Voltaire remarked that it is possible to kill a flock of sheep by witchcraft if you give them plenty of arsenic at the same time. The sheep, in this figure, may well stand for the complacent apologists of capitalism; Marx's penetrating insight and bitter hatred of oppression supply the arsenic, while the labour theory of value provides the incantations.
Chapter III, The Labour Theory Of Value, p. 22 - An Essay on Marxian Economics (Second Edition) (1966)