The opponents of contemporary socialism see in it, or wish to see in it, merely a reproduction of the sentimental socialism of the first half of the Nineteenth Century. They contend that socialism is in conflict with the fundamental facts and inductions of the physical, biological and social sciences, whose marvelous development and fruitful applications are the glory of our dying century. To oppose socialism, recourse has been had to the individual interpretations and exaggerations of such or such a partisan of Darwinism, or to the opinions of such or such a sociologist--opinions and interpretations in obvious conflict with the premises of their theories on universal and inevitable evolution. It has also been said--under the pressure of acute or chronic hunger--that "if science was against socialism, so much the worse for science." And those who thus spoke were right if they meant by "science"--even with a capital S--the whole mass of observations and conclusions _ad usum delphini_ that orthodox science, academic and official--often in good faith, but sometimes also through interested motives--has always placed at the disposal of the ruling minorities. I have believed it possible to show that modern experiential science is in complete harmony with contemporary socialism, which, since the work of Marx and Engels and their successors, differs essentially from sentimental socialism, both in its scientific system and in its political tactics, though it continues to put forth generous efforts for the attainment of the same goal: social justice for all men. I have loyally and candidly maintained my thesis on scientific grounds; I have always recognized the partial truths of the theories of our opponents, and I have not ignored the glorious achievements of the bourgeoisie and bourgeois science since the outbreak of the French Revolution. The disappearance of the bourgeois class and science, which, at their advent marked the disappearance of the hieratic and aristocratic classes and science, will result in the triumph of social justice for all mankind, without distinction of classes, and in the triumph of truth carried to its ultimate consequences. The appendix contains my replies to a letter of Herbert Spencer and to an anti-socialist book of M. Garofalo. It shows the present state of social science, and of the struggle between ultra-conservative orthodoxy, which is blinded to the sad truths of contemporary life by its traditional syllogisms and innovating heterodoxy which is ever becoming more marked among the learned, as well as strengthening its hold upon the collective intelligence. ENRICO FERRI. Brussels, Nov., 1895. Introduction.