A classic challenge to ethical naturalism is the fact-value distinction. Most of the proposed values do not look like they could be facts. If one accepts this distinction, that values are not facts, then one could end up accepting tentatively that the facts and values are distinct types of stuff - a form of dualism. If this is the outcome of the failure for claimed values to be shown as facts, then this an a posteriori conclusion which is still provisional and revisable based on new evidence. However I think it is a problematic conclusion and not the default - being contrary to the successes of methodological naturalism and science and it is better to assume that those attempts have failed and we do not know if other attempts will work and that is the real default. It becomes worse when instead of this being a conclusion it is taken a starting assumption.This fact-value dualism usually invokes the a priori definitional practice of defining facts and values as mutually exclusive and together exhausting the space of possibilities. Granted this, when an ethical naturalist attempts to develop a theory arguing for specific values as facts, one can claim this is committing a version of the Naturalistic Fallacy, the fallacy of defining or reducing a non-natural property - a value - to a natural property - a fact.
So there are two issues here, the first is to argue that assuming a priori that fact-value dualism is true is a metaphysical mistake. The second is to propose a candidate for a value that is a type of fact
When one defines facts as not values and values as not facts, so that they are mutually exclusive, one is making a metaphysical claim, the assertion of fact-value dualism (in Moore's terms facts are natural and values are non-natural). This makes the challenge of ethical naturalism analytically impossible and is a philosophical not scientific position. Is this a sound assumption? Now Dennet has said "Philosophy is what you do when you don't yet know what the right questions to ask are". It is reasonable to wonder that by keeping to this form of dualism that one is preventing the correct questions being asked.
The Mind-Body Problem So let us first ask is this a false dichotomy? Well this type of challenge has repeatedly occurred in the sciences. The best known is example is the mind-body problem. This was first made explicit by Descartes and so is sometimes called Cartesian Dualism - where "mind" and "body" are two distinct stuffs. In modern terms this is often stated as the physical and the mental being defined as mutually exclusive and .together exhausting the space of possibilities. However, even as these debates persist in the fields of the philosophy of mind, cognitive psychology and neuroscience, the better approach is to say whatever is mental (and there is still dispute over that) is a type or species of physical processes, events or states, say such as a brain state.
Monism versus Dualism
Note this is the position held by those who call themselves monist (even as they disagree with each other) - and as a (methodological) naturalist this is the default position to take in any investigation. Now there are others who openly endorse dualism both there and/or here but then the burden on them is to argue for such a dualism and not to assume it, since such a dualism is contrary to the most successful endeavor to date to understand reality, namely methodological naturalism, specifically science. This is not the place to be distracted by disputes in other areas but only to retain the insight that only be dropping the a priori definitional practice is work able to more clearly proceed in this area.
Mental as a type of physical
So one could say that the mental overlaps the physical and the dispute now revolves around what are the mental process that are physical, other proposed ones being fictions, if they are not (physical) facts. That is the dispute is over the border that distinguishes over what is deemed as mental and physical (facts) and mental and non-physical (fictions). And this was achieved by dropping the practice of defining them a priori as mutually exclusive. Of course, this does not prevent disputants on both sides of accusing the others of dualism and related problematic claims, so one needs to treat this parallel example with caution. Nonetheless given that this is one growing way to deal with that problem , this could be an answer to our issue here.
Values, fact and fiction.
What philosophy can contribute to any empirical inquiry is to ensure that there are no philosophical misconceptions that could bias or doom the endeavor. I argue that assuming fact-value dualism is such a mistake. Instead one can see that the space of facts and values can overlap. It may be that empirical research will show the this overlapping space is tentatively empty and we could provisionally conclude that they are distinct without invoking dualism. Still this position is only provisional and it is allows the possibility of being refuted and error corrected - two of the hallmarks of science or methodological naturalism. I agree that many if not all the previously offered attempts do not fill the overlap and that they are fictions. The challenge of any ethical naturalist - and axiologist, any form of value theory in general - is to show that their model of value is fact not fiction. I will complete this post by making such an argument. However if even you disagree with the to be proposed model the point here still stands, that it is a metaphysical mistake to assume that these are distinct stuffs.
Value as Fact
So what is being proposed as value that is not a fiction here?
We start with desire - an attitude to make or keep something true. This the core common structure shared by what we variously call appetites, tastes, needs, wants, intentions, preferences, goals, projects, interests and so on. How far can we get with just this core concept of desire rather than needing to expand upon this in terms of additional factors such as duration, finality, scope, necessity and so on that the other items on the list could incorporate? First of all desire exists. Now we do not know exactly how and this is up to those aforementioned neuroscientists and so on. Desires are motivational brain states at the very least. To deny desire in this sense is to deny that there are motivational brain states, and without such brain states human (and animal) agents could not interact with the world. And this is quite consistent with any conception of determinism.
Given that desires interact via agent's actions with the world, these desires - states of brains - serve to change the states of the world via actions of the agent or other agents. And certainly such states of the world exist. This is not to deny that some desires attempt to make an impossible state of the world, still many desires are successful in achieving such a suitable change, where we can say the desire is fulfilled. Others are not, not because they are impossible, but due to a myriad of real confounding factors, leading to the desire state of the world failing to obtain, where we can say the desire is thwarted.
So is value in the desire? No. The desire is the capacity of the agent to dynamically impose value on the world. So is the value in the resultant state of the world? No it is just a state of the world. The value is in the interaction, or imposition itself, including and between the desire and the target of the desires, the state of the world to be obtained. One could say the value is in the relation between the desire and the state world. It is not anything additional, it just is this quite physical and material interaction. The use of such relational reasoning is quite standard and unexceptional in science and methodological naturalism and indeed would be impossible without it. This approach assumes there is nothing special in the challenge here, difficult yes given the history of work in this area but not in any way special. So the value is in the interactions between the states of brain and states of world - as to whether the desires state of the world obtains. Thats is valuing is primary and is the process, "value" is the derivation of this process. Or value is what leads to where the state of the world obtains - the desire is fulfilled, disvalue what leads to where it does not - the desire is thwarted.
So desires exist, states of the world that are the targets of desire could or do exist, and the interactions that affect these state of the world exist. That is these are all physical and a material. The empirical challenge is to ascertain these facts and no others are required. That is there may be easy or hard epistemological challenges (how one knows) to stating or determining the ontological (what there is ) facts, but these facts exist regardless. And one of these facts is a value, the value in the interaction leading to desire fulfillment or thwartment.
Finally we have only spoken about generic value not any particular form of value such a prudential,moral or aesthetic value. However the challenge at this stage was to make a case for value as fact, once that is made one can proceed to develop other forms of value based on such facts.