The slave owners kept them from having an established identity. If they saw themselves as individuals with a purpose and sense of self, they would then have the desire to question authority. They kept parents and children seperate due to human instinct to develop kinship ties; the slaves would form a support system and thus establish a group identity, leading to potential uprising and questioning their positions. Education is freedom, as is knowledge. Why give such a gift to those in servitude? (Rhetorical question...)
