This post addresses the issue of how we could discover existing moral facts, or acquire any moral knowledge at all - a big problem for moral realists.

Given that moral facts exist, and are arranged reasonably, there must be some possible ethical formulations that would at least be partially correct and compatible with all of the relevant moral facts. Consider the potential moral fact: “insulting sentient creatures is wrong”. We could come up with the formulation: “it is wrong to disparage rats”. This formulation is true, yet it does not capture the entire moral fact. Nonetheless, we have a particular, true moral claim that is totally compatible with the relevant moral fact(s).

In terms of processes, we have a better chance of achieving such true moral claims if we start with a reasonable, intuitive assumption - even if our intuition is wrong - and then use reason to create a formulation. Incoming, admittedly insane example:

If I were to tell you that there are a large number of balls in a bag, and that there are balls that are painted both red and white, and that those that are not red and white are painted garish colors, and that those that are painted garish colors are not the same size as the balls painted red or white, you might assume that the balls that are painted garish colors could only be bigger or smaller than the balls that are both red and white. This is not a correct assumption, as all of the garish balls could be larger than all of the red balls and smaller than all of the white balls, or vice versa. But that doesn’t really matter when considering the following:

If I then told you that the white balls are bigger than the red balls, and then asked if more balls are bigger than the red balls or smaller than the red balls, you could come to the correct conclusion that it is more likely, based on the earlier, incomplete assumption, that there are more balls bigger than the red balls than smaller than the red balls, as all of the white balls are bigger than the red balls and the garish balls could be bigger or smaller than the red balls (the earlier, incorrect assumption would dictate this in light of the new fact). Therefore, one can move from a reasonable - if incomplete or partially incorrect - assumption to a more specific conclusion that is correct, all the while not looking inside the bag (the exact way such a process would work in reality is unknown; I am just using this as an example of a hypothetical process. That our own attempts to nail down such a process would be bumbling is a semi-valid counter-argument).

How is this applicable to discovering moral truth? If we have an assortment of moral propositions that could be true or false related to general moral facts based on some sensible distribution, we can rule out certain propositions via the application of logic or come to some conclusions about what could be right given some sort of discovered structure of morality, even if we start with some intuitive and seemingly reasonable but not necessarily correct or complete first principles.

Such a structure could be semantic, empirical, or even mathematical - we just need some discernible patterns and some basic knowledge of how moral claims work.