What led Marx to launch out upon the troubled sea of economic science, when all his predilections were for the study of pure philosophy, was the fact that his philosophical studies had led him to a point whence further progress seemed impossible, except by way of economics. The Introduction to "A Contribution to the Critique of Political Economy" makes this perfectly clear. Having decided that "the method of production in material existence conditions social, political, and mental evolution in general," a study of economics, and especially an analysis of modern industrial society, became inevitable. During the year 1845, when the theory of the economic interpretation of history was absorbing his attention, Marx spent six weeks in England with his friend Engels, and became acquainted with the work of the Ricardian Socialists already referred to.[138] Engels had been living in England about three years at this time, and had made an exhaustive investigation of industrial conditions there, and become intimately acquainted with the leaders of the Chartist movement. His fine library contained most of the works of contemporary writers, and it was thus that Marx came to know them. Foremost of this school of Socialists which had arisen, quite naturally, in the land where capitalism flourished at its best, were William Godwin, Charles Hall, William Thompson, John Gray, Thomas Hodgskin, and John Francis Bray. With the exception of Hall, of whose privately printed book, "The Effects of Civilisation on the People of the European States," 1805, he seems not to have known, Marx was familiar with the writings of all the foregoing, and his obligations to some of them, especially Thompson, Hodgskin, and Bray, were not slight. While the charge, made by Dr. Anton Menger,[139] among others, that Marx took his surplus value theory from Thompson is quite absurd, and rests, as Bernstein has pointed out, upon nothing but the fact that Thompson used the words "surplus value" frequently, but not at all in the same sense as that in which Marx uses them,[140] we need not attempt to dispute the fact that Marx gleaned much of value from Thompson and the two other writers. While criticising them, and pointing out their shortcomings, Marx himself frequently pays tributes of respect to each of them. His indebtedness to any of them, or to all of them, consists simply in the fact that he recognized the germinal truths in their writings, and saw far beyond what they saw.