Slaves cannot speak.

Kendrey, in essence, as Kant argues, what makes us human is our capacity to reason as individuals.  Animals have instincts; they cannot reason.  Therefore to deny individuals the right to use, exercise, apply their reason, which involves making choices on the basis of honest facts, is to deny their humanity.  Yes, Kant bases his ethics on reason.  He, of course, lived in the Age of Reason.  In effect, he attacks slavery, for the ancient Greeks defined a slave as a person who could not speak, unless asked, and then the slave better say what the master wanted to hear!  Speaking of course is an expression of our reason.  To not let us "have a voice" is to deny our reason, which, in turn, denies our dignity as humans.