What Marx wished to do, or mistakenly thought himself to be doing is, I think, of interest to criticism up to a certain point; although the history of science shows that thinkers have not always had the clearest and plainest knowledge of the whole of their thought; and that it is one thing to discover a truth, and another to define and classify the discovery when made. It may be allowed that he who confuses ideological with historical research thus best reproduces Marx's _spirit_; but in this case the work will be an artistic recasting or a psychological reproduction, not a criticism; and will gather up with the sound also the unsound portion of Marx's thought. To go into details. Labriola tries to prove the emptiness or vagueness of some of my definitions and the falsity of some of my reasoning. I having asserted that capitalist economics is a special case of general economics, Labriola remarks, '_en passant_,' that it is nevertheless the only case which has given rise to a theory and to divisions of schools; and I acknowledge that I do not understand the point of this remark, although it is said to be made '_en passant_.' Both Marx and Engels lamented that the ancient and medieval economic systems had not been studied in the same way as the modern. Thus there are conceivable at least three economic theories, ancient, medieval and modern, and is it not lawful to construct a general economics; _i.e._ to study in isolation that common _element_ which causes these three groups of facts to be all three denoted by a common name? Labriola then asks what this general and extra-historical economics can consist of, and whether it can never be of service to the conjectural psychology of primitive man: he jests after the manner of Engels, who in truth has sometimes joked too much during a discussion on serious matters. Is it incredible that I too should jest? But I do not think there is occasion to do so! He wonders at my _insatiability_, because having accepted the hedonistic theories, I wish to accept Marx's theories too: as though my entire proof was not intended to make it plain that the antithesis between these theories exists only in imagination; and that Marx's theory is not an economic system entirely opposed to other systems ('_quelque chose de tout-à-fait opposé_' are Labriola's own words), but a special and partial inquiry; and as though by hedonism I meant all the personal convictions, philosophical, historical and political, of those who follow, or say that they follow, its guidance, and not indeed _only what follows legitimately from its axiom_. When I call the explanation of the nature of profits, offered by the hedonistic school, an _economic explanation_, he inquires sarcastically: 'Could it possibly be _non-economic_?' But my statement contains no pleonasm: the adjective _economic_ is added to mark off the hedonistic explanation from that of Marx, which, to my thinking, is not purely economic, but historical and comparative, or