So spoke Lenin :

That is just the whole trouble. Bogdanov (like the rest of the Russian Machians) has never looked into the idealist views originally held by Mach and Avenarius, has never examined their fundamental idealist premises, and has therefore failed to discover the illegitimacy and eclecticism of their subsequent attempts to smuggle in materialism surreptitiously. Yet, just as the initial idealism of Mach and Avenarius is generally acknowledged in philosophical literature, so is it generally acknowledged that subsequently empirio-criticism endeavoured to swing towards materialism. Couwelaert, the French [?] writer quoted above, asserts that Avenarius� Prolegomena is �monistic idealism,� the Kritik der reinen Erfahrung (1888-90) is �absolute realism,� while Der menschliche Weltbegriff (1891) is an attempt �to explain� the change. Let us note that the term realism is here employed as the antithesis of idealism. Following Engels, I use only the term materialism in this sense, and consider it the sole correct terminology, especially since the term �realism� has been bedraggled by [those] who vacillate between materialism and idealism. For the present I will suffice to note that Couwelaert had the indisputable fact in mind that in the Prolegomena (1876) sensation, according to Avenarius, is the only entity, while �substance�—in accordance with the principle of �the economy of thought!�—is eliminated, and that in the Kritik der reinen Erfahrung the physical is taken as the independent series, while the psychical and, consequently, sensations, are taken as the dependent series. (pages 53-4, the Pekin edition.)

Couwelaert, F. van, "L'empiriocriticisme", Revue neoscholastique, February 1907-41, 148.