I expect that probably what he's getting at is Seneca's argument in favor of humane treatment of slaves, though some other philosophical schools had similar ideas about slaves. The most well known formulation of Seneca's argument is in Epistle 47, in which Seneca famously argues that slaves are human beings the same as their masters, that slavery is not indicative of a failure on the part of the enslaved, and that there is nothing inherent to being a free person that makes one morally better than a slave. But Seneca (as often) is taken out of context here. He's not arguing for equality between free persons and slaves, nor is he making the much later Enlightenment argument that human beings have inalienable rights--Seneca doesn't really have an idea of human rights at all. Seneca's argument is typically Stoic. Since fortune dictates the lives of all human beings and we have no control over our own states, and just as every slave may one day become a master so too may every master become a slave. Hence he calls slaves "conservi," fellow slaves, as all men are enslaved to fate. Moreover, his argument isn't really about slaves at all, it's really about the masters. Seneca argues that a master who mistreats his slaves or who lives in excess at the expense of his slaves does moral and ethical damage to his own character. It's similar to his argument in Epistle 7, when he argues that the bloodshed of the midday games is distasteful not because of the loss of life but because it does moral and ethical damage to the audience, which is influenced by the crowd and the spectacle and loses its ability to control its mental state, which is one of the few things that Senecan Stoicism argues we can control about our lives. Seneca's certainly not arguing against the status quo, he's only arguing against a prevailing attitude, not an institution