What argument does stephens offer for the idea that blacks are innately suited for the condition of slaves?

In his Cornerstone Speech, Stephenson had emphasized that its "natural" for the African Americans to serve the Whites. Stephenson believed that it is right and just for the Condeferate states to have slavers. However, the said idea was against the belief and the principles of the forefathers of the American Constitution.

Dbecause i feel like that’s the correct answer