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Proslavery is an ideology that perceives slavery as a positive good.
Accordingly, those who are as different [from other men] as the soul from the body or man from beast—and they are in this state if their work is the use of the body, and if this is the best that can come from them—are slaves by nature. For them it is better to be ruled in accordance with this sort of rule, if such is the case for the other things mentioned. For he is a slave by nature who is capable of belonging to another–which is also why he belongs to another–and who participates in reason only to the extent of perceiving it, but does not have it. 
Thomas Aquinas argued that slavery was not part of natural law, but nonetheless he defended it as a consequence of human sinfulness and necessary for the good of society. He viewed the natural state of humanity as that which had existed prior to the fall of man, in which slavery was non-existent; on those grounds, many commentators see him as rejecting Aristotle's claim that some people were naturally slaves, although it is a matter of controversy as to whether he fully rejected Aristotle's views on the matter. 
John Locke discusses slavery in his Second Treatise of Government. He rejects the idea that a person could voluntarily consent to enslavement, saying "a man, not having the power of his own life, cannot, by compact or by his own consent, enslave himself to any one, nor put himself under the absolute, arbitrary power of another" (emphasis in original).  However, he goes on to argue that enslavement of those who are guilty of capital offences is permissible.  He also defends the enslavement of those captured in war: "This is the perfect condition of slavery, which is nothing else, but the state of war continued, between a lawful conqueror and a captive" (emphasis in original). 
James Farr describes John Locke as "a merchant adventurer in the African slave trade and an instrument of English colonial policy who proposed legislation to ensure that 'every freeman of Carolina shall have absolute power and authority over his negro slaves'" (the Fundamental Constitutions of Carolina).  Farr argues that Locke's theoretical justifications of slavery were inadequate to justify his practical involvement in the enslavement of Africans. He sees this contradiction as ultimately unsolvable:
Locke never addressed, much less resolved, this contradiction. On Afro-American slavery, silence seems to have been his principal bequest to posterity. Locke's silence is all the more difficult to fathom inasmuch as in the Two Treatises he developed a general theory and justification of slavery for captives taken in a just war... I hope to show that this theory is woefully inadequate as an account of Afro-American slavery and, further, that Locke knew this... Locke's silence about the Afro-American slave practices that he helped forward remains profoundly unsettling and poses one of the greatest problems for understanding Locke as a theorist and political actor 
While Locke criticised slavery as "so vile and miserable an estate of man",  Farr argues that this statement was meant primarily as a condemnation of the enslavement of the English (which Locke accused advocates of absolute monarchy as effectively proposing), not necessarily as a judgement of the African slave trade.
While Islam traditionally permits slavery, most contemporary Islamic authorities argue that the practice is inapplicable in the modern world. However, a minority of contemporary Islamic jurists defend slavery as still relevant and permissible today, and it is actively practiced by Islamist extremist groups. 
Pro-slavery apologists fought against the abolitionists with their own promotion which invariably stressed that slaves were both well treated and happy and which included illustrations designed to prove their points. 
Larry E. Tise argues against what he views as the mistaken position that proslavery thought was unique to the antebellum American South:
Among the numerous myths perpetuated by traditional interpretations of proslavery history, none has stood on shakier ground or proved more misleading than the notion that, for all practical purposes, proslavery was a phenomenon that appeared only in the antebellum South. From the age of the abolitionist through recent historiography the impression has remained that the Old South alone deserved the dubious distinction of fostering proslavery thought and building a culture largely defined by its buttressing of slavery. 
Tise goes on to describe in detail the history of proslavery thought in the British West Indies and the broader British Empire. He argues that the debate over slavery is more accurately seen, not as something exclusively American, but as a debate occurring throughout the English-speaking world. 
Until the middle of the 18th century, slavery was practiced with little challenge anywhere in the world. For centuries philosophers as varied as Aristotle, Thomas Aquinas, and John Locke accepted slavery as part of a proper social system. However, across Europe through the last part of the 18th century there were intellectual antislavery arguments based on Enlightenment thought, as well as moral arguments (notably among Quakers, in Great Britain and the United States) which questioned the legitimacy of slavery. Only in the American Revolutionary War era did slavery first become a significant social issue in North America.  In the North, beginning during the Revolution and continuing through the first decade of the next century, state by state emancipation was achieved by legislation or lawsuit  although in the larger slaveholding states such as New York and Pennsylvania emancipation was gradual. By 1810, 75% of Northern slaves had been freed and virtually all were freed within the next generation. 
In the United States, the antislavery contention that slavery was both economically inefficient and socially detrimental to the country as a whole was more prevalent than philosophical and moral arguments against slavery.  In Virginia, as the economy shifted away from tobacco towards less labor-intensive wheat crops, more slaves were freed between 1783 and 1812 than any time until 1865. There was the potential, in many Southern minds, for a relatively short transition away from slavery. However this perspective rapidly changed as the worldwide demand for sugar and cotton from America increased and the Louisiana Purchase opened up vast new territories ideally suited for a plantation economy. 
Only in the early 19th century did abolitionist movements gather momentum, and many countries abolished slavery in the first half of the 19th century. The increasing rarity of slavery, combined with an increase in the number of slaves caused by a boom in the cotton trade, drew attention and criticism to the Southern states' continuation of slavery. Faced with this growing 'antislavery' movement, slaveholders and their sympathizers began to articulate an explicit defense of slavery.
The famous Mudsill Speech (1858) of James Henry Hammond and John C. Calhoun's Speech to the U.S. Senate (1837) articulated the pro-slavery political argument during the period at which the ideology was at its most mature (late 1830s – early 1860s). These pro-slavery theorists championed a class-sensitive view of American antebellum society.  They felt that the bane of many past societies was the existence of the class of the landless poor. Southern pro-slavery theorists felt that this class of landless poor was inherently transient and easily manipulated, and as such often destabilized society as a whole. Thus, the greatest threat to democracy was seen as coming from class warfare that destabilized a nation's economy, society, government, and threatened the peaceful and harmonious implementation of laws.
This theory supposes that there must be, and supposedly always has been, a lower class for the upper classes to rest upon: the metaphor of a mudsill theory being that the lowest threshold (mudsill) supports the foundation for a building. This theory was used by its composer Senator and Governor James Henry Hammond, a wealthy southern plantation owner, to justify what he saw as the willingness of the non-whites to perform menial work which enabled the higher classes to move civilization forward. With this in mind, any efforts for class or racial equality that ran counter to the theory would inevitably run counter to civilization itself.
Southern pro-slavery theorists asserted that slavery eliminated this problem by elevating all free people to the status of "citizen", and removing the landless poor (the "mudsill") from the political process entirely by means of enslavement. Thus, those who would most threaten economic stability and political harmony were not allowed to undermine a democratic society, because they were not allowed to participate in it. So, in the mindset of pro-slavery men, slavery was for protecting the common good of slaves, masters, and society as a whole.  These and other arguments fought for the rights of the propertied elite against what were perceived as threats from the abolitionists, lower classes and non-whites to gain higher standards of living. Economic self-interest of slaveholders certainly played a role, as slaves represented a massive amount of wealth – at the time of the Civil War some historians estimate the over 20% of private wealth in the US was slaves.  They saw the abolition of slavery as a threat to their powerful Southern economy: an economy that revolved almost entirely around the plantation system and was supported by the use of black slaves.
Passages in the Bible on the use and regulation of slavery have been used throughout history as justification for the keeping of slaves, and for guidance on how it should be done. Therefore, when abolition was proposed, many Christians spoke vociferously against it, citing the Bible's acceptance of slavery as 'proof' that it was part of the normal condition. George Whitefield, famed for his sparking of the Great Awakening of American evangelicalism, campaigned, in the Province of Georgia, for the legalisation of slavery,   joining the ranks of the slave owners that he had denounced in his earlier years, while contending that they had souls and opposing mistreatment and owners who resisted his evangelism of slaves.  Slavery had been outlawed in Georgia, but it was legalised in 1751 due in large part to Whitefield's efforts. He bought enslaved Africans to work on his plantation and the orphanage he established in Georgia. Selina Hastings, Countess of Huntingdon inherited these slaves and kept them in bondage. 
In both Europe and the United States many Christians went further, arguing that slavery was actually justified by the words and doctrines of the Bible.
[Slavery] was established by decree of Almighty God...it is sanctioned in the Bible, in both Testaments, from Genesis to Revelation...it has existed in all ages, has been found among the people of the highest civilization, and in nations of the highest proficiency in the arts.
... the right of holding slaves is clearly established in the Holy Scriptures, both by precept and example.
In 1837, southerners in the Presbyterian denomination joined forces with conservative northerners to drive the antislavery New School Presbyterians out of the denomination. In 1844, the Methodist Episcopal Church split into northern and southern wings over the issue of slavery. In 1845, the Baptists in the South formed the Southern Baptist Convention due to disputes with Northern Baptists over slavery and missions. 
In the 20th century, the American philosopher Robert Nozick defended the notion of voluntary slavery, whereby a person voluntarily sells themselves into slavery. In Anarchy, State and Utopia, Nozick writes that "The comparable question about an individual is whether a free system will allow him to sell himself into slavery. I believe that it would."  Commenting on Nozick's views, David Ellerman (writing under the pseudonym "J. Philmore") notes the parallels with provisions of the Institutes of Justinian permitting an individual to sell themselves into slavery. 
Another 20th-century advocate of legal slavery was Rousas Rushdoony. Rushdoony as an adherent of theonomy believed that Old Testament laws should be applied in the present day, including those laws permitting slavery. Unlike Nozick, who believed that slavery should be limited to those who voluntarily agreed to it, Rushdoony supported the forcible enslavement of all those who rejected Christianity.   Rushdoony also asserted that antebellum American slavery, while unbiblical, was a positive good. 
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