Take this moral proposition:
"I think it's wrong to make cows suffer in slaughter houses."
We could conduct research into the nervous systems of the animal, illustrating these animals 'feel' the same sort of pain we do. We could conduct research into the natural habitat of such an animal, illustrating how it's 'unnatural' to keep cows pent up (leading to physical complications). And so forth and so on, but in the end, it would be our moral judgment that decides. Every premise set forth to support our conclusion doesn't seem to support our conclusion, as there doesn't appear to be any objective method with which to evaluate. Is illustrating how animals suffer a valid premise to support "I think it's wrong to make cows suffer in slaughter houses", in the case of a moral proposition?
What kind of justification would you seek from the proposition I just noted?
By the is-ought principle, your argument must contain a moral premise in order to stand a chance at being
valid. So a justification may come in this way, and I might question it, agree with it, etc. Or you might show that it is logically inconsistent to hold your statement in the background of some other beliefs (Socratic style). A justification as patent inconsistency is a good justification. So I might seek to show that your statement is inconsistent with your other views or that it presupposes some principle that generates other inconsistencies.
Also, I might argue that there's nothing empirical about it. We don't need to delve into the nervous system to determine that animals feel the same sort of pain we do. If we did, then it would not have been until fairly recently that we determined that other humans
suffer the same kind of pain that we do. This is manifestly absurd, and so we should reject the premise that "research into the nervous system illustrates that species X feels the same sort of pain as species Y". We have had and have used evidence (of a certain kind) to determine rightly that animals feel the same kind of pain we do, and that evidence was not based on research into the nervous system. But this is a tangent.
There's a distinction to be had in the concept of justification. "to be justified" and "to give justification" might both be interpreted from the statement "X has justification."
Thus, I might say "X is a justified conclusion" and "X has available justification." The former sense is evaluative while the latter descriptive. Up 'til now, xris has failed to give justification in the latter sense. Justification for his argument is not available in the sense that it has not been given. So it would be silly to speak of evaluating his justification because there's none to speak of.
His arguments are not unjustified; they are non-justified. Any ol' justification will do. But saying of an utterance or expression "it is an opinion" is not to provide a justification for it. Thus, I cannot say his argument is unjustified in the evaluative sense; it lacks justification in the way that a car lacks a driver. I cannot say it is a fast or a slow car until I see it in action, where a driver is essential to that judgment. To say an argument is unjustified in the evaluative sense is just to say it is bad. I am not saying his argument is bad. I am saying it is not an argument.
Again, a moral justification will do. Name calling is not a moral justification. "They are despicable" is not a justification. It presupposes a justification (in order words, evidence). The evidence can be slim, minute, great, multifaceted, etc. Calling someone a criminal is not to give a moral justification but to presuppose one, a justification for one's claiming such.
So, onto the search for moral premises, then, yeah? "All living things ought to be treated as equal" or "Cows share some property P with things we typically do not put into slaughter houses (say, humans)."