Labriola distinguishes between historical materialism as an interpretation of history, and as a general conception of life and of the universe (_Lebens-und-Weltanschauung_), and he inquires what is the nature of the _philosophy immanent_ in historical materialism; and after some remarks, he concludes that this philosophy is the _tendency to monism_, and is a _formal_ tendency. Here I take leave to point out that if into the term _historical materialism two different things_ are intruded, _i.e._: (1) a method of interpretation; (2) a definite conception of life and of the universe; it is natural to find a philosophy in it, and moreover with a tendency to monism, because it was included therein at the outset. What close connection is there between these two orders of thought? Perhaps a logical connection of _mental coherence_? For my part, I confess that I am unable to see it. I believe, on the contrary, that Labriola, this time, is simply stating _à propos_ of historical materialism what he thinks to be the necessary attitude of modern thought with regard to the problems of ontology; or what, according to him, should be the standpoint of the socialist opinion in regard to the conceptions of optimism and pessimism; and so on. I believe, in short, that he is not making an _investigation_ which will reveal the philosophical conceptions underlying historical materialism; but merely a _digression_, even if a digression of interest and importance. And how many other most noteworthy opinions and impressions and sentiments are welcomed by socialist opinion! But why christen this assemblage of new facts by the name of historical materialism, which has hitherto expressed the well-defined meaning of a way of interpreting history? Is it not the task of the scientist to distinguish and analyse what in empirical reality and to ordinary knowledge appears mingled into one? IV OF SCIENTIFIC KNOWLEDGE IN FACE OF SOCIAL PROBLEMS _Socialism and free trade not scientific deductions: Obsolete metaphysics of old theory of free trade: Basis of modern free trade theories not strictly scientific though only possible one: The desirable is not science nor the practicable: Scientific law only applicable under certain conditions: Element of daring in all action._ It has become a commonplace that, owing to Marx's work, socialism has passed _from utopia to science_, as the title of a popular booklet by Engels expresses it; and _scientific socialism_ is a current term. Professor Labriola does not conceal his doubts of such a term; and he is right. On the other hand, we hear the followers of other leaders, for instance the extreme free traders (to whom I refer by preference _honoris causa_, because they, too, are amongst the _idealists_ of our times), in the name of science itself, condemn socialism as _anti-scientific_ and declare that free trade is the only scientific opinion.