In every system of morality, which I have hitherto met with, I have always remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprised to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, it's necessary that it should be observed and explained; and at the same time that a reason should be given, for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it. But as authors do not commonly use this precaution, I shall presume to recommend it to the readers; and am persuaded, that this small attention would subvert all the vulgar systems of morality, and let us see, that the distinction of vice and virtue is not founded merely on the relations of objects, nor is perceived by reason.
A Treatise of Human Nature, David Hume, 1793, book III, part I, section I.
This is the basis of the is-ought gap,
The truth of "is" statements is determined by their correspondence to reality - they are descriptive of reality. "ought" statements on the other hand are defined by their correspondence to the "right desire" in the context of some goal or value.
It is easy enough to bridge the gap in a functional way (my goal is to create a blog post, therefore I ought to write something to go in the blog), but less easy to do so for a moral "ought", since that would depend on making a judgement about the moral value of the goal in the first place (the moral value would not be derived from the existence of the goal, but from the goals morality). Religious apologists may like to try to get around this by using a goal independent of our minds/judgement in the shape of an edict from a deity, but this doesn't really solve the problem, it just moves it from the human to the deity - on what basis did the deity determine that this is a moral thing?
A functional bridge however may show a route to creating a thing that we would perceive as morality, or so it seems to me at least, as follows:
Gene propagation is the fundamental driver of the individual and species.
Among the species level evolutionary "strategies" for maximising propagation is the formation of social species.
Homo Sapiens Sapiens is self aware, sophisticated, social species, of relatively low genetic diversity.
In such a species, wanton killing of other individuals is likely to reduce propagation, as it will inhibit social behaviour.
Therefore individual humans ought to have a strong moral intuition that killing other humans is wrong, but not so strong as to inhibit defence against aggressive out-groups or aberrant individuals.
So in this way, morality would be an emergent property of a social species. An intelligent and sophisticated eusocial species, like bees perhaps, if we could communicate at all, would likely have a different idea of morality, while apes might be rather similar.
