It seems to be essentially a bit of wordplay, in that he uses it to mean two different things. Initially he is contrasting “is/is not” statements with “ought/ought not” statements. Later he talks about things that exist vs. things that don’t exist. It doesn’t seem to be very helpful though; in the earlier sense, there is no distinction between the “world of is” and the “world of is not”. So this seems like it was a bad idea.
I think there may be a good idea behind it though: view it as a cryptic appeal to Occam’s Razor. Various moralists (e.g., Railton, Craig) were shown to be speaking of real things and properties, or of imaginary ones, with their moral language. Why not then hypothesize that all are—albeit less transparently than these two—and do away with the need of a special metaphysics or semantics (or both) for “ought” questions as “opposed” to “is” questions.
It seems to be essentially a bit of wordplay, in that he uses it to mean two different things. Initially he is contrasting “is/is not” statements with “ought/ought not” statements. Later he talks about things that exist vs. things that don’t exist. It doesn’t seem to be very helpful though; in the earlier sense, there is no distinction between the “world of is” and the “world of is not”. So this seems like it was a bad idea.
I think there may be a good idea behind it though: view it as a cryptic appeal to Occam’s Razor. Various moralists (e.g., Railton, Craig) were shown to be speaking of real things and properties, or of imaginary ones, with their moral language. Why not then hypothesize that all are—albeit less transparently than these two—and do away with the need of a special metaphysics or semantics (or both) for “ought” questions as “opposed” to “is” questions.