@fast,
fast;158537 wrote:Well, I think the proposition "Sherlock Holmes is not a detective" is true, and so do you--I thought.
No, it's false because there does not exist an entity, in this case, named "Sherlock" that does not have the property of being a detective to make that statement true.
"Sherlock Holmes is a detective" is false.
"Sherlock Holmes is not a detective" is false. But,
It's
not the case that "'Sherlock Holmes is a detective' is true" is true.
There is a scope ambiguity here about "not."
If either of the former statements above were true, then you would be committing to the existence of something you didn't want to commit to, which is precisely the problem.
"Extrain is not rich" is true. But the statement is true only if Extrain exists and is not rich.
Here are some possible Russellian formulations:
∃x(Sx & ∀y(Sy → y=x) & Dx)
says, there exists one and only one Sherlock and that thing is a detective. This is false.
∃x(Sx & ∀y(Sy → y=x) & ~Dx)
says, there exists one and only Sherlock and that thing is
not a detective. This is also false.
~∃x(Sx & ∀y(Sy → y=x) & Dx)
says, it is not the case there exists one and only one Sherlock and that thing is a detective. This is
true.
Though I am not committed to formulating proper names this way, the last formulation is your best bet.