Thomas Clarkson said this quote

Neither can men, by the same principles, be considered as lands, goods, or houses, among possessions. It is necessary that all property should be inferiour to its possessor. But how does the slave differ from his master, but by chance?

~ Thomas Clarkson

We cannot suppose therefore that God has made an order of beings, with such mental qualities and powers, for the sole purpose of being used as beasts, or instruments of labour.

~ Thomas Clarkson