I think you are right that I was misusing the phrase "...by definition". What I was trying to get at was this--when we first had the urge to call things wrong or immoral (and when we have it today) we have in mind something in the real world. If we were to win a race and the prize was given to someone else, we would call it unfair, and that's where the word comes from. That makes the meaning of "unfair" pretty solid. The person giving the prize would clearly by factually incorrect if they said "it may be "fair to you" that the fastest runner gets the 1st place prize, but according to me it is fair to give it to my son".

So what I was suggesting then was that "killing innocent children for fun" is a paradigm case of something that is wrong. Similar to how being burnt by fire is a paradigm case of something that is painful. That since our conception of pain and fairness and wrongness are built off of such cases, they have to be discussed in that framework. To do otherwise is use the same words but act like they refer to something different, which is why I said that about definition. And I think this shows that moral claims are things that can be true or false. What you are talking about is a different problem. For example, astrological claims are things that can be true or false, but they fail at being factual. So it becomes the much tougher question of when we are justified in believing something.

I don't think anyone would argue that we aren't justified in believing something is painful (even though some people may be tougher or less sensitive than others). But I think that is because it is not at all a confusing topic. Whereas morality is something that people can become confused about, and thus there is widespread disagreement. But I generally agree with the SEP's summary:

Some moral realists argue that the disagreements, widespread as they are, do not go very deep—that to a significant degree moral disagreements play out against the background of shared fundamental principles with the differences of opinion regularly being traceable to disagreements about the nonmoral facts that matter in light of the moral principles. On their view, the explanation of moral disagreements will be of a piece with whatever turns out to be a good explanation of the various nonmoral disagreements people find themselves in.

Other moral realists, though, see the disagreements as sometimes fundamental. On their view, while moral disagreements might in some cases be traceable to disagreements about nonmoral matters of fact, this will not always be true. Still, they deny the anti-realist's contention that the disagreements that remain are well explained by noncognitivism or by an error theory Instead, they regularly offer some other explanation of the disagreements. They point out, for example, that many of the disagreements can be traced to the distorting effects of the emotions, attitudes, and interests that are inevitably bound up with moral issues. Or they argue that what appear to be disagreements are really cases in which the people are talking past each other, each making claims that might well be true once the claims are properly understood (Harman 1975, Wong 1984). And they often combine these explanatory strategies holding that the full range of moral disagreements are well explained by some balanced appeal to all of the considerations just mentioned, treating some disagreements as not fundamentally moral, others as a reflection of the distorting effects of emotion and interest, and still others as being due to insufficiently subtle understandings of what people are actually claiming. If some combination of these explanations works, then the moral realist is on firm ground in holding that the existence of moral disagreements, such as they are, is not an argument against moral realism. Of course, if no such explanation works, then an appeal either to noncognitivism or an error theory (i.e. to some form of anti-realism) may be the best alternative.
Quote Originally Posted by TheStranger
however i think what we need to establish first is this, in the case that there would be moral facts, what kind of facts would they be? most people would say that they are metaphysical facts, but perhaps you are a moral realist and you would say that they are empirical facts and they can be determined by experiment.
I'm afraid I can't answer this properly. It's a difficult question, compared to moral realism which can be arrived at merely by rejecting sophistry. I think W.D. Ross gave a good description of how we acquire moral knowledge:

That our responsibilities are self-evident does not entail that they are obvious to everyone who reflects on them. Ross maintains that a responsibility is self-evident ‘not in the sense that it is evident from the beginning of our lives, or as soon as we attend to the proposition for the first time, but in the sense that when we have reached sufficient mental maturity and have given sufficient attention to the proposition it is evident without any need of proof, or of evidence beyond itself. It is self-evident just as a mathematic axiom, or the validity of a form of inference, is evident’ (RG 29; also 12, 32). The analogy with mathematics is instructive, for we acquire our moral knowledge in the same way we acquire knowledge of mathematical axioms. We apprehend that 2+2 = 4 by apprehending that 2+2 matches makes 4 matches and that 2+2 balls makes 4 balls, and so on. We apprehend the algorithm in the particular cases after repeated exposure to particular instances of its application, by a process of intuitive induction (FE 170). We apprehend that it is prima facie right to keep promises by apprehending that it is prima facie right to fulfill this or that particular promise. ‘What comes first in time is the apprehension of the self-evident prima facie rightness of an individual act of a particular type. From this we come by reflection to apprehend the self-evident general principle of prima facie duty’ (RG 33; also FE 170).
But in general it's a complicated subject that's a little beyond me. But I find the practical theory's like ross's and aristotle's virtue ethics far more sensible and genuine than many other attempts which are often biased, sophist, or attempt to be too systemized.

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On a personal aside from the argument, I doubt the sincerity of people who claim they don't think "killing innocent children for fun is wrong" is factual, and that we are justified in believing it.