What is dialectical materialism?

(This was part of a longer article on revolution and free will posted to alt.politics.socialism.trotsky by "Tecumseh")

Historical materialism is the understanding that human society is part of material reality which develops over time according to the laws of physics, chemistry, biology etc. Human society is a subset of animal society, life on Earth, planetary evolution, and the evolution of the universe. Human ideas are products of, and part of, human societies.

Dialectical materialism is the understanding that the evolution of all matter and energy does not take place in a mechanical way that can be explained arithmetically (by formal logic), but through a process of internal and external conflicts and transformation of one form to another. For example, the evolution of the chemical elements through the process of the birth and death of stars, or the evolution of new species of life from other species of life, or the transformation of an embryo into a fetus into a human being, or new forms of human society from old forms of human society. Quantitative changes can lead to qualitative changes under the right conditions.

An important component of dialectical materialism is its understanding that human consciousness is an incomplete reflection within individuals and societies of the totality of reality. This incompleteness could be called simply "ignorance", although it is more complex. Humanity has, for its four or five million year existence on this planet, slowly, and now more rapidly, overcome its ignorance of the material reality it is a part of, and of itself. However, Lenin argued - and I agree, that the rapidly changing nature of material reality makes it impossible for our consciousness ever to be complete.

(Mechanical materialism is the term used by Engels and others to define those materialists who saw the development of matter as mechanical and arithmetical: change meant more or less of a particular kind of matter or motion, not the production of new kinds of matter and motion through a process conflict and development. Mechanical materialism consequently saw human thought as a simple reflection of reality, rather than a contradictory process. Diderot was a mechanical materialist, the British empiricists approached mechanical materialism at times, although their principal leaders like Bishop Berkeley were philosophical idealists.)

Dialectics is the idea that history develops through contradictory stages. Before Marx and Engels, this idea was developed to its then highest point by idealist philosopher Hegel. As I understand his ideas (and I am neither a Hegelian, nor an expert on this arcane - though important - cranny of human thought) Hegel believed that all existence was the dialectical unfolding of the absolute idea, and that every stage in that development was necessary, preordained by the previous stages, back to the beginning when God (or the absolute idea) created existence. For Hegel, material existence was a pale reflection of the absolute idea. Marx was trained by Hegelian philosophers against whom he rebelled.

Scientific socialism is the set of ideas that inclu