“If the sentence ‘the ghosts exist’ is objective, then there exists a (perhaps unknown) method to know it”.
Strange, is it not? In reality, this is not to say anything about the existence of ghosts.
There is not a doubt in my mind that that sentence is objective. I think that you're confusing "objective" with "true".
The sentence, "A god carries the sun in his (visible) chariot across the sky" is objective. Any person in a position to examine the sun can see there is no such chariot.
The sentence “The Earth turns around the Sun” was not objective four billion years ago because there was not anybody to utter it. A sentence is the product of the human brain and it is not an eternal entity similar to the platonic ideas. You can imagine an alien uttering this sentence four billion years ago. You can imagine God uttering this sentence four billions years ago. (Not the Bible god which has not the ideas about planets too clear). You can imagine that science is wrong and the Earth was created five thousand years ago. Why not? In this sense all is “open”. But not everything you imagine is useful to investigate. Or to believe.
This utterly dull side issue about sentences is easily avoided. In 3000 B.C., a suitable translation of that sentence could be uttered. No one in that time had any method for determining the truth of the sentence. There are two options.
(1) The sentence was subjective then, and objective now.
(2) The sentence was objective then, though the method for verifying it was unknown at the time.
As far as I'm concerned, sentences do not go from subjective to objective. Now, a person 3000 years ago may have been expressing a subjective opinion about the truth of that sentence, but the meaning of the sentence itself was objective.
If you think (1) is the case, then we may amend my claim: moral norms are currently subjective, but it is possible that they will become objective. It's a fairly unnatural way of saying what I mean, but perhaps you will understand.
You can imagine all the possibilities you want. But knowledge is not imagination. We know our sentences are objective if we have any evidence (not absolute evidence!) that they are so. If we have not this evidence, provided by an objective method, we can say that our sentence is not caused by any positive evidence, but by subjective reasons such as wishes, fears or delusions.
No, the last part is false, unless you're willing to say that the objectivity of sentences such as, "The earth orbits the sun," is something that changes over time, that such sentences were once subjective and now are objective.
Re-reading that last sentence, I suppose it is true as stated, but introduces a new use of "subjective", namely the "subjective" reasons which cause one to utter the sentence. This is distinct from the question whether the sentence itself is subjective. (I think there's a type-token misunderstanding here. I'm not speaking of any particular utterance of the sentence, but the sentence itself. This is, after all, the approach that logicians take to understanding properties of sentences.)
On your account, when an astronomer says, "The earth orbits the sun," that sentence is objective, whereas when an uneducated person reads that same sentence aloud because he has been prompted to, it is "subjective"? Or meaningless? This is not a useful level of abstraction for understanding properties of assertions about the world around us.
By the way, pages ago I gave my definition of "objective sentence". It is clear from my definition that a sentence's objectivity does not change.
And this is the case of moral principles. We have not any real (not fanciful) evidence of their objectivity but we have a lot of evidences of their dependence on subjective reasons.
Look, as long as you admit the possibility that tomorrow, we may find such evidence, then there is no argument here. It takes a subtle argument, but I've shown that there are objective non-moral norms, and hence there is no contradiction in the concept of objective norm.