David Hume
A statue of David Hume sits in front of St Giles Cathedral in Hume's hometown, Edinburgh. (Richard Croft)

With all this talk about justice and rationality in ethics, what should we say about the role of passion? David Hume, a moral sentimentalist, firmly believed morality couldn’t be defined without it. Morality, he said, can’t be boiled down to external or objective facts, discoverable through reason or other modes of representation. Instead, morality is something found within us, whereby our moral sense—our response to our perception of things—allows us to define right and wrong, not empty legalities.

Why did Hume think this? To understand his position we first need to consider his ‘science of man’, which focuses on our perceptions of things and their relations. To him our perceptions come in two forms: impressions (sensations, desires, passions, emotions) and ideas (copies and combinations of impressions). With this philosophy-of-mind strategy, Hume built a desire-based theory of ethics, wherein the foundations of moral beliefs are born in impressions.

Reason alone, he thought, cannot determine a moral action but our passions can. So while it’s unreasonable of me to prefer the destruction of the whole world to the scratching of my finger, it’s only immoral if I assign value to human life. Hence what we value defines the morality of our actions; and what we value is born in our experience of the world through our impressions and ideas.

'Take an action allow'd to be vicious … Wilful murder, for instance. Examine it in all lights, and see if you can find that matter of fact, or real existence, which you call vice. In whichever way you take it, you find only certain passions, motives, volitions and thoughts. There is no other matter of fact in the case. The vice entirely escapes you, as long as you consider the object … You never can find it, till you turn your reflexion into your own breast, and find a sentiment of disapprobation, which arises in you, towards this action. Here is a matter of fact; but 'tis the object of feeling, not of reason. It lies in yourself, not in the object.' — David Hume, A Treatise of Human Nature

What do you think?