system
Given the following post, respond as if you're commenting on Reddit:
human
Moral Realism: How can the normative affect the non-normative? How can we know of the normative by non-normative means?

Similarly to substance dualism within the philosophy of science debate, moral realism seems to invoke a sort of dualism. 

There are normative ("Killing is wrong") and non-normative properties and utterances ("Jim kills Tom") which are both real, and normative properties seem to affect non-normative ones in particular ways. After all, assuming moral realism to be true, we don't banish normative properties to an isolationist existence within the normative realm never to affect anything non-normative, e.g. whether Jim actually kills Tom or not, as in whether his neurons are fired in such a way or not that he actually does kill Tom or not.

As moral realists, we want "Killing is wrong" to affect "Jim kills Tom" (preferably, in a way for the latter to be wrong as Jim refrains from killing) and we don't want "Killing is wrong" to be purely subjective or even non-normative, e.g. as in "My neurons are arranged in such a way that I dislike killing".

However, there is no "ought" value of neurons or the such. It either fires or it doesn't. How do normative properties affect non-normative ones? How do they determine or increase the likelihood of neurons to be fired in such a way that Jim kills Tom or not? Or even more importantly: How do we even know that "Killing is wrong" in the first place if our whole capacity of knowing is based on a very much non-normative instrument, the brain, consisting of very much non-normative entities, neurons, who either fire or not, but surely don't "ought" to fire or not. How can normativity arise from such non-normativity?
gpt
There's a lot to say here. First, not all realists agree that normative and natural properties are distinct. Naturalist normative realists argue that normative properties just are natural properties. But you seem like you're assuming some form of non-naturalist normative realism that denies this, so let's work with that.

This leads to the second thing, which is that no non-naturalist normative realist thinks that normative properties have causal powers that work at the physiological level. It's our *beliefs about what we have reason to do* that move us to act, not *the reasons themselves.* The reason that we are moved by our beliefs about what we have reason to do is that we're rational agents.

Some normative realists have argued that normative properties themselves figure in causal explanations (as opposed to our beliefs about normative properties). For instance, common examples include that Hitler's depravity led him to orchestrate a genocide, injustice causes revolutions, and trust engenders friendship. But usually those normative realists who accept these views are naturalist normative realists who think that depravity, injustice, and trust just are natural properties, and hence there's no puzzle about how they can cause people to do things.

Regarding your remark about how we know that killing is wrong since the brain is a physical mechanism, the framing here is problematic. Obviously our brain is a physical mechanism, but it's not your brain that grasps and responds to reasons. It's rational agents that grasp and respond to reasons, and while our rational agency depends in someway on our brains, it's a mistake to conflate the two.