During the Paris years, Marx plunged into the study of various reformist and socialist theories that had been inaccessible in Germany. He read Proudhon and Louis Blanc, Cabet and Fourier, Saint-Simon and the Saint-Simonians, as well as the revolutionary disciples of Babeuf such as Blanqui. In addition, he became familiar with the British political economists from Adam Smith to Ricardo and with their liberal and radical critics such as Sismondi.

In Paris Marx not only had an opportunity to study novel doctrines, but he also was able to meet a number of radicals in person. Among the emigres, he was especially attracted to the Russian revolutionary Michael Bakunin, and among the Germans, he frequented the radical poets Heinrich Heine and Ferdinand Freiligrath, the revolutionary itinerant tailor Wilhelm Weitling, and the radical left-Hegelian writer Arnold Ruge. Among the Frenchmen Marx met in person, Proudhon may have made the strongest impression. Marx had already read his What Is Property? in Cologne and had praised it very highly. At first the two seemed to be made for each other, but after a fairly short period the friendship dissolved. A few years later Marx savagely attacked Proudhon's Philosophy of Misery in his The Misery of Philosophy, charging him with a misuse of Ricardo's economic concepts and with doing away with the movement of history by neglecting and neutralizing the thrust of dialectical contradictions.

Above all, it was in Paris that the remarkable lifelong friendship with Friedrich Engels began. Here Marx became intimate with the textile manufacturer's son who had turned socialist from revulsion about the conditions of the working class, which he had observed both in his native Rhineland and in England, where he was now a manager of one of his father's enterprises. It was through Engels and his work that Marx was introduced to an understanding of the concrete conditions and the misery of working-class life.

Besides the leading intellectuals of the radical and liberal movement whom Marx had an occasion to meet in Paris, he also encountered for the first time those artisan and craftsman radicals, German and French, who, in alliance with intellectuals, were the mainstay of the socialist and revolutionary movement. In almost daily commerce with them, Marx, although often contemptuous of their simple-mindedness and lack of intellectual distinction, was impressed by this new type of man, so very different from the academically trained intellectual with whom he had associated before.

Marx, the radical liberal, completed his conversion to socialism in the heady atmosphere of Paris. It was here that, sometimes alone and sometimes in collaboration with Engels, he wrote those early works that served to define his new philosophical and political position and helped to sever the ties that had bound him to his erstwhile Young Hegelian companions. Some of these writings appeared as articles in a short-lived review, Deutsch-Franzoesische Jahrbuecher, which he edited with Arnold Ruge. Most, however, like the now famous Economic and Philosophical Manuscripts and The German Ideology (which was completed in Brussels), were never published during his lifetime, having been written primarily as a means for intellectual self-clarification. The Holy Family, his final settling of accounts with the key figures of the Young Hegelian "family," appeared in Frankfort in 1845. It received little attention since it appeared to most readers, not without reason, as a tedious family quarrel within the ranks of the Hegelian Left. The Misery of Philosophy was published in French in 1847.

In the beginning of 1845 Marx was expelled from Paris by the Guizot government. Just as the Prussian government had once terminated Marx's editorial career as a result of protests from Russia, so the French government now acted to expel him upon representations of Prussia, which had been offended by the antiroyalist comments of the socialist paper Vorwaerts on which he collaborated. Marx moved to Brussels and established contacts with the German refugees who had taken shelter there. In particular, he sought out the remaining members of the dissolved League of the Just, an international revolutionary movement and eagerly cultivated relations not only with German but also with Belgian and other socialist individuals and organizations. He had become a professional revolutionary, writing, lecturing, and conspiring in the service of a revolution which he, like his newly found comrades, believed imminent. From then on, as Isaiah Berlin has said, "His personal history which up to this point can be regarded as a series of episodes in the life of an individual [became] inseparable from the general history of socialism in Europe."

From Coser, 1977:61-62.