The Scottish empiricist philosopher David Hume and the German philosopher Immanuel Kant are most frequently mentioned when we discuss the Enlightenment's validation of racist notions about black people. To give you an idea of how strongly Hume felt about the supposedly inherent differences between black and white people, he wrote the following in the nineteenth century:
I am apt to suspect the Negroes, and in general all other species of men to be naturally inferior to the whites. There never was any civilized nation of any other complection [sic] than white, nor even any individual eminent in action or speculation.
This statement from Hume reveals an astounding level of ignorance from one of the most prominent thinkers of the Enlightenment era. His statement fails to account for the great philosophical and literary traditions of Asia and the Middle East, the innovative talents of the Chinese, the Arabic founding of the numeric system, the architectural wonders of the Aztecs, Incas, and Mayans, the existence of Timbuktu as a great center of learning, and, of course, the indelible influences of the Egyptians on culture and religion.
Similarly, a couple of decades later, Immanuel Kant, who is often revered as the most important modern philosopher, once said, in response to a report that an African had once said something intelligent, that "this fellow was quite black from head to toe, a clear proof that what he said was stupid." Supposedly, Kant later recanted his racist views, but the damage was already done.
Both Kant and Hume were speaking on the basis of the scientific racism that developed during the Enlightenment. Scientific racism sought explanations for the "black" skin of Africans and the "red" skin of indigenous people, wondering why they were so different from whites (in the view of such thinkers, whiteness was clearly the norm against which others were measured). The conclusion they drew was that these differences in skin color were signs of these groups' primitive characters, which included an inability to develop an intellect. On these grounds, it was seen as just to enslave them. Firstly, the Europeans observed the existence of some systems of slavery in West and Central Africa, though those systems neither enslaved people for lifetimes nor categorically dehumanized particular groups. Secondly, some asserted that they were doing Africans a favor by taking them out of Africa and bringing them either to Europe or the New World, where they would be instilled with Christianity.
In sum, some Enlightenment thinkers justified slavery by insisting on notions of natural inferiority drawn out of scientific racism. Though Hume rejected slavery and Kant later changed his mind about his own racism, their ideas provided supporters of slavery with an intellectual basis for enslaving indigenous people and, later, Africans. Thomas Jefferson would later repeat similar views about the supposedly intrinsic differences between races in his polemic "Notes on Virginia."