If the slaves were “diffused,” enslavers would be more likely to free them, for whites were afraid to live surrounded by large numbers of free black people. Thus, moving enslaved people into new regions where their enslavement was more profitable would lead to freedom for said enslaved people. Make slavery bigger in order to make it smaller. Spread it out to contain its effects. And those most eager to buy this bogus claim were the Virginians themselves. Jefferson became the most prominent advocate of diffusion.
