From Alexander Bogdanov’s Faith and Science (a long response to Lenin’s Materialism and Empiriocriticism):
If we take into consideration that the larger part of Lenin’s book, about two thirds, is dedicated precisely to the accusations of idealism directed against “Machists” and Lenin provides an infinite amount of citations from the works of “Machists” themselves and from the works of philosophers who do or do not sympathize with them, then in addition to the psychological puzzle – how our author could come up with his polemics while in essence expressing the same point of view – there is also a logical puzzle: how this polemics could be formally combined in his head with his own views. The key to the solution of the first puzzle is already available to us: it is religious thinking, solid in its verbal expression but vague in its concepts. The second puzzle can be explained by the extremely peculiar philosophical-critical method that is systematically used by our author, the one I would call, choosing the softest expression, the method of the “substitution of concepts.”
It is a very simple method. “Machists” reduce all reality to “elements of experience.” What are these “elements”? Colors, forms, tones, smells, touch and so on. But Hume thought that all these colors and tones are sensations! Therefore, “elements” are the same as sensations. But Berkeley considered these same colors, forms and so on to be ideas! Therefore, sensations are the same as “ideas”! So, “elements” are ideas, and “Machism” is the purest form of idealism, i.e. “fideism,” “obscurantism” and so on.
Mach and empiriocritics understand experience realistically: experience is things and images, physical and mental complexes. Elements are the same in both cases; in some complexes they are elements of things, in others, elements of images or sensations. The elements of things (or of “environment”) are colors, forms, rigidity, softness and so on, taken as independent of an individual, in objective connection – in a complex of a “rose petal” the color red is connected to the softness, the oval shape, the certain smell and so on objectively, i.e. completely independently from whether “I” look at it or not, whether “I” can distinguish between colors or not and so on. In the complex “perception of petal” the red color is present, but if “I” close my eyes, it changes to something different, if “I” am colorblind, it is accompanied by the sensation of “softness” only if while touching it I am also looking at it; here the red color, the softness or the smell are my sensations.
Hume, Kantians and Plekhanov understand experience individualistically-psychologically: experience consists of “my” mental images, and nothing else. “My” here means that we cannot speak of any independent connection between the elements, that this connection is always subjective, and all the component parts of experience are always only “sensations,” only individual “my” sensations. “It is ridiculous,” writes, for example, Plekhanov, “to ask what color rose has when no one is looking at it, what smell is has when no one is smelling it…” (“Materialismus militans”, Second Letter). Here the rose has no color and no smell because no “subject” is “sensing” them.
And finally Berkeley understands experience idealistically, and therefore all the component parts of experience are conceived by him as elementary “ideas.”
In other words, since these are different understandings of experience, naturally, they produce different concepts of what experiences consists of.
Lenin’s conclusion: we can use these different concepts interchangeably!