Actually, you could technically count a slave's food/clothing/housing provision as income, though it wouldn't come up to much.
PS I did find this, but my knowledge of economics isn't enough to appreciate if it is really what I meant. Member of the Anti-Fabulousness League since 1987.
Seriously. At least in certain circumstances, a slave is an investment that must be maintained, rather than someone that can simply be sacked and forgotten about. This is interestingly brought out in a passage I saw quoted in G.E.M de St. Croix's 'The Class Struggle in the Ancient Greek World', referring to the institution of slavery in the US. In this passage, a traveller noted that in a ship being loaded, the black slaves were all throwing the objects into the hold, and the white Irish were at the bottom, catching the stuff and moving it about. On enquiry, he was told: 'No one cares if the Paddies get their backs broken' - because they were hired by the day, and no loss resulted if they were injured permanently. It was different for the black slaves, item of property, chattels ... who, ironically, and unbelievably from our point of view, were better off than the paid labour ...
