The most formidable hurdle facing all readers of Marx is his ‘peculiar’ use of words. Vilfredo Pareto provides us with the classic statement of this problem when he asserts that Marx's words are like bats: one can see in them both birds and mice. No more profound observation has ever been offered on our subject. Thinkers through the years have noticed how hard it is to pin Marx down to particular meanings, and have generally treated their non-comprehension as a criticism. Yet, without a firm knowledge of what Marx is trying to convey with his terms, one cannot properly grasp any of his theories.

How, for example, are we to understand the startling claim that ‘Value is labor’ (my emphasis), or Marx's assertion that the ‘identity of consumption and production … appears to be a threefold one’, or his allusion to theory which under certain circumstances becomes a ‘material force’? Marx's statements frequently jar us, and instances of obscurity in his work, occasions where two or more interpretations seem equally applicable, are more numerous still.

Engels was well aware of the trouble people had in coming to grips with Marx's terminology. In his Preface to the English edition of Capital I, he says, ‘there is, however, one difficulty we could not spare the reader: the use of certain terms in a sense different from what they have, not only in common life, but in ordinary political economy.’