How did English slave owners justify slavery?

England was technologically more advanced than Africa at the time Africans were being enslaved by the English, which was taken as evidence of English racial superiority. It seemed reasonable that superior races should control and be served by inferior races. From our present perspective we can see that even though one culture may advance technologically beyond another, this has to do with many kinds of historical accidents, rather than being a measurement of the capabilities of any ethnicity.