There are moral propositions. From Michael Huemer

ETHICS IS A PRIORI

That knowledge of moral principles is also a priori follows from the following two theses:

(1) Moral principles are not observations. The content of every observation is descriptive.

That is, you do not literally see, touch, hear, etc. moral value.

(2) Moral principles can not be inferred from descriptive premises. This principle is just an instance of the general fact that you cannot derive a conclusion within one subject matter from premises in a different subject matter. Just as you cannot expect to derive a geometrical conclusion from premises in economics, or derive a conclusion about birds from premises that don't say anything about birds, you should not expect to derive a conclusion about morality from non-moral premises.

(3) Therefore, moral knowledge requires an a priori basis.

(1) There are moral propositions.
(2) So they are each either true or false. (by law of excluded middle) (3) And it's not that they're all false. Surely it is true, rather than false, that Josef Stalin's activities were bad. (Although some communists would disagree, we needn't take their view seriously, and moreover, even they would admit some moral judgement, such as, "Stalin was good.")
(4) So some moral judgements correspond to reality. (from 2,3, and the correspondence theory of truth)
(5) So moral values are part of reality. (which is objectivism)

And we have some knowedge of moral propositions

As far as I can tell, this claim follows from the proposition that there is moral knowledge, just as some analogous, more general claim follows from the premise that there is any knowledge at all. For if we know some particular thing, then there are only three possibilities as regards its justification:

(a) it is infinitely regressive. That is, there is a reason for it, and a reason for the reason, and then a reason for that, and so on indefinitely.
(b) it is circular. That is, it is based on some chain of reasoning in which something ultimately is supposed to (directly or indirectly) justify itself.
(c) it is foundational. That is, the item of knowledge itself is, or is based upon, a fact that is known directly and without any argument or reason given.

We know moral principles not based on Empiricism. Empiricism--roughly, the idea that all 'informative' knowledge, or knowledge of the mind-independent, language-independent world, must derive from sense perception--has been fashionable for the last century, though less so, I think, in the past decade. I cannot do justice to this subject here; nevertheless, I will briefly report how things seem to me. First, it is so easy to enumerate what appear on their face to be counter-examples to the thesis of empiricism, and at the same time so difficult to find arguments for the thesis, that the underlying motivation for the doctrine can only be assumed to be a prejudice. Second, I think that in the last several years, if not earlier, the doctrine has been shown to be untenable.⁽²⁹⁾ Here, I will give two of the better-known counter-examples to empiricism.

First example: Nothing can be both entirely red and entirely green.⁽³⁰⁾ How do I know that? Note that the question is not how I came upon the concepts 'red' and 'green', nor how I came to understand this proposition. The question is why, having understood it, I am justified in affirming it, rather than denying it or withholding judgment. It seems to be justified intuitively, that is, simply because it seems obvious on reflection. How else might it be justified?

A naive empiricist might appeal to my experiences with colored objects: I have seen many colored objects, and none of them have ever been both red and green. One thing that makes this implausible as an explanation of how I know that nothing can be both red and green is the necessity of the judgment. Contrast the following two statements:

Nothing is both green and red.
Nothing is both green and a million miles long.

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The way I wrote about this before is this: There are universals. Morals are universals. Function of reason is to recognize universals. How do we know there are universals. Prime facie. E.g. There are trees.