When travelling on the river Ohio in 1841, Abraham Lincoln was struck by the sight of twelve shackled slaves who were being taken down river to be sold. ‘A small iron clevis’, he later wrote in a letter describing his experience, ‘was around the wrist of each, and this fastened to the main chain by a shorter one at a convenient distance from the others: so that the negroes were strung together precisely like so many fish upon a trot-line.’ He thought of the way the slaves had been uprooted from their homes, of the families and friends they had left behind, of the cruelty they would endure from their new owners, finding himself at a loss as he did so to understand how the slaves could continue in such a plight to seem so cheerful. Lincoln was long disturbed by the episode, and as he recalled the sight to a fellow traveller in another letter fourteen years later he remarked what ‘a continual torment’ the image of the shackled slaves had been to him ever since.

In the Americas slavery was destroyed with astonishing speed. From 1807, when Britain placed a ban on the transatlantic slave trade, to 1888, when the last slaveowning State in the New World, Brazil, abolished slavery, less than a century was required to eradicate an institution that for hundreds, even thousands, of years had gone unchallenged until early in the eighteenth century philosophers and religious thinkers began to raise questions about the moral legitimacy and acceptability of slavery.