Slavery has existed as the most extreme form of unfree labour in most human societies. It has assumed a myriad of legal, economic and social forms, and still exists today in many countries. Slavery has not only existed in almost all human societies, but has historically also done so without arousing outrage or abhorrence, or undergoing any radical challenge to its existence. Even the Greek philosopher Aristotle had argued that for some men, slavery was natural to their being.

In the centuries between Aristotle and the Enlightenment no one argued that the slave's lot was enviable, and many had pleaded for the humane treatment of slaves. But it was not until the eighteenth century that there were more than isolated attacks on the institution of slavery itself. The complex history of the growth of radical opposition to slavery as an institution from around the time of the French thinker Montesquieu's attacks in his 1748 De l'esprit des lois, will be one of the main themes of this chapter. Even so, it was only from the 1770s onwards that compassion for the lot of the slave began to coalesce into organised pressure groups such as the Société des Amis des Noirs (Society of the Friends of Black People), which attacked the institution of slavery itself. In the young United States of America, this growing feeling was sometimes put into practice. Quakers in Pennsylvania and Massachusetts refused to hold slaves. In Pennsylvania gradual emancipation began in 1780, and in Rhode Island and Connecticut in 1784. There were bans on participation in the slave trade in Connecticut, Massachusetts, New York and Pennsylvania in 1788, and in Delaware in 1789.