is a good reply, but I thought Lewis had a simpler reply available: metaphysical priority is not conceptual priority. To say that what it is for it to be possible that p is for p to be true at some world does not commit us to saying that our epistemic access to the fact that p is possible must go via epistemic access to there being a world at which p. The Lewisian realist needn’t claim we have any way of discovering what’s true at a world independently of discovering what’s possible. Why can’t the Lewisian simply say she knows there’s a world where there’s a talking donkey because she knows that there could be talking donkeys (here appealing to whatever story about modal epistemology that any realist appeals to), and she knows that everything that could be the case is the case at some world (and here she cites the familiar Lewisian reasons for believing that claim)? What’s the problem?
That still seems convincing to me. Here’s my problem. I also find convincing an epistemological objection to consequentialism: were consequentialism true we couldn’t know what’s right or wrong because we can’t know what the full consequences of our actions would be. And it doesn’t seem to me in the least bit satisfying for the consequentialist to say: I know that murdering X will have the worst consequences because I know that murder is wrong – metaphysical priority isn’t epistemic priority, so my knowledge that it is wrong can ground my knowledge about the consequences even though what it is for it to be wrong is for it to have the worst consequences.
What I’d like is for the two cases to be disanalogous so I can consistently do what seems to me intuitive: hold the epistemological objection to consequentialism and reject the epistemological objection to Lewisian modal realism. I haven’t been able to convince myself that they’re analogous yet, so any thoughts on this are welcome (even if they’re of the form: they’re obviously analogous, and you’re wrong about the epistemological objection to ____). (Incidentally, I barely know the literature on consequentialism, so if anyone knows what consequentialists say about the epistemological objection, please enlighten me!)
Question 2. I was reminded by Brian’s post about the autonomy in logic issue. There’s a thought that every logical truth should be provable using only the rules governing the connectives in that truth. This is meant to be bad for classical logic because there are classical tautologies like Pierce’s law where the only connective is the conditional but one can’t prove Pierce’s law using only the rules for the conditional. I was thinking about this briefly, and I couldn’t see how the objection could possibly be right. We can do classical logic with just one logical connective: the Sheffer stroke, e.g. Every wff of classical logic – a fortiori every theorem – has a translation into a sentence statable using only the Sheffer stroke, and the translations of the theorems will be provable using only the rules governing the Sheffer stroke, as those are the only rules you have. But it can’t be the case that the acceptability of a logic depends on what connectives you allow yourself to use to state its theorems. The defenders of the objection are obviously going to be unimpressed with such a simplistic response, so my question to those who know more about this than me (= those who know than is written in this paragraph!) is: why not?