The classical Greek dialectic was synonymous with the notion of synthesis. A does or says X, B does or says not X, they duke it out and eventually come to someplace in the middle. That's a very flippant presentation of the Greek dialectic, but it summarises everything I remember reading about it. There is also a Confucian dialectic, with a long, complicated history of which I am almost completely ignorant, and ought to know more.
Hegel's dialectic, IIRC, is built on the idea that change is the product of internal forces in things. Since I don't doubt there are some Hegel scholars out there and I have read only a little of Hegel, I'll leave him there.
Dialectical materialism, at least as I've always thought of it, flows from certain propositions.
_There is nothing in the universe except normal matter (or mass/energy for the Einstein-enabled)_
No gods, no disembodied minds floating around in Cartesian ideal places. Everything that can have causative effect is in the universe and made of the same stuff as everything else.
_Everything is in a dynamic state_
Where something seems to have stability and permanence it is the product of a dynamic, and temporary, equilibrium. The sun exists because of a dynamic equilibrium between the forces crushing and the forces blowing it up. Capitalism exists because of a dynamic conjunction of forces (economic expansion, technological change in productivity, consumer debt, a billion other things) enables it to continue. Someday, the sun will blow up. Someday, capitalism will blow up.
_Qualitative change only happens as a result of some physical action involving real, regular matter_
People develop out of dead things because of physical activities. The transformation of dead matter into living matter is due to physical, chemical processes. "Life" or "non-life" is a property of an assemblage of matter, not of matter itself.
_Entities and categories only exist in the human mind_
Humans have no unmediated, "God's eye" access to the universe, which apart from the human assignment of categories is just a big place where stuff happens. Our only true knowledge of the universe comes from practice. If I drop things, they fall. The only way I can know about gravity it is because the practices informed by my theory of gravity (like not dropping things) are successful. So human categories aren't arbitrary, but they are still made by humans and make sense only in a human centred way. They don't have some extra-human existence. If I had some kind of God's eye view of the universe, all I could say is that stuff moves around.
_Things don't exist independently of their environment_
Analysing things abstractly, in isolation from the systems they're embedded in, can lead to misleading and inaccurate conclusions. If I want to understand how human hands work, I can chop one off of somebody and subject it to whatever tests I want, but as millennia of dissection shows, you won't learn much about how hands evolved, or how people use them, and you won't learn squat about the neural organisation of human motor control.
These aren't fully independent propositions, but I think they are better than some mystical sounding quasi-Hegelian vocabulary, which seems to be the favoured way of talking about dialectical materialism. Marx can be excused for using that kind of vocabulary. Hegel was still widely read and debated in his time. Today, it's just anachronistic and mystifying.
None of these propositions is a scientific theory. I am hard pressed to see how any falsifiable statement can be derived from any of them. All of them have been held to be true, at one time or another, by non-Marxists. Marx and some later Marxists are, I believe, the only ones to stick to all of them and build a philosophical system out of it.
Dialectical materialism is not, therefore, a scientific theory, but then neither is materialism, nor mathematics, nor realism, nor any other philosophical conjecture held to be necessary by scientists. Dialectical materialism isn't a religion, or at least shouldn't be, any more than scientific realism is. It is a philosophy, and you can't not have a philosophy.
It is possible for someone who does not believe in materialism to accept quantum theory, but if physicists allowed themselves to accept the intervention of extra-material forces every time they encountered something they couldn't explain I don't see how a scientific theory like quantum mechanics could ever have been developed. Therefore, it should hardly be surprising that non-Marxists can say that Marx, or some committed Marxist, did good work in his analysis of some particular phenomenon. It seems to me Popper praised Marx' analysis of the business cycle. Social scientists everywhere (or at least honest ones) praise Marx for calling so much attention to the economic factors in cultural development. Vygotsky, a seriously Marxist psychologist who plainly stated that dialectical materialism was behind his work, is at the root of a whole school of psychology where nearly no one credits dialectical materialism.
There are a lot of corollaries and consequences to the ideas I've outlined. One of them is the importance Marxists usually place on the mastery of history and development. If everything comes into being and changes over the course of its existence, then trying to understand things statically, as if they could stay the same forever, is clearly never good enough. Thus the emphasis on process. Another is the tendency to prefer viewing things holistically and to mistrust reductionist explanations, since things don't exist outside of their environment and their functioning is specific to their environment. Thus, the Marxist belief that consciousness is socially determined.
Anyway, that is the kind of thing I mean by dialectical thinking.
Scott Martens