The circle of sympathy
On the axis: Partiality · human sympathy
Our concern for others weakens as they grow distant from us, in space, in blood or in likeness: should morality build on this natural slope, or correct it like an inborn prejudice?
Justification
Sympathy, the source of moral approval, is by nature unequal: we feel more keenly for those near us than for the distant. Hume acknowledges this limited sympathy as a primary fact of human nature; the rules of justice and institutions serve precisely to correct the effects of this partiality, without abolishing it. The leaning is thus partialist, tempered by an artificial corrective.
A Treatise of Human Nature, III, part II, sect. 1-2 (the origin of justice, limited sympathy)
Justification
Hume credits animals with learning and with reasoning from experience, and grants them the gentle treatment that the "laws of humanity" prescribe. Yet he separates these duties of humane treatment from the obligations of justice: imagining rational creatures entirely incapable of resisting us, he grants them humane treatment without placing them under the protection of justice. Their inability to constrain us, rather than any lack of sentience, explains that exclusion. Benevolence thus reaches beyond the human species without extending to equal consideration for all sentient beings.
A Treatise of Human Nature, I, III, 16 · An Enquiry Concerning the Principles of Morals, III, 1
Anchors: Hume · Singer
leads to: Moral circle