Answer: to show how dehumanizing slavery was
Step-by-step explanation:
In comparing the lack of knowledge forced upon enslaved people to horses who also lack knowledge, Douglass was trying to show that slavery was so dehumanizing because humans were treated in the same way that animals were treated in this particular regard.
Slave owners felt that the more ignorant an enslaved person was, the easier they would be to control and this was a very accurate view. Once they saw the success of keeping the enslaved ignorant, they pursued it as a strategy ever so diligently.