I think the question may be poorly phrased but I'm not sure how to phrase it so I'll elaborate a little bit.

I would like to know when did opposition to slavery begin not just in mainstream political discourse but in general, so for example were there philosophers in ancient Greece, Rome or Egypt who opposed it? Was abolition of slavery not even suggested till the 17th century? Has the treatment and emancipation of slaves always been an ongoing conversation or were they just considered property and nothing else since civilisation began up until a few centuries ago?