Hume famously laid out this argument in his Treatise On Human Nature:
In every system of morality, which I have hitherto met with, I have always remark’d, 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 surpriz’d 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 shou’d be observ’d and explain’d; 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.
This Is / Ought argument is the principal objection to any attempt to ground ethics in naturalistic accounts of human nature – human nature IS like this, therefore humans OUGHT to behave like that. You can’t move from the descriptive to the prescriptive, Hume argues.
Now let’s just mention in passing that Hume didn’t follow this rule himself. His most famous quote is: ‘Reason is, and ought to be, the slave of the passions’, which has since been used by many an evolutionary psychologist and social intuitionist to justify moral claims based on anthropological and neuroscientific insights into human emotions.
But let’s ignore that inconsistency, and ask if we can get over the Is / Ought chasm, to arrive at some sort of natural foundation for moral claims.
I’ve written before about how modern communitarians such as Alisdair MacIntyre, Sam Harris, Martin Seligman or Martha Nussbaum are trying to go beyond moral relativism, and to return to an idea of Aristotelian ethics grounded in Aristotle’s idea of human flourishing.
Aristotle, like Plato and the Stoics, built his ethics on a biological account of human nature – on an ‘Is’. The Greeks then said that humans should develop this human nature into its full flowering, which they called eudaimonia, which means ‘highest happiness’, ‘fulfilment’ or ‘flourishing’.
That’s the ‘Ought’, the goal or telos. Philosophy is the bridge from the Is of human nature to the Ought of eudaimonia.
What was the Greeks’ description of human nature, and how do we develop it into eudaimonia?
1) Humans ARE usually unconscious animals driven by animal passions and automatic beliefs. However we ARE also capable of rationality, uniquely so among the animals, which enable us to reflect on our beliefs, passions and behaviour, and therefore we SHOULD develop this uniquely-human higher rationality.
2) Because of humans’ unique higher cognitive powers and their capacity for language, humans ARE capable of knowing themselves, rationally discussing and scrutinizing their beliefs, and thereby also changing their emotions and behaviour (this is the cognitive theory of emotions). Therefore they OUGHT to develop this ability to know themselves, examine themselves and take responsibility for their thoughts, emotions and behaviour.
3) Humans ARE creatures of habit. Therefore, they OUGHT to get into good habits, engraining their rational choices into habits of thought, emotion and behaviour.
The Aristotelians and the Stoics at this point part company. Aristotle said the following:
4A) Humans ARE capable of using their reason to know themselves and choose their behaviour. Therefore they OUGHT to find the golden mean between excesses of emotion and behaviour, so that they think, feel and respond in the right way at the right time. This will lead to eudaimona, or highest happiness.
While the Stoics go from the same Socratic foundation in a different direction.
4B) Humans ARE capable of using their reason to know themselves and choose their behaviour. This reason is a divine gift from the divinely intelligent universe. Therefore they OUGHT to accept the divine will of the Logos, and never feel any negative emotions, which are judgements that things should be other than they are. This will lead to eudaimonia.
5) Humans ARE social and political animals. Therefore to achieve eudaimonia, they OUGHT to engage in the social and political life of their society.
The Stoics added to this:
5B) Humans ARE members of the human race, and therefore they OUGHT to use their reason to widen their affectional attachments to include wider and wider groups of fellow humans, until they feel sympathy with the entire human race (this has been called cultivating cosmopolitanism).
It seems to me that almost all of these Is / Oughts still stand up, and are still supported by psychology and neuroscience. They’re pretty limited in their moral recommendations – they have nothing to say about sexual preference, for example, other than not being excessively into sex. They have nothing to say about abortion. So they don’t necessarily give us an entire moral framework.
But they do give us a natural ethics based on what Richard Rorty called a ‘core self’, or a core idea of human nature. To Hume’s anti-rational argument that reason IS and OUGHT to be the slave of the passions, the Greeks would reply:
Yes, reason often is the slave of the passions. Often our automatic emotional self responds in appropriate ways to our environment, especially if someone has been brought up well.
But often our emotional responses to the world are wrong, and destructive to ourselves and our social, political and natural environment. Our animal nature, upbringing, our family and our society can ingrain false or self-defeating beliefs into us, and this can mean our passions cause us and those around us big problems.
In that situation, we have to use our reason to educate, guide and control our passions. So there are clearly instances when our reason needs to steer our passions. We can’t trust them to simply carry us to eudaimonia.
Indeed, the history of human culture is nothing less than the millennia-old effort to steer the passions through reason, discussion, religion and the arts.