For the slaveowner implement, beast and slave are all only tools, but the slave knows he is a man if the master doesn't, and he proceeds to demonstrate this on the bodies of the horse and plow. Hence it was not possible to introduce the steel plow and the horse into southern agriculture until after the elimination of slavery; before they had to use mules and wooden plows which could stand the punishment the slave visited on both.
That is incredibly wordy, & I'm sure Marx was much more precise & cutting.
Carrol