This is the concept of objective knowledge by Kant:
For Kant, a defining feature of our representations of objects is their objective validity. For a representation to be objectively valid it must be a representation of an objective feature of reality, that is, a feature whose existence and nature is independent of how it is perceived. ( Pereboom, Derk, "Kant's Transcendental Arguments",
The Stanford Encyclopedia of Philosophy (Fall 2014 Edition),
http://plato.stanford.edu/archives/fall2014/entries/kant-transcendental/).
As you can see, Kant’s concept of “objectivity” is similar to other quotations I have put here and similar to mine. Consensus and progress are a mere signs of objectivity and they must to be actual, not hypothetical. They are the sign that scientific statements have objective validity and the metaphysical claims have not.
Sorry, I am speaking of "objective propositions". I don't think that representations of objects are the same as propositions, and I don't know that the word "objective" applied to one is precisely the same as the others. I suppose it all depends on what counts as a representation of objects.
In any case, I was thinking of Kant's treatment in Metaphysics of Morals. What he says is actually less informative than I remembered, but he wrote:
That is practically good, however, which determines the will by means of the conceptions of reason, and consequently not from subjective causes, but objectively, that is on principles which are valid for every rational being as such. It is distinguished from the pleasant, as that which influences the will only by means of sensation from merely subjective causes, valid only for the sense of this or that one, and not as a principle of reason, which holds for every one.
I will admit that my definition is rather loosely based on this passage.
But this is not our problem now. You have chosen a personal definition of “objective” and I am doing a semantic analysis in order to understand if our disagreements are only a question of words or we disagree on facts.
I think my position is easier than yours because I have taken the usual meanings in philosophy. They are already done.
I dispute this. You have not defined "objective proposition" at all, with the exception of one attempt which reduced to "true proposition about external stuff". And I do not find a common definition of objective proposition thus far. The various quotes you have given are either not about propositions or fail to give an explicit definition or both. This is why I have tried to come up with my own working definition.
Please, if you know what "objective proposition" means, state the meaning as clearly as possible. And if it means "true proposition about external stuff", then clearly that is not what anyone could possibly mean when they speak of objective morality (where "stuff" here evidently means the physical realm).
Your definition of rationality sounds very pragmatist to me: “A rational person is one who chooses what to do on the basis of its fitness to achieve his goals”, and relativist: “given his background beliefs”. I have a problem with the last part of the definition because it allows the qualification as “rationalist” to someone with absolute irrational beliefs (“given his background beliefs”). A fundamentalist, a believer in the car of the Sun or similar would be “rationalist” if they were coherent with any set of absurd beliefs (their background). I think this is in contradiction with the usual meaning of the word, both in human sciences as in the common language (this forum, for example). And this is useless for distinguishing the points (a) and (b) in my previous question.
I think that the second part of your definition is more accurate: “He also comes to believe on the basis of evidence and reason, and on methods which are effective in coming to true beliefs”. I agree now. And it is important to me, because you have introduced the rational method (“effective in coming to true beliefs”). So, an objective assertion must be accepted by persons that use “methods which are effective in coming to true beliefs”. I am sorry if I insist on your sentence. It is of capital importance to me in the matter we are discussing and I think it is well said.
I took it implicitly that a rational person who has come to bad beliefs could, in the presence of appropriate arguments and evidence, disavow those beliefs, so I think the difference matters little for our purposes.
But let me point out this: rational persons necessarily rely on authority (we cannot all participate in experiments for the Higgs Boson) and hence rational persons can come to false beliefs. Appeal to authority is one of the methods a rational person employs, subject to the usual concerns of reliability of the authority, and this is an effective method. You can't exclude religious persons from rationality, because the fact is that they've come to their beliefs in more or less the same manner that I've come to believe in the Higgs Boson.
Let that pass, and let's move on.
We know what the objective-rational methods are the cause of a real consensus between scientists. We don’t know any similar method in moral. So, we can say with Kant (Critic of Pure Reason) that moral sentences have not “objective validity”. (Another Kant is the philosopher of the moral imperative, but no one has defended this aprioristical theory here).
I wasn't aware of that statement from Kant in Critique. Could you provide a quote? (Evidently, he changed his mind.)
And I am not concerned with claims about the possibility of moral objective knowledge in God’s mind, nor super-intelligent aliens, nor humans of the 30th Century, if this kind of animals will exist at this date (really, I doubt it).
Ah, but recall the definition of objective. It deals with rational beings with the requisite observations, arguments, etc. It is a hypothetical claim, and if you and I are unaware of the "right" method, should it exist, perhaps we would converge on moral claims.
This is just the same argument over and over. You point out that we do not have a method for deciding moral claims. I agree, and point out that this does not conflict with the hypothesis that such claims are nonetheless objective -- though we lack evidence that this is the case. I've never once said I could show moral realism is true, so there is no contradiction here.
I wonder if our discussion has played out.
Finally, note that the universal imperative “everybody prefers truth to falsehood” you have proposed above, if really universal, it is not a moral imperative. Moral imperatives are referred to my relation to other people.
Right. That's why I have repeatedly referred to it as an objective,
non-moral norm. I use it merely to show that the concept of objective norm is not self-contradictory. I have never called it a moral norm.