David Hume provided the classical statement of the view that moral values are the product of certain natural human desires. Hume argued that human behaviour is a product of passion and reason. Passions set the ends or goals of action; and reason works out the best available means of achieving these ends. Under this view, passions determine what humans find agreeable, desirable and valuable. Values are projected onto the world of objects and events by the passions in much the same way that colors are projected onto the world by the visual system. For this reason, Hume is said to have a subjectivist or projectivist theory of value. Whereas Hobbes argued that natural human passions were entirely selfish and that morality was an artificial invention, Hume argued that human nature included some passions – such as familial affection, sexual fidelity, sympathy and pride -- that promoted the common good. Hume called these moral passions, and argued that they constituted the basis of human morality.
[...] Although Hume’s theory is primarily a meta-ethical account of the nature and ontological status of morality, it segues into normative or substantive ethics in the following way. The ends supplied by the passions provide the first premises of chains of means-end reasoning. [...] It follows from this view of human psychology – and in particular, from this instrumentalist account of reason – that, in the absence of any passions, desires, or ends, reason alone cannot tell you what you ought to do. [...] one cannot go on justifying statements forever, one must come to a stop somewhere. And where one comes to a stop constitutes one’s meta-ethical theory. Theologians stop at divine commands, relativists stop at social conventions, Humeans stop at certain passions.
[...] The Humean-Darwinian argues that humans are equipped with a suite of adaptations for cooperation, that these adaptations constitute what have been called the moral passions or moral sentiments, and that these adaptations determine what people deem morally good and bad. If one accepts this argument, it makes no sense to complain that evolution may have explained why humans find certain things morally good, but it cannot tell us whether these things are really morally good or not. It follows from the premises of the argument that there is no criterion of ‘moral goodness’ independent of human psychology, and hence this question cannot arise.
Oliver Curry, Who’s Afraid of the Naturalistic Fallacy?, Evolutionary Psychology, 2006. 4: 234-247 [pdf]