CB :in _Materialism and Empirio-Criticism_, Lenin analyzes the Empirio-Critics as subjective idealists, Berkeleyians. Berkeley and Hume are the purist and most explicit subjective idealists. In philosophy 101 class, Stan Scheinbaum and I used to call ourselves "Humean Beings": we don't really know if anything outside us really exists. Just sense-data. Absolute skepticism about objective reality. Doubt everything. Criticize everything .

Anyway, Lenin analyses The Empirio-Critics as Neo-Kantians , too. Kant ultimately reduces to a subjective idealist, but he's , as Engels termed him, a "shamed faced materialist ", agnostic. The thing-in-itself exists, but it's unknowable. Objective reality exists , but we can't (Kant) know it ! Now there's a revolting development.

By the way, Einstein , for whom Mach was a mentor, disagreed with Mach on the objective reality of atoms . (See _Einstein:The Life and Times_ by Ronald W. Clark; p. 62). Einstein also criticized Mach's positivism or "facts speak for themselves." Lenin criticizes the positivism of the Empirio-critics, including Mach.

Hegel critiqued Kant's extreme skepticism : practice is how we know objective reality. Lenin cites Engels who credits Hegel's critique of Kant's "Critical Philosophy", Kant's Critique of Pure Reason and Practical Reason.

Marx's Second Thesis on Feuerbach is this principle. Lenin cites the Second Thesis in his critique of the Empirio-Critics. Actually the central idea of the Theses on Feuerbach is practice as the test of theory , proof of theory ; the epistemological test, key to Marx's theory of knowledge. The First Thesis criticizes all previous materialist philosophies , including Feuerbach's , for being passive contemplative , not active , like idealism, for not being involved in practical critical activity (praxis). The famous 11th says philosophers have interpreted the world; the thing is to change it. Don't just contemplate it ; act and practice in it .

Charles Brown says : Most of objective reality is other people , including especially natural , not super - natural, wishes from dead generations.

The talking dead

Natural soul

V. I. Lenin

MATERIALISM and EMPIRIO-CRITICISM

Critical Comments on a Reactionary Philosophy Conclusion There are four standpoints from which a Marxist must proceed to form a judgment of empirio-criticism. First and foremost, the theoretical foundations of this philosophy must be compared with those of dialectical materialism. Such a comparison, to which the first three chapters were devoted, reveals, along the whole line of epistemological problems, the thoroughly reactionary character of empirio-criticism, which uses new artifices, terms and subtleties to disguise the old errors of idealism and agnosticism. Only utter ignorance of the nature of philosophical materialism generally and of the nature of Marx’s and Engels’ dialectical method can lead one to speak of “combining” empirio-criticism and Marxism.

Secondly, the place of empirio-criticism, as one very small school of specialists in philosophy, in relation to the other modern schools of philosophy must be determined. Both Mach and Avenarius started with Kant and, leaving him, proceeded not towards materialism, but in the opposite direction, towards Hume and Berkeley. Imagining that he was “purifying experience” generally, Avenarius was in fact only purifying agnosticism of Kantianism. The whole school of Mach and Avenarius is moving more and more definitely towards idealism, hand in hand with one of the most reactionary of the idealist schools, viz., the so-called immanentists.

Thirdly, the indubitable connection between Machism and one school in one branch of modern science must be borne in mind. The vast majority of scientists, both generally and in this special branch of science in question, viz., physics, are invariably on the side of materialism. A minority of new physicists, however, influenced by the breakdown of old theories brought about by the great discoveries of recent years, influenced by the crisis in the new physics, which has very clearly revealed the relativity of our knowledge, have, owing to their ignorance of dialectics, slipped into idealism by way of relativism. The physical idealism in vogue today is as reactionary and transitory an infatuation as was the fashionable physiological idealism of the recent past.

Fourthly, behind the epistemological scholasticism of empirio-criticism one must not fail to see the struggle of parties in philosophy, a struggle which in the last analysis reflects the tendencies and ideology of the antagonistic classes in modern society. Recent philosophy is as partisan as was philosophy two thousand years ago. The contending parties are essentially, although it is concealed by a pseudo-erudite quackery of new terms or by a feeble-minded non-partisanship, materialism and idealism. The latter is merely a subtle, refined form of fideism, which stands fully armed, commands vast organisations and steadily continues to exercise influence on the masses, turning the slightest vacillation in philosophical thought to its own advantage. The objective, class role of empirio-criticism consists entirely in rendering faithful service to the fideists in their struggle against materialism in general and historical materialism in particular.

https://www.marxists.org/archive/lenin/works/1908/mec/concl.htm

The physicist Mach ( Mach speed is named for him) who is a main subject of Lenin's criticism in this book, was admired much by Einstein. However, on one of the philosophical issues Lenin criticizes Mach on - the reality of atoms as opposed to their existence only as sense datums - Einstein broke with his mentor Mach ( not aware of Lenin's book though). Einstein and Lenin had the same position on that philosophical question independently of each other. ( I have to dig up my copy of one of the biographies of Einstein in which I read about the disagreement between Einstein and Mach).

Lenin criticizes the Empirio-critics as neo-Kantians - things-in-themselves exist but they are unknowable. We only know our sense datums of them. I think the so-called Post-modernists of our era can be similarly critiqued as neo-Kantians. They deny the knowability of objective reality. All knowledge is symbols. There is no objective science only " the endless play of signs."