Here’s Xaro’s argument:

“We curse the rain when it falls upon our heads, yet without it we should starve. The world needs rain … and slaves. You make a face, but it is true. Consider Qarth. In art, music, magic, trade, all that makes us more than beasts, Qarth sits above the rest of mankind as you sit at the summit of this pyramid … but below, in place of bricks, the magnificence that is the Queen of Cities rests upon the backs of slaves. Ask yourself, if all men must grub in the dirt for food, how shall any man lift his eyes to contemplate the stars? If each of us must break his back to build a hovel, who shall raise the temples to glorify the gods? For some men to be great, others must be enslaved.”

This was an argument that was used historically to justify slavery as a “positive good,” in the leadup to the American Civil War. People like George Fitzhugh would argue that, without slavery, you wouldn’t have Plato or Aristotle or all the other wonders of Ancient Greece or Rome, or indeed civilization itself.

Anti-slavery writers responded to this particular line of attack by arguing that free labor was inherently more productive than slave labor, following Adam Smith:

“The wear and tear of a slave, it has been said, is at the expense of his master; but that of a free servant is at his own expense. The wear and tear of the latter, however, is, in reality, as much at the expense of his master as that of the former. The wages paid to journeymen and servants of every kind must be such as may enable them, one with another, to continue the race of journeymen and servants, according as the increasing, diminishing, or stationary demand of the society may happen to require. But though the wear and tear of a free servant be equally at the expense of his master, it generally costs him much less than that of a slave…It appears, accordingly, from the experience of all ages and nations, I believe, that the work done by freemen comes cheaper in the end than that performed by slaves.”

“The liberal reward of labour, as it encourages the propagation, so it increases the industry of the common people. The wages of labour are the encouragement of industry, which, like every other human quality, improves in proportion to the encouragement it receives. A plentiful subsistence increases the bodily strength of the labourer, and the comfortable hope of bettering his condition, and of ending his days perhaps in ease and plenty, animates him to exert that strength to the utmost.” (Work of Nations, Book 1, Chapter 8)

Free laborers work hard because their labor can directly improve their material standard of living and, ultimately, the opportunity for upward mobility. Because a slave is fixed in position and cannot hope to be anything but a slave, they work only hard enough to avoid a whipping. Thus, anti-slavery writers argued, free societies are more productive than slave societies, and would produce a greater surplus to invest in the refinement of civilization than slave societies could.

A second line of argument follows more Rawlsian lines. Since, in a slave society, freedom and slavery are ultimately an accident of birth, many people with the talents to make great advancements in the arts and sciences, trade and industry, politics and warfare, all the occupations and professions denied to the slave, would be born into slavery and thus be unable to share their gifts with the world. Whereas in a free society, all are free to pursue their dreams and ambitions, and civilization benefits from the additional contributions of those who would have otherwise been slaves.