So I want to explain something that was well known in the Middle Ages but since then forgotten:that Reason recognizes universals. What is a universals? Lets say I have two blank white pieces of paper in front on me. Do they have something in common? Yes. Whiteness. So Whiteness is a universal. Something that particulars have in common. Do universals exist? If you think so, then you are a realist. There are two kinds of realists. One that holds universals do not depend on particulars, and the other that hold they do. Plato was the first kind. Aristotle, the second.
What are some examples of universals? Numbers, colors, laws of physics. Moral principles are also examples of universals. They are rules that apply to particular situations. Not rules of "must" but rules of "ought". Never the less they are still rules.
Reason can recognize these rules. For that is the function of reason--to see things in common among particulars.
So we get what was fairly well known in the Middle Ages: that reason recognizes moral principles and that Torah is meant to make us aware of moral principles that are objective.
[You can see this more in detail in Professor Michael Huemer's writings.]
[One important point here is that there is no reason to exclude reason from the "synthetic a priori".This original idea came from Hume who held reason can do nothing but locate about contradictions in definitions. He was a teacher of Euclidean Geometry so he got this idea from there. But it is not the case that he showed this to be true. he just asserts it
