In their German Ideology Marx and Engels argue that humans are distinguished from animals from the moment they start producing their livelihoods. Individuals are the product of the way they make their livelihoods and what those livelihoods are. Thus, according to Marx and Engels, the nature of individuals depends on material conditions that determine their production.
The extent to which a nation's productive forces have developed can be judged by the degree to which a nation has implemented the principle of division of labour. In addition, there is a direct link between the social division of labor and forms of ownership.
The ruling class, which governs the material dimension of society, is thus at the same time the class which governs the intellectual dimension. It regulates the production and dissemination of ideas of its era. As the ruling class changes over the years, the ideals it produces and disseminate change, it is therefore incumbent upon the ruling class to make society believe that its ideals are universal in nature. This system persists as long as a society is organized around the need for a ruling class.
To clarify this theoretical framework, Marx and Engels introduce the image of the substructure and superstructure. Historical developments, which are part of the superstructure, are only the reflections of changes of the substructure, which consists of the economic and material relations of a society. When there is a change in those proportions, the superstructure follows automatically. Here the work of Marx and Engels leads to a form of ideology critique: ideas and thus ideologies are not the cause of historical changes, but the consequence thereof. This view of ideology enables Marx and Engels to dismiss views of the proletariat that are clearly against its own interests as forms of false consciousness., that is, as ideas emanating from the ruling class.
During revolutions, changes in the substructure lead to eruptions of the superstructure. The ideas that are central during a revolution are nothing but the reflection of tensions that were already present in the less visible substructure.
The core of The German Ideology can be summarized as follows. Morality, religion, metaphysics and all other forms of ideology and their attendant forms of consciousness lose any form of independence. They are only a reflection of the way people generate and distribute their livelihoods, and only by focusing on the production of their livelihoods is it possible for them to change their ideas and daily lives.