Gerritsen
2023 raises a good worry about revolutionary moral fictionalism,
as advocated in Joyce
2001.
The (revolutionary moral) fictionalist says that our naive moral
practice rests on the mistaken assumption that there are
mind-independent moral facts. There are no such facts. But this doesn't
mean that we should stop caring about morality. It is in our personal
interest, according to the fictionalist, to adopt a fiction of morality
and continue talking and acting much like before.
Standard Bayesianism assumes that agents are "logically omniscient":
they assign credence 1 to all necessary truths. This seems wrong.
But we need to be clear about the data (as Stalnaker has emphasized,
for example in 1991 and 1999). It's
not a datum that we don't give credence 1 to all necessary truths.
"Credence" is a theoretical term. The data that cast doubt on logical
omniscience are things like this:
Imagine a community of people who pass encrypted messages to one
another, without knowing what they mean. Agent X has encrypted a message
and handed it to messenger A, who passes it to messenger B, who passes
it to agent Y, who has the codebook to decrypt the message. When A
utters the message to B, she has no idea what it says; neither does
B.
Intuitively, the meaning or content of A's
utterance is the content of the decrypted message. That's why A and B
don't know what the utterance means.
I finally got around to adding the papers from Janssen-Lauret and Macbride
2023 to the search corpus at https://www.david-lewis.org. It's
a wonderful collection with lots of treasures. I want to comment on an
intriguing passage on pp.71f., from an abandoned 1969 textbook project
on confirmation theory.
First, some context. At this point in the manuscript, Lewis has
introduced \(\mathcal{M}\) as a
probability measure on the propositions expressible in a language \(\mathcal{L}\) with classical boolean
connectives; \(\mathcal{C}\) is the
associated conditional probability measure, defined by the ratio
formula. Lewis notes that conditional probabilities are often read as
"the probability of C if A", which suggests that \(\mathcal{C}(C/A)\) might equal \(\mathcal{M}(C\textit{ if }A)\), where
'\(C\textit{ if }A\)' is the material
conditional. But that's obviously false. Lewis continues: