Historical slavery

Both secular and religious philosophers going back to Plato had seen the mundane physical world as being far less important than the ideal or spiritual world, so that being right and free in one’s mind was more important than one’s fate in the physical world. Dissipating one’s energies trying to reform the practices of a sinful world was considered less important than bringing one’s own soul into line with spiritual imperatives. However, as a humanistic philosophy began to affect both secular and religious thought, what happened in the mundane physical world began to assume greater importance than it had before in the eyes of intellectuals, philosophers, and religious leaders. Religious minorities, such as the Quakers or the Evangelicals within the Anglican Church, could not simply rely on religious tradition and authority because their very existence was based on a questioning of, and in some cases a break with, those traditions and authorities. These insurgents had to think independently about slavery, as about other things, and derive their own conclusions. The rising class of secular intellectuals in the West could even less rely on the authority of established religious institutions. This did not mean that either secular or religious insurgents were automatically anti-slavery. What it meant was that they both had to evolve some intellectually and morally defensible position because they could not simply base themselves on existing beliefs or practices. Different individuals resolved the issues differently but out of this process came some who began to see slavery as an intolerable evil. 

—Thomas Sowell, Black Rednecks and White Liberals