Part 1: Materialism and Idealism
The concept of dialectics and dialectical materialism is complicated, subtle, and, in my not-at-all humble opinion, not well understood even by canonical communist writers, including Lenin, writing in Materialism and Empirio-criticism or Mao writing in On Contradiction. We can gain some insight from these brilliant thinkers, but more is needed.
I don't feel confident in my own understanding of dialectical materialism; I'm writing these posts to explore and expand my own understanding. Also, I'm not much of a philosophologer. I'm not interested in interpreting or even criticizing earlier work on the subject. If you want to find out what Mao himself believed about dialectical materialism, read On Contradiction yourself and draw your own conclusions.
The materialism part of dialectical materialism seems fairly straightforward: nothing exists except the physical, material world. Materialism stands in opposition to idealism; idealism holds that some "ideas", such as the idea of a circle, or the idea of physical law, or the idea of numbers, or our minds themselves, exist independently of material reality and exert some discernible influence on the material world or our understanding of it. Plato's theory of forms is the most obvious example of idealism. As the Stanford Encyclopedia of Philosophy notes, in Plato's middle period dialogs,
we find remarks about the immortality of the soul, about special entities called ‘Forms’ that exist outside of space and time and that are both the objects of knowledge and somehow the cause of whatever transpires in the physical world, and the doctrine of recollection, the thesis that the immortal soul, in a disembodied state prior to its incarceration in a body, viewed these Forms, knowledge of which is then recalled by incarcerated souls through a laborious process.Materialism, on the other hand, denies an separate existential category to abstract or universal properties; these properties are immanent or reducible to immanent properties.
One way of thinking about the difference is thinking about the "laws of physics": Do the laws of physics exist independently of the material world, or are they ways of talking about the immanent properties of reality? An idealist takes the former view; the materialist the latter view.
Philosophy has struggled for millennia to deal with the paradox of change and constancy. The controversy goes back to the earliest written Western philosophy, with Parmenides and Heraclitus. The Stanford Encyclopedia of Philosophy notes that
Parmenides argued that there is and could be only one thing, Being. One could not even think or say what is not. Moreover, since change implies that something comes to be what it was not—I change from not being tan to being tan, nothing can change. Reality is static. ...If reality is static, why does it appear to change? If reality is change, why do there appear to be things that don't change, and why are is there any consistency at all as things change?
[In contrast] Heraclitus is the apostle of change. For Heraclitus, the ordinary objects of the physical world seem to be continually changing. The only constant, the underlying commonality, is the pattern of change itself. That there are entities that do not change is, for Heraclitus, an illusion. Heraclitus' notion of ‘flux’ seems to have influenced Plato's thinking about ordinary material objects.
Because things manifestly appear to our senses to change, one of the earliest philosophical positions was that our senses are fatally unreliable, that all "true" philosophy consists of purely abstract reasoning uncontaminated by the senses. To make this view work, abstract ideas, unsullied by material and perceptual c, must exist and exert a direct causal influence on our minds; mind itself (or some portion of it) must likewise exist independently of the material, perceptual world: The idea of an entirely disembodied intelligence, with particular properties and characteristics but lacking any material instantiation whatsoever, must in some sense be coherent and conceivable. A disembodied mind, for example, could contemplate pure mathematics, and deduce mathematical theorems.
Of course, materialism faces the same problems from the opposite position: Why does mathematics even appear to yield actual truth that appears to be independent of the material world? We can, for example, contemplate a material universe with large or small difference from our own, but it does not appear physically or psychologically possible for us to contemplate a material universe where we could put two stones in a bowl, put two more stones in the bowl, and actually count five stones in the bowl. We would suspect our sanity (or stage magic) before believing there were actually five stones in the bowl.
There's a lot of philosophy discussing idealism vs. materialism; I myself have little to contribute. It's enough to say that on the available evidence, materialism wins hands-down as the simplest scientific theory to explain the available evidence. Neurology and human consciousness are still poorly understood, however, and further evidence might compel a change in the evaluation; even so, the limited amount of evidence we presently have points strongly to a materialist interpretation of human consciousness. Non-materialists (idealists, dualists, and empirio-critics) presently have nothing more than "god of the gaps" arguments.
It must be said of materialism that 20th century scientific study of quantum mechanics has substantially changed our notion of what constitutes "material". Before quantum mechanics, it seemed obvious that ordinary matter — atoms of various elements, from hydrogen to uranium — constituted the material of the universe. However, quantum mechanics has shown us not only that the elements of atoms are themselves divisible (to subatomic particles such as protons, neutrons and electrons) but also that these so-called "particles" behave in ways alien and completely foreign to our understanding of classical mechanics. But this just means that we have a new way of describing the material of a materialist universe.
The most important distinction between idealism and materialism concerns the human mind. The materialist view is that whatever the nature of the material of the universe, human minds are made up of that material, and the behavior of the human mind is ontologically reducible to that material: minds are fully of and within material reality. In contrast, idealism holds that minds are, at least in part, apart from the material, i.e. the material that obviously makes up our brains.