Was in a conversation with someone who argued that slaves don't produce surplus value, that surplus value is unique to a worker-owner relation as in liberalism. Is anyone familiar with this idea? It didn't make a great deal of sense to me. One would think that slaves would give you as much if not more surplus value than workers you have to pay.
Surplus value = the value generated - cost of labor power
Slaves do create much more surplus value than workers. Remember that earlier supporters of classical liberalism were slave owners or supported the slave trade
Note that "slaves would give you as much if not more surplus value than workers you have to pay" is one of the central problems here. They surely would give more surplus value in %, but in absolute values they would only do so in very primitive level of development, and even then not necessarily since even in antiquity slave labour was often considered inefficient.
I guess you could make a really stretched semantic argument that surplus isn't the correct term, as it implies that the labourer also receives some of the value of their labour. Even so, the labourer produces something of value, which an owner then reaps the rewards of. That's surplus value.
I guess you could also make a historical argument in that the concept of surplus value only came about contemporaneously to wage labour, but that's also quite weak. Value and labour existed before wage labour, so surplus value also existed, regardless of when it was first formally conceptualized.