This work is a testimony with regard to the method employed by Marx and Engels in arriving at their philosophical conclusions. It is the statement of the philosophical foundations of modern socialist theory by one who helped to lay them; it is an old man's account of the case upon the preparation of which he has spent his entire life, for, this work, short as it is, represents the results of forty years of toil and persevering effort. With Marx's eleven theses on Feuerbach printed here as an appendix.