Marx's answer to the "Philosophy of Poverty" is indicated at once by the title "The Poverty of Philosophy." He deals first of all with the economic details of Proudhon's work, and proves with documentary evidence that the theses and antitheses it contains partly spring from a half-understood reading of English and French political economists, and in part have been taken direct from the English Communists. Marx already displays in this section an extensive knowledge of economic literature. Then he confronts Proudhon's philosophical and social theories with his own deductions and gives many positive results. Marx's main object was to induce the Socialists to give up their Utopianism and think in terms of realism, and to regard social and economic _categories_ in their historical setting: "Economic _categories_ are only the theoretical expressions, ideal conceptions of the conditions of production obtaining in society.... Proudhon has grasped well enough that men manufacture cloth, linen, etc., under certain conditions of production. But what he has not grasped is that these social conditions themselves are just as much human products as cloth, linen, etc. Social conditions are intimately bound up with productive power. With the acquisition of new productive power men change their methods of production, and with the change in the methods of production, in the manner of obtaining a livelihood, they change their social conditions. The hand-mill gives rise to a society with feudal lords, the steam-mill to a society with industrial capitalists. But the same men who shape the social conditions in conformity with the material means of production, shape also the principles, the ideas, the _categories_ in conformity with their social conditions. Consequently these ideas, these _categories_, are just as little eternal as are the conditions to which they give expression. They are the transitory and changing products of history. We are living in the midst of a continuous movement of growth in productive power, of destruction of existing social conditions, of formation of ideas."--("Poverty of Philosophy," Stuttgart, 1885, pp. 100-101.) Here it should, above all, be noticed that Marx ascribes to industrialism a powerful revolutionary effect, and that he characterises the different forms of society by their different methods of labour. Or, as he says later in "Capital," "not _what_ is produced, but _how_ it is produced distinguishes the various forms of society." What he means to say, then, is that ideas and systems are limited by their time, that they are conditioned by the prevailing means of production. To understand them one must study the times which have preceded them, as well as investigate the ideas and systems themselves, and find out whether new forms have not arisen which stand in contradiction or in contrast to the old one. Or, as Marx says: