I feel like I understand communist theory pretty well at a basic level, and I believe in it, but I just don't see what part of it requires belief in an objective world of matter. I don't believe in matter and I'm still a communist. And it seems that in the 21st century most people believe in materialism but not communism. What part of "people should have access to the stuff they need to live" requires believing that such stuff is real? After all, there are nonmaterial industries and they still need communism. Workers in the music industry are producing something that nearly everyone can agree only exists in our heads. And they're still exploited by capital, despite musical instruments being relatively cheap these days, because capital owns the system of distribution networks and access to consumers that is the means of profitability for music. Spotify isn't material, it's a computer program. It's information. It's a thoughtform. Yet it's still a means of production that ought to be seized for the liberation of the musician worker. What does materialism have to do with any of this?
I think it's a matter of scope you're not considering. Mechanical materialism is what you are referring to when you are creating a division between mechanical substance and metaphysical substance. Marx draws on Hegel who draws on Spinoza who says that mechanical substance and metaphysical substance are composed of the same thing, while understanding that metaphysical substance is self generative and not determined by mechanical substance in and of itself.
Marx's dialectical materialism is a unity of social reality meaning it's an understanding that there is both a true form of existence in the material world with complex social concepts existing as a part of that reality. The point of this epistemology is that it helps us understand where truth comes from (that is beyond metaphysical symbolic truth), which is a useful tool in actually changing the world.
Sure there are mystic truths beyond the scope of Marxism, but they are functionally useless in changing the world which is the primary goal of Marxism.
Are you saying that property dualism is compatible with Marx's materialism?
Oh, now this seems like a concrete claim we can test. So, would propaganda fall within one of these mystic truths or within Marxian materialism?
Marxian materialism. It is not property dualism because in my view Marx agrees with Hegel that property dualism subjects the metaphysical to be subordinate to the physical. Propaganda is a metaphysical notion informed by physical observations but those also physical observations get their character from the notion. It's a bit ridiculous to assume propaganda, which is defined by its capability to propagate ideology, is a purely physical thing and would involve a ridiculous amount of loopholes to explain within a mechanical materialist worldview. Marxian materialism doesn't hold a primacy of one or the other but doesn't claim an agnosticism to the difference, rather there is a very specific dialectic between the two.
"It is dualist because it is monist. Marx’s ontological monism consisted in affirming the irreducibility of Being to thought, and, at the same time, in reintegrating thoughts with the real as a particular form of human activity." Sartre, Critique of Dialectic Reason
Philosophy is not exactly my strong point but I think you might get a kick at least out of Critique of Dialectic Reason if you are trying to triangulate how you feel about Marxian materialism. As you are now, you are completely denying the character of the real as possible to be understood at all and reducing it to a matrix of symbols completely detached from the real at all, which doesn't incorporate that while the symbolic and social reality is the lens with which our minds functions to make "sense" of the real there still exists a real that informs those symbols at the same time.
Or in other words how can you possibly hope to change anything when you can only ever know nothing.
There's an old saying from chaos magic, and maybe you've heard it in Assassin's Creed as the philosophy of the Assassins too: "nothing is true. everything is permitted."
If I believe in nothing, then I can choose to believe in anything. I find unrealism to be revolutionary.
I think that is a great basis for revolutionizing our ideas, and in many many ways I adhere to that same ethos. I think it needs to be dialectically balanced however with the need to enact real social change on a society wide scale, where things are true given certain assumptions. While the assumptions may be problematic in certain contexts, the outcomes are undeniably real and that is the strength of Marxism. We can deny the symbolic as "truth" but we can't deny the real no matter how we try.
Why not a simple relativist answer to the problem?
"I want to have a revolution because capitalism causes me to perceive myself and others as suffering. I have a subjective distaste for suffering and choose to impose my personal views upon the world by supporting communism. I will use the scientific method to determine which actions of mine reduce perceived suffering, and then I will do those actions."