I am interested in understanding the psychology of how humans could have convinced themselves that slavery is alright. To this end, I am looking for any recorded public debates on the subject from any time period after or during the 17th century.
For millenniums, humans did not need to convince themselves that slavery is allright. Nobody objected slavery in general, people just did not like to be slaves personally. Relatively modern idea that “all man are created equal” demanded explanation of slavery existence. Last and most vocal proponents of slavery in civilized world were USA slave owners.
1. John C. Calhoun
February 06, 1837
“I hold that in the present state of civilization, where two races of different origin, and distinguished by color, and other physical differences, as well as intellectual, are brought together, the relation now existing in the slaveholding States between the two, is, instead of an evil, a good–a positive good” http://teachingamericanhistory.org/library/document/slavery-a-positive-good/
2. Supreme Court of the United States
Dred Scott v. Sandford (Dred Scott decision), March 6, 1857
"They had... been regarded as beings of an inferior order, and altogether unfit to associate with the white race, either in social or political relations; and so far inferior, that they had no rights which the white man was bound to respect; and that the negro might justly and lawfully be reduced to slavery for his benefit. He was bought and sold, and treated as an ordinary article of merchandise and traffic, whenever a profit could be made by it" https://en.wikipedia.org/wiki/Dred_Scott_v._Sandford#Opinion_of_the_Court
3. Alexander H. Stephens, vice-president of CSA
“Corner Stone” Speech
Savannah, Georgia, March 21, 1861
“The prevailing ideas entertained by him (Thomas Jefferson - A. Barhavin) and most of the leading statesmen at the time of the formation of the old constitution, were that the enslavement of the African was in violation of the laws of nature; that it was wrong in principle, socially, morally, and politically. It was an evil they knew not well how to deal with, but the general opinion of the men of that day was that, somehow or other in the order of Providence, the institution would be evanescent and pass away. This idea, though not incorporated in the constitution, was the prevailing idea at that time... Those ideas, however, were fundamentally wrong. They rested upon the assumption of the equality of races. This was an error. It was a sandy foundation, and the government built upon it fell when the “storm came and the wind blew.”
Our new government is founded upon exactly the opposite idea; its foundations are laid, its corner- stone rests, upon the great truth that the negro is not equal to the white man; that slavery subordination to the superior race is his natural and normal condition. This, our new government, is the first, in the history of the world, based upon this great physical, philosophical, and moral truth” http://teachingamericanhistory.org/library/document/cornerstone-speech/
Remember that all societies have institutionalization. People today who are considered mentally retarded, criminal, and so forth are placed in an institution. Indeed they are sometimes given work as in slavery. Furthermore, liberal societies could be considered to be wage slave societies as well.
You can read more here about how Marx considered slave labor to be no worse than any other type of labor.