The Enlightenment emphasized rationality and logical thinking. It wanted things to be proved, not just taken for granted because of tradition. These ideas of the Enlightenment were used to argue for and against the trans-Atlantic slave trade.
In favor of the slave trade were Enlightenment thinkers who believed in the ideas of scientific racism. Such people believed that they could use scientific observation to prove that whites were superior to other races of people. This was used to justify the slave trade on the idea that Africans were an inferior race and could therefore be enslaved.
On the other side were Enlightenment thinkers who did not believe that racism was logical. Such people believed more in the ideas of universal human equality. These were the sorts of ideas that had led Enlightenment thinkers to oppose monarchy and they could also lead people to oppose slavery. In addition, Enlightenment thinking called for the reform of society along rational lines. This was one of the things that motivated people like William Wilberforce to oppose the slave trade.