Kateb's article was sincerely argued and reached some important conclusions, and I appreciate the attempt to explain many of these technical concepts to the lay public such as myself, but it starts from a common but erroneous concept of the science of science and its relationship to the school of Marxism, which I felt the need to attempt to address for your review, in the hopes to warn against misunderstanding and that an unwarranted historical rigidity can give way to a better motivated (in the words of 'our philosopher' Dietzgen) "admiration by the reflection that the whole world is wonderful, that everything is beautiful, so that nothing ugly remains." At the risk of being accused of 'idealism' within the ranks, I would like to entice the reader to open their minds about what the materialism of Marx may have meant as a theory. Marx did not use the term Dialectical Materialism, though it is not inappropriate. Materialists were many philosophers before and after Marx, and Engels calls our theories the "materialist conception of history" in Socialism: Utopian and Scientific. While I will address the topic of the quantum realm in the end, I am here more concerned with the matters of scientific principle and the legacy that the proletarian class struggle, as it has expressed itself in Marxism, has left on the concepts of scientific knowledge.

Now, it ought to be mentioned to start with that "idealism" is not quite the open-ended concept which this article infers, that "instrumentalism" in its worst interpretation does not exist (except perhaps by a small subset of Silicon Valley AI 'simulation-reality' conspiracists), and that "realism" as asserted here has conceptually nothing to do with Marxism. Let us address these points.

Insofar as idealism exists among the theories of Kant and Hegel, it is not epistemological skepticism - i.e., the idea that reality may not really exist outside our minds, therefore that our minds create reality, or solipsism, if you will. ("Is there a conflict between realism and idealism? The origins and structure of an error" by Isabelle Thomas-Fogiel may delve deeper for you into this subject.)

That the external world exists is an everyday fact of life, and we can congratulate ourselves for this great discovery ipso facto that we think and move through it. Kant and Hegel were not particularly blind to the existence of the world, either. Were they to deny that reality is knowable and be skeptical of the possibility of epistemological truth, they likely would have had short careers as philosophers, and this would have been odd for two men who claimed the legacy of and belief in the Christian God as external-world force, in an era of reaction where their theories gained relevance for the need of the Church to reassert itself post-Death of God, or more specifically with Hegel the restoration of the Bourbons and the Catholic Church in France and the seemingly-final victory of reactionaries in Europe, to be dispelled in 1848.

Engels addressed this issue himself, where he considered the question of skepticism of reality and concluded that almost nobody believed such a thing:

Is our thinking capable of the cognition of the real world? Are we able in our ideas and notions of the real world to produce a correct reflection of reality? In philosophical language this question is called the question of identity of thinking and being, and the overwhelming majority of philosophers give an affirmative answer to this question. With Hegel, for example, its affirmation is self-evident; for what we cognize in the real world is precisely its thought-content — that which makes the world a gradual realization of the absolute idea, which absolute idea has existed somewhere from eternity, independent of the world and before the world. But it is manifest without further proof that thought can know a content which is from the outset a thought-content. It is equally manifest that what is to be proved here is already tacitly contained in the premises. But that in no way prevents Hegel from drawing the further conclusion from his proof of the identity of thinking and being that his philosophy, because it is correct for his thinking, is therefore the only correct one, and that the identity of thinking and being must prove its validity by mankind immediately translating his philosophy from theory into practice and transforming the whole world according to Hegelian principles. This is an illusion which he shares with well-nigh all philosophers.

Ludwig Feuerbach and the End of Classical German Philosophy, Engels

Hegel claims not that the world is non-existent or impossible to know and produced by the mind, but the very opposite: that God comes into real being external to and before the world, that the world (but more specifically the history of men) independently exists, and the absolute idea shapes the self-realization of itself within the historical process. What Engels is quick to point out is that this philosophy is correct for his thinking, that is, for his class and society, but he fails to realize that he is simply repeating himself: "the idea forms the idea" tells us more about his ideas than the world of other real, living actors.
But let us dig deeper to ask whether this knowable reality, which nobody denies, is deeply knowable, and is that the essential nut to crack at the core of Marxism and our necessary prerequisite for opposing all eternal metaphysics? This is missing the forest for the tree, the equivalent of asking if the actor on the TV screen is really dead - because our question is more about the medium of our understanding of reality than it is about reality. Because measurement is an act, this is a true and very important conclusion; we should not forget that experience is an act, and that the medium is the experience. This is an article brought up often in media: that we are telling a story through a stage play means we are creating something unique to that medium, but let us imagine that concept briefly instead for humanity and ourselves.

Our idealist Kant's 'Transcendental Idealism' here means to remind us that as we experience phenomena, we experience it as a product of our mind, which has created innate mechanisms of processing sensations, which is self-evident when it comes to emotions which we all understand to be innate (we don't think pain is a certain electrical feeling produced by a nerve, but an emotion which we understand from birth, and later may learn the biology behind it) but, he also argues, it has also given rise to spacetime, math and an array of existences as we uniquely know it.

If general relativity has shown us anything, it is that the assumptions of the human experience as mathematical beings are not wholly representative, such that we divide the whole world into parts to explain them by the function of our minds. Particles are an ancient concept. Einstein discovered that time could not be divided away from space, and indeed it presented to us in the real world as one whole which acts on our experiences, so that if we divide time from space we are unable to calculate simple things like the orbit of Mercury. With the addition of spacetime as a whole, defined through general relativity, the equation made sense after two centuries of persistent error.

Next is where we firmly, as Marxists, must depart from Kant, as he means, as a good Prussian monarchist should, to impart on us with the Thing-in-Itself, or his idea of the external world if we wish (that includes that which is innate), the quality of morality and God as a part of the product of our reason, i.e. the self-realization of the idea without sensuous activity. Historical materialism has disproven the independence of morality, God, and such concepts from the relations of society. And let us go further: historical materialism has disproven the conception of our innate senses as independent of our societies. Do we not all grow numb to certain pains, whether from work or the emotional turmoils of our late capitalist era? Is our personal perception of time not changed by work, schooling, and the ubiquity of clocks? Does learning Western geometry not impart to us the predisposition to see triangles and squares everywhere?

I wish to emphasize here that we are not denying the reality of time, space, or of pyramids and cubes as they exist in reality, but simply to point out that to see the piled-up rocks in the middle of a sandy desert as a great pyramid, you were taught to do so by your society. Perhaps you would have figured out triangles on your own as a feral child, but the fact that you didn't shaped you. Equally, your emotions and pain receptors as a biological fact can only ever be predisposed to certain numbness or change, but your sensibilities, as Marx details in Grundrisse, are a much more complicated and wonderfully interesting question, which I would hope we do not shut down in the interests of rigid understanding of realism.
Kant attempts to connect his transitory historical morality to a fact (and that is a mind-independent, God-dependent external reality) of reason, innate to the rational mind. Equally, when he sees spacetime as a Godly product of intuition, which has led some Kantians to struggle with any theories of physics, he asserts a kind of independence of the mind and the idea from the material in a Marxist sense. This is where his idealism departs from the material, and why we say we are opposed to idealism and mysticism. The point of departure is not realism, but the denial of dialectical materialism, which is not unique to idealists.

The chief defect of all hitherto existing materialism [...] that the thing, reality, sensuousness, is conceived only in the form of the object or of contemplation, but not as sensuous human activity, practice, not subjectively. Hence, in contradistinction to materialism, the active side was developed abstractly by idealism – which, of course, does not know real, sensuous activity as such.

Theses On Feuerbach, Marx

Such is to say equally that the old materialism, the vulgar materialism of the bourgeois philosophers, is not so much concerned with the characteristics of humanity as a living, breathing organism, that is, a society, but with the object of man as isolated from its environment and innate, the study of rats in a cage to determine the inherent 'ratish quality' - but even a rat will behave differently in a cage by itself compared to a happy colony. Ironically, the idealists here are the ones who assert self-realization, and the materialists present an eternal view of mankind. This is why vulgar materialism, without a dialectical understanding that neurons may create consciousness as a whole, tends to overexplain the processes of the brain, or in our day, attempts to call the various advancements in learning algorithms a real consciousness.

It is worth, as an aside, to point out that in the real world, for human beings who live and act, the abstract question of the external world is irrelevant, as they produce and change a world through their physical body, which they control through their brains and thought-processes, and within which they can self-realize - even if not for the first time in history, then for the first time in a manner which can lead to the radical change of their social organization - the social basis for the development of these physical realities which they experience, so that there is no more mystic thought. That is to say that the proletariat is not very concerned with the question of "realism" in practical terms; solipsism is mostly reserved for monks and niche AI forums, but it is greatly concerned with the question of historical materialism.

Reality, though, for us, does not exist where we do not experience it in our labour and efforts. We do not make scientific theories about what lies beyond the observable universe; we only speculate. We have reached such a magnitude of domination over the natural world, though domination should not be confused with control as the climate emergency demonstrates, that our experiences have grown to a massive scale, to where we can prove theories about Hawking radiation millions of lightyears away. Despite this, human action does not dominate all; it does not control the market as the product of blind profit-motive between ignorant and competing actors, nor does it have direct social control of production, nor of the state, which finds itself a product of intersecting systems of capital and politics as both interest and bureaucracy. We can attribute to this lack of experience the phenomenon of commodity fetishism, which hides Marxism's great revelation of the past relationships interwoven into every coat: who was paid, who commanded, and what wasn't paid - a commodity presents itself to us born naked, defined by its price tag, yet it contains a living history. And we can attribute much spiritualism and ignorance around the state and the economy - perhaps we can attribute to this the often staggering poverty of economics and psychology as a science, and the richness of Marxism, which experiences exploitation directly.

That the external world, as it may, enforces limits on us and we do not choose the circumstances of physics or reality is a truism of the human experience, and agreed upon by every living person who did not jump off the Eiffel Tower to try to fly, but these limits are not rules insofar as 'rules' are what humans create. For we may use Newtonian gravity, general relativity, or even alternative ideas such as the principle of least action; these rules are defined in the limitations the theory determines, which we now know are incompatible with Quantum Mechanics in one way or another. And yes, before I am accused of the worst solipsism imaginable, nature provides 'rules' in your meaning of the word. I would also like to provoke you into considering when the rules change. Disappearing polymorphs have left medical science clueless several times, and false vacuum decay has inspired many horror writers. My attempt here is not to deny that nature and its laws exist; in fact, both of these concepts are based on scientific understandings of the laws of nature, but instead it is to beg you to allow the creativity of humanity and let yourself question: "If instead of our current understanding of natural laws, another theory had risen to dominate the sciences, how different would our understanding of the world be? What would be more truthful to our experiences and what less, what would be more complicated and what would be simpler to calculate?" General relativity and quantum physics, as explained in the article, are not compatible. There is no Great Theory of Everything as it stands. According to Marxists, the question of truth is not a theoretical question but a practical question; a person has to prove a working model of our reality, and yet the model, even if proven, may have competition with other proven but simpler models, which will take precedence. So we must question in both the quantum and the bigger realm: what of our approach is falsifiable and what is opportune, and since it would seem both theories are wrong and neither are strictly speaking universal 'rules of nature', is it even possible for us to unite all of physics into one theorem, as I would hope one day?

The fact that I think, and I am sure at least some Cosmonaut readers, in terms of forces, impulses, accelerations, momentum, etc., that we see the apple falling because of gravity, is a product of the long history of science and technology, stemming even before Aristotle and crystallizing first in the Newtonian revolution, “seeping” into the wider culture, influencing the very way we perceive. The notion that everything is made of particles, and that these have a definite state, is not a universal feature of all human culture; it may seem trivial to you and I, but it was a tremendous revolution in human cognition. That quantum seems to therefore violate some of these assumptions is not a testament to a violation of “common sense”, or some one standard of “human perception”. The CP is not inbuilt into the human mind.

In this article, it is this which inspires in me the hope that these points may not be lost, as we both understand that what we see as natural laws are the interpretation of a phenomenon through our scientific, and therefore inherently cultural, Western lens. There are no imps forcing apples to fall from a tree, but an apple falls, and we have invented a whole plethora of theories to calculate the trajectory or speed of any future apple that may fall, because that is what is useful to industry. But it is not the argument among almost any, if any, person that an external reality does not exist, with the extension of this idea to Quantum Mechanics.

It must be said that if we are denouncing absolute realism, that a mind-independent world is fully knowable, to be false, then we start to devolve into petty arguments about what 'deeply knowable' exactly means as the core of Marx - which I do not wish to engage with. Here I feel I must return to Kant for a single reason: if it is true that we divide the world up from our innate sense, then scientific inquiry, as Dietzgen supposed, is infinite. That is to say, quantum particles, like the atom before it, are not the smallest object of study for science may deal with, and the observable universe is not the largest, but these are the limits of our current sensuous experiences, and science is always evolving in such a way that we are able to experience. This includes the limits of measurement, which we have discovered in our experiences with quantum mechanics. Whether we will expand our realms of experience is unknown, except with the victory of socialism, upon sociology. If so, we ought to consider that our knowledge is not so much deep (what depth to infinity have we descended upon?) but simply useful for the social reasons of human knowledge, which in economics currently serves capital, and in quantum physics serves a complex web of reasons, including the desire to make a comprehensive model of the universe, if nothing else.

If we consider this concept instrumentalism, then I would encourage thought on what exactly science is if not an instrument for 

a profoundly social enterprise in which persons not only must know and discover, but show others of their discovery and knowledge, and furthermore, be skeptical and open to revising their beliefs. The fruit of this dialectical process is the human race's collective ability to profoundly change the world, for good or bad, epitomized in the tragic destruction of our planet.

As Kateb mentions, "even then it is logically possible to assume the obverse of the mind-dependent content of knowledge." Kant, Dietzgen, or myself here, did not ever propose that the mind creates all of reality as if we are in the animus from Assassin's Creed. If the mind processes knowledge in such a way, say even by the society in which it inhabits, it still processes knowledge which is given to it independent of itself, an exterior mind-independent world (though Kant and Hegel saw this as divine). What Marxism tells us instead, the materialist conception of history, is that (to borrow from Hegel's concept of master-slave dialectics) the proletariat will be changed by its necessity to sell labour-power, it will self-realize to learn its place in history, a history which is materially determined by the relationships between people (that is defined by their classes), while the bourgeoisie becomes truly dependent on the cycle of M-C-M'. God, in the same fashion, is a product of material history and not an external reality. 

If we understand, as I have laid out here, what the idealists really proposed, which is not a skepticism about reality existing outside of our mind, but the method of the mind of processing reality and the creation and self-development of ideas as inherent to ourselves, history and God, it is easy to see why a mind like Marx's was attracted to these philosophers and managed to turn them from their heads to really establish the living consciousness of mankind, that is, to determine the material conception of history as real relationships, that relationships make more than their whole, such that a brain can create consciousness without there being a literal material-substance of consciousness, and that a capitalist economy can create a market not as some kind of God or eternal natural law but independent of its own direct control by the relations between its sellers and buyers, as a social law which is upheld solely by the inherent conditions of its relationships.
What does dialectical materialism, as far as Marxists have incorporated it into their corpus, mean then? Perhaps, we can describe Dialectical Materialism as seeing the moving and changing live relationships within and between humans as a part of the material whole, such that we assert dialectical materialism as a tool at the proletariat's disposal for self-realization and class consciousness, to know their position in history as a struggling underclass, that built-in to the systems of capitalism itself there is the opportunity that we may free ourselves to seize power, for all of humanity, and to really topple all old civilization as we know it - that is, to change the world, and not only interpret it in various ways. To make the whole world a product of human thought and action, without delegation to a mystical law or idea.

By understanding the differences between idealism, mysticism, vulgar materialism, and epistemological skepticism on a deeper level, we can understand how each influences the class conflicts of our time, even if the conspectus of sources and books on the subject may as well be infinite.

Here I have simplified many concepts in the interests of readability (and it is still too long), and I have to acknowledge I am not educated at a university-level in regards to physics, but I hope these leaps can be forgiven to emphasize the ways in which a historical misunderstanding continues to today to limit our interactions with the sciences, and impoverishes our thought; if we only had to think about proving that reality really exists to the enemies of Marx and our apathetic coworkers - instead of the much deeper and invasive problem of what the products of our mind entail - perhaps the bourgeoisie of the world would hand over the keys to the kingdom without a fight.

I hope this letter is received well and in the spirit it is intended. If my thoughts are shown to be wrong, I welcome the dialectic.

Thank you,

F. V.