And you missed an even MORE relevant excerpt!!

    Man and Slave
    
    The problem, and it is a moral prob-
    lem, with which we are here faced is
    very close to one of the great problems
    of slavery. Let us grant that slavery
    is bad because it is cruel. It is, how-
    ever, self-contradictory, and for a
    reason which is quite different. We
    wish a slave to be intelligent, to be able
    to assist us in the carrying out of our
    tasks. However, we also wish him to
    be subservient. Complete subservience
    and complete intelligence do not go
    together. How often in ancient times
    the clever Greek philosopher slave of
    a less intelligent Roman slaveholder
    must have dominated the actions of his
    master rather than obeyed his wishes!
    Similarly, if the machines become
    more and more efficient and operate
    at a higher and higher psychological
    level, the catastrophe foreseen by
    Butler of the dominance of the ma-
    chine comes nearer and nearer.