by Massimo Pigliucci
Similarly with ethics: we need an evolutionary understanding of where a strong sense of right and wrong comes from as an instinct, and a neurobiological account of how our brains function (or malfunction) when they engage in ethical reasoning. But it is the moral philosopher, not the evolutionary biologist or the neurobiologist, we should check with if we want to know whether a particular piece of ethical reasoning is logically sound or not.
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 ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when all 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, 'tis 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.Nowhere does Hume say that there is an in principle unbridgeable gap between is and ought. He is simply, very reasonably, pointing out that one cannot “imperceptibly” slide from one type of consideration to the other without providing explanations and reasons.