On 1st August 1834 slavery was abolished in most of the British Empire, as the Slavery Abolition Act (1833) came into force. (It only applied in India after the 1843 Indian Slavery Act.)
There were many rationales used to support slavery. One common one was Aristotle’s theory of the “natural slave”. In the Politics, Aristotle said:
For that some should rule and others be ruled is a thing not only necessary, but expedient; from the hour of their birth, some are marked out for subjection, others for rule…
In other words, it was right that some people were slaves and others their masters. Such natural slaves were lacking certain qualities which would enable them to rule themselves. This justified “chattel” slavery, where the person and all their descendants were property in perpetuity.
This theory, though criticised by some, was widely accepted. Even those who did not accept it argued for slavery on other grounds. Locke, despite asserting that “every Man has a Property in his own person” (in his Second Treatise, §27), and calling slavery “vile and miserable” such that he cannot believe an Englishman would argue for it (Second Treatise, Introduction), justifies some forms of slavery. Locke defined slaves as (§85) “Captives taken in a just War […] by the Right of Nature subjected to the Absolute Dominion and Arbitrary Power of their Masters” and as such cut off from civil society. Moreover despite his theoretical requirement for a just war, in practice Locke tolerates chattel slavery. In Constitutions for Carolina Locke writes: “Every freeman of Carolina shall have absolute power and authority over Negro slaves, of what opinion and Religion soever” (see Sypher, 1939 and a discussion on Locke’s attitude to slavery in the notes of the SEP article about him).
The view of slavery as “vile” slowly started to spread in the early 18th century. Steel portrayed slaves sympathetically in the Spectator, though he falls short of attacking the institution, unsurprisingly since he owned a plantation in the West Indies through his wife. Hans Sloane’s account of harsh punishments for slaves also had an effect. However here too we should not read too much into Sloane’s accounts: he also derived income from sugar plantations and says nothing to suggest chattel slavery should be abolished (see more here.)
Also writing in the early 18th century, Hutcheson’s theory of morality (following the lead of Shaftesbury) was based on the idea of a moral sense, which allowed us to approve of moral actions and disapprove of immoral ones. Moral actions are rooted in benevolence – a feeling of goodwill towards others. Unlike Locke (and like Hume), for Hutcheson emotion is our guide for moral behaviour, not reason.