The basis of this candidate ethical naturalism is in desires and beliefs, we examine here the distinctions and relations between them and their relations to evolutionary processes.
A desire is an attitude to make or keep something true. Only agents have desires, that is can have an attitude to make or keep something true - by acting on that attitude on the world. This is the core common structure shared by what we could call other world changing attitudes held by agents - appetites, tastes, needs, wants, intentions, preferences, goals, projects, interests and so on. Lets see 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. Now, although we do not know exactly how desire works biologically - this is up cognitive psychologists and neuroscientists to work out - all we need here to develop a theory of value is that 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 would not interact with the world as there would be nothing to motivate them to do so.
A belief is an attitude that something is true. Only agents have beliefs, that is can have an attitude that something is true - by acquiring that attitude from the world. This is the core common structure shared by what we could call brain changing attitudes acquired by agents - convictions, ideas, notions, opinions, positions, views and so on. Lets see how far can we get with just this core concept of belief rather than needing to expand upon this in terms of additional factors such as certainty, accuracy, probability and so on that the other items on the list could incorporate. Now we do not know exactly how belief works biologically - this is up cognitive psychologists and neuroscientists to work out - still all we need to develop a theory of value is that beliefs are descriptive brain states, at the very least. To deny belief in this sense is to deny that there are descriptive brain states, and without such brain states human (and animal) agents could not successfully interact with the world, as they would lack a description (presentation or representation) as to what it is they are interacting with.
The distinction made here between beliefs and desires is that only desires motivate. This is technically called motivational non-cognitivism, meaning desires are not true or false (non-cognitive) and they are attitudes to make something true or false (motivational). By contrast beliefs are cognitive, they are true or false (cognitive) and they are attitudes that something is true or false (descriptive). To state what one wants to make true is a prescription, that is what a desire does. So we could say that beliefs are descriptions of what it is, desires are prescriptions of what could be. Beliefs describe, they do not prescribe. Desires prescribe, they do not describe.
Descriptions of Prescriptions
Given the last statement how can we talk about desires without describing them? Well we can talk about desires by, indeed, describing them. If a belief is a statement of something that is true, that something could be a desire and that it is true that that particular desire exists. A proper description of such a prescription would be a statement of who has the desire -which agents - and what its conditions of fulfillment are. If or when those conditions are met, then the agent's desire is fulfilled. If or when those conditions are prevented or denied, the agent's desire is thwarted. Note also that to describe prescriptions one needs a capacity to symbolize such as language. Non-human agents lack this capacity and so cannot describe their own prescriptions in more than a rudimentary sign or behavioral fashion, if at all. Still, lacking this capacity animals can still be said to have beliefs and desires, operate according to those and be understood as operating accordingly. Indeed, and also, human agents may be unaware of quite a few the desires and beliefs they themselves hold and may be unable to describe some of these - say when their attention is drawn to them - still this does not mean these do not exist, only that the agent is unaware of or unable to describe them.
The Evolutionary Basis of Desire
We are the resultant descendants of over 4 billion years of successfully reproducing survivors. We have a desire for sex - a desire which is an end in itself - that is fulfilled in the successful act of sex, it is a desire-as-end. A side effect of this is procreation, we can comprehend this - foresee it and choose to avoid this side effect, but animals cannot, they just have the desire for sex. Reading any comprehensive naturalist guide on sex describes the extraordinary ways, to us, over how organisms achieve orgasm - e.g. male bumble bees explode in mid air and die! Any individual or population that did not have this desire for sex would not have benefited from this side effect and not have reproduced. We are the evolutionary result of those that did and do have this desire-as-ends, a desire for sex. Similar arguments can be made for other desire-as-ends such as hunger resolved in feeding, thirst resolved in drinking and so on. As biological beings we have a shared set of desire-as-ends.
A Desire to avoid predators
One particular desire-as-ends one that helps in showing the general relation of desire imposing capacities to our belief acquisition capacity is the "desire to avoid predators". When one sees a (relevantly specific) predator one flees, hides or otherwise deals with the threat. This is a desire-as-end, there is no additional abstract notion of "life" that one is trying to keep, this concept is unavailable to animals and they did not need it nor do they have the abstract notion of "predator" per say, this is just our way of describing it (please bear this in mind in what follows). Certain animals scare them and they flee, that is it. That is the presence of a (relevantly specific) predator generates fear feelings, the release of adrenalin (in higher animals) and the flight responses (or equivalent) or, if not, eventually dies. Again we are the result of a long line of successful ancestors that did have a "desire to avoid (relevantly specific) predators".
The Evolutionary Basis of Belief
However this is also related to belief, the belief that there is a predator. This belief is an attitude that something is true, in this case that there is a predator. This can be mistaken - but there are two types of mistake. A false negative is to believe there is a predator when there is not, a false positive is to believe there is no predator when there is. Now there is a different cost to committing these two types of mistake. A false positive has a far higher cost - in terms of injury and death - than a false negative - loss of feeding or mating time or wasted energy in running for no real reason and so on. We have not evolved to be generate perfectly accurate beliefs - and maybe could not - but to have suitable error-minimization belief capacities - a balance between being over cautious and over reckless. And the same argument applies to our belief acquisition capacities in general. That is most, if not all, our unconsidered beliefs are heuristic based on rules of thumb, approximate and pragmatic rather than entirely rational and sound.
The Evolutionary Basis of Evaluative Beliefs
Now we can now make explicit how beliefs and desires interact. An agent acts on its desires, given its beliefs. If an animal believes there is a predator (whether accurate or not) - which it experiences as fear- it will act on the desire to avoid that predator - through the behavior of,say, fleeing or hiding. If an animal does not believe there is a predator (whether accurate or not), it will not act on the desire to avoid that predator - and it is mistaken it might die. This is how evaluative beliefs work, since a belief that there is a "threatening predator" is both a description (accurate or not) of the state of the world and a factor and trigger for a pre-existing desire. If there was no pre-existing desire, the animal would have no basis to respond to the predator qua predator - it would not mean that to the animal and it would most likely die. That is "predator" is a value-laden term, which we can make clear using language but this still applies even when believed and responded to via desire, by non verbal animals. Indeed the response is the evaluation.
The Extinction of the Dodo
The Dodos was a case in point, having no natural predators they lost that desire-as-end to avoid predators and so were unable to deal with new predators - man and the animals brought with him - without that desire. Their beliefs as to what was occurring in their world might have been descriptively quite accurate but they did not have the capacity for these beliefs to be evaluative in this way and so failed to recognize the presence of predators. "Predators" had no meaning for them without the desire to avoid predators, that is they lacked or failed to trigger the appropriate biological predator detectors and behavioral responses, which had fallen into disuse. Lacking the capacity to quickly adapt to these new circumstances they became extinct.
Post-Adaptive Beliefs and Desires
And this leads to final key contribution of evolution to the biological belief and desire capacities of the brain. The Dodos not only did not have enough inter-generational time to regenerate their desire to avoid predators - a desire they had but had lost and but also lacked an intra-generational capacity to develop new desire-as-ends. If they had the latter they might have adapted quick enough to survive. Given two competing populations, one of which has a heritable variation of adapting faster than inter-generational time to new threats - amongst other things - and the other population lacks this heritable variation, then given new threats to both populations the first is more likely to survive and the latter more likely to go extinct. What is inherited is not a specific set of desire-as-ends but the capacity to develop new desire-as-ends (and the associated generalizing capacity to have heuristic beliefs about anything including beliefs themselves). If the Dodo had had that, then they could have quickly learnt to deal with this new threat - including learning the "idea" of a predatorial threat. They did not have this but we do. We are the result of having such heritable variations, although we are not alone, it is particularly developed in our brains. That is our brains have the post-adaptive capacity to develop new beliefs and desires faster than animals who rely on inter-generational (evolutionary) time to develop such capacities. Still this capacity is an evolved capacity and only animals who have this evolved capacity can do this and in our case, amongst other things, it has enabled us to develop culture. More on this in the next post.