What was the rationale for slave-holding?

There are two rationals for slavery. One is obviously cheap labour. Industrialization essentially ended slavery, a machine can work with out getting tired, it dose not revolt on you and try to kill you and it dose not consume vast amounts of food and it never tires. Think about it this way, one of the reasons for slavery ending in the south no joke was the cotton gin. There was no need to keep a vast amount of men to get the fibres out of the husks from cotton plants. The other reason for slavery, in ancient times....10 slaves forced manual labour freeded up one man to produce special goods like metal work, alchemy (chemistry in the modern sense) or any other of the sciences.